1004 Chapter Eighteen LINE INTEGRALS across a cross-section of the wire. The magnitude of B is constant along any circle which is centered on and perpendicular to the z-axis. The direction of B is tangent to such circles. Show that the magnitude of B , at a distal)Ce r from the z-axis, is given by { al -- I\B II= 2;;;. for r 2:: ro -- 21rr0 2 for r < 1·0. (b) A torus is a doughnut-shaped surface obtained by rotating around the z-axis the circle (x - (3)2 + z2 = o:2 of radius a and center ((3, 0, 0), where O < a < (3. A toroidal solenoid is constructed by wrapping a thin wire a large number, say N, times around the torus. Experiments show that if a constant current I flows though the wire, then the magnitude of the magnetic field is constant on circles inside the torus which are centered on and perpendicular to the z-axis. The direction of B is tangent to such circles. Explain why, at all points inside the torus, 11.B II= aNI 21rr ' and 11.B II = 0 otherwise. [Hint: Apply Ampere's Law to a suitably chosen surface S with boundary curve C. What is the net current passing through S?] •�11.... 4. Conservative Forces, Friction, and Gravity The work done by a conservative force on an object does not depend on the path taken but only on the endpoints-in other words, the vector field representing the force is path-independent. Only for a conservative force is the work done stored as energy; for a non-conservative force, the work may be dissipated. (a) The frictional drag force f on an airplane flying through the air with velocity iJ is given by f = -cl\v \\v. The constant c is positive and depends on the shape of the airplane. One airplane takes off from an airport located at the point (2, 0, 0) and follows the path r(t) = (2cost)T + (2sint)j+ 3tk, from t = 0 tot= 1r. An identical airplane takes off from the same point and follows the path - 2- 4t r(t) =--tr+ 3tk 7r also from t = 0 tot= 7r. (i) Compute the total work done by the drag force f on each airplane. (ii) Is the drag force conservative? Explain. (b) Suppose the two airplanes of the preceding problem each have mass m, while the earth has mass !YI. The gravitational force f acting on either airplane at a point with position vector i" is given by - F (r) = -GMm r l\r\l3 , where G is the gravitational constant. (i) Compute the total work done by the gravitational force f on each airplane as it moves from the point (2, 0, 0) to the point (-2, 0, 31r ). (ii) Is the gravitational force conservative? Explain. 1006 Chapter Nineteen FLUX INTEGRALS AND DIVERGENCE 19.1 19.1 THE IDEA OF A FLUX INTEGRAL THE IDEA OF A�� FLUX-----INTEGRAL ---�------------------------------------ Flow Through a Surface Imagine water flowing through a fishing net stretched across a stream. Suppose we want to measure the flow rate of water through the net, that is, the volume of fluid that passes through the surface per unit time. Example1 1007 s Direction of orientation: Positive flow Figure 19.2: An oriented surface showing directions of positive and negative flow A flat square surface of area A, in m , is immersed in a fluid. The fluid flows with constant velocity 2 v, in m/sec, perpendicular to the square. W rite an expression for the rate of flow in m3/sec. Figure 19.3: Area vector A = n A of flat smface with area A and orientation n The Area Vector !he flux through a fl �t surface depends both on the area of the surface and its orientation. Thus, 1s useful to represent its area by a vector as shown in Figure 19.3. The area vector of a flat, oriented surface is a vector A such that • The magnitude of A is the area of the surface. • The direction of A is the direction of the orientation vector 11, . The Flux of a Constant Vector Field Through a Flat Surface Figure 19.1: Fluid flowing perpendicular to a surface Solution Suppose the velocity vector fieid, v, of a fluid is constant and A is the area vector of a flat surface. The flux through this surface is the volume of fluid that flows through in one unit of time. The skewed box i': Figure l�.4 has cross-sectional area II.A II and height llv II cosB, so its volume is (llv II cosB) IIA II= v ·A. Thus, we have the following result: In one second a given particle of water moves a distance of llv II in the direction perpendicular to the square. Thus, the entire bcidy of water moving through the square in one second is a box of length llv II and cross-sectional area A. So the box has volume llv IIA m3, and If v is constant and A is the area vector of a flat surface, then Flow rate = llv IIA m 3/sec. Flux through surface = This flow rate is called the flux of the fluid through the surface. We can also compute the flux· of vector fields, such as electric and magnetic fields, where no flow is actually taking place. If the vector field is constant and perpendicular to the surface, and if the surface is flat, as in Example 1, the flux is obtained by multiplying the speed by the area. Next we find the flux of a constant vector field through a flat surface that is not perpendicular to the vector field, using a dot product. In general, we break a surface into small pieces which are approximately flat and where the vector field is approximately constant, leading to a flux integral. v · A. Orientation of a Surface Before computing the flux of a vector field through a surface, we need to decide which direction of flow through the surface is the positive direction; this is described as choosing an orientation.' At each point on a smooth surface there are two unit normals, one in each direction. Choosing an orientation means picking one of these normals at every point of the surface in a contin­ uous way. The unit normal vector in the direction of the orientation is denoted by ii . For a closed smface (that is, the boundary of a solid region), we choose the outward orientation unless otherwise specified. We say the flux through a piece of sur ace is positive if the flow is in the direction of the orientation and negative if it is in the opposite direction. (See Figure 19.2.) Figure 19.4: Flux of iJ through a smface with area vector A is the volume of this skewed box Example2 Water is flowing down a cylindrical pipe 2 cm in radius with a velocity of 3 cm/sec. Find the flux of the velocity vector field through the ellipse-shaped region shown in Figure 19.5. The normal to the ellipse makes an angle of e with the direction of flow and the area of the ellipse is 41rj (cosB) cm2 . �-- A (where IIA 11 = 41r/ (cos B) cm2 ) ---- iJ ( where llil 11 = 3 cm/sec) f 1 Although we will not study them, there are a few surfaces for which this cannot be done. See page !012. Figure 19.5: Flux through ellipse-shaped region across a cylindrical pipe I -I 1008 Solution Chapter Nineteen FLUX INTEGRALS AND DIVERGENCE 19.1 THE IDEA OF A FLUX INTEGRAL There are two ways to approach this problem. One is to use the formula we just derived, which gives Flux through ellipse= iJ ·A= llv IIIIAII cosB = 3(Area of ellipse) cosB In computing a flux integral, we have to divide the surface up in a reasonable way, or the limit might not exist. In practice this problem seldom arises; however, one way to avoid it is to define flux integrals by the method used to compute them shown in Section 21.3. = 3 (�) cosB = 127T cm 3 /sec. cose The second way is to notice that the flux through the ellipse is equal to the flux through the circle perpendicular to the pipe in Figure 19.5. Since the flux is the rate at which water 1s flowrng down the pipe, we have Flux through circle= Velocity of water x Flux and Fluid Flow If iJ is the velocity vector field of a fluid, we have Rate fluid flows through surface S Area of circle a If the vector field, F, is not constant or the smiace, S, is not flat, we divide the surface into e 6:.A, area with patch particular a For 19.6.) "'!._, patchwork of small, almost flat pieces. (See Figure pick a unit orientation vector n at a point on the patch and define the area vector of the patch, 6:.A , as t:.A = n t:.A. Example3 Flux through patch � Flux through whole surface � -yi + xj B(x,y,z)= x 2 +y 2 z s L · F 6:.A. l�m ll6A 11-+0 Thus, provided the limit exists, we make the following definition: b.A --!--- y x Solution L F . t:.A. If Sis a closed surface miented outward, we describe the flux through Sas the flux out of S. Area b.A 2 y Figure 19.8: The vector field i +xi B (x , y , z) = -Y_ x2+y2 The flux integral of the vector field F through the oriented surface S is jS z s 2 L F · 6:.A, As each patch becomes smaller and 116:.A II ----+ 0, the approximation gets better and we get l�m llll.A 11-+0 - - i F · 6:.A, so, adding the fluxes through all the small pieces, we have = h v. dA Find the flux of the vector field B (x, y, z) shown in Figure 19.8 through the square S of side 2 shown in Figure 19.9, oriented in the direction, where (See Figure 19.7.) If the patches are small enough, we can assume that F is approximately constant on each piece. Then we know that r F . dA Flux of iJ through S The rate of fluid flow is measured in units of volume per unit time. The Flux Integral Flux through S= 1009 3 Figure 19.9: Flux of B through the square S of side 2 in xy-plane and oriented in J direction Figure 19.10: A small patch of surface with area lib.A 11 = b.xb.z Con�ider a small rectangular_patch with area vector t:.A in S, with sides 6:.x and 6:.z so that 116:.A II = 6:.x 6:.z. Since 6:.A points in the J direction, we have t:.A = 6:.xt:.z. (See Fig­ ure 19.10.) At the point (x, 0, z) in S, substituting y = 0 into B gives B (x, 0, z) = (1/x)i. Thus, we have Flux through small patch � B · t:.A = -j · (j 6:.xt:.z) = - 6:.x 6:.z. x x Therefore, i - Flux through smiace = 1B · S dA = - ( 1-) - l�m ll6A 11-+0 1 LB · t:.A= lim l\.x-+ o L ! 6:.x 6:.z. X l\.z-+ 0 Figure 19.6: Surface S divided into small, almost flat piece�, showing a typical orientation vector n and area vector b.A ,, , '. V_§ctor field 1lF(x,y, z ) Figure 19.7: Flux of a vector field through a curved surface S This last expression is a Riemann sum for the double integral JR 1 :S x :S 3, 0 :S z :S 2. Thus, Flux through surface= {B Js · dA = { � dA= JR x ! Jo 2 i dA, where R is the square 3 { 11 � x dx dz = 2 ln 3. The result is positive since the vector field is passing through the smia�e in the positive direction. 1010 Chapter Nineteen FLUX INTEGRALS AND DIVERGENCE Example 4 19.1 THE IDEA OF A FLUX INTEGRAL Each of the vector fields in Figure 19.11 consists entirely of vectors parallel to the xy-plane, and is constant in the z direction (that is, the vector field looks the same in any plane parallel to the x y ­ plane). For each one, say whether you expect the flux through a closed surface smTmmding the origin to be positive, negative, or zero. In part (a) the surface is a closed cube with faces perpendicular to the axes; in parts (b) and (c) the Ulface is a closed cylinder. In each case we choose the outward orientation. (See Figure 19.12.) W Y M "- \ \ Y � I I/ l \ -- x +-·---"I---- x I Y 1 -- - - ,,... __ .- /-- ��- x \ I I I Figure 19.13: Flux of E = q1' /117' 113 through the surface of a sphere of radius R centered at the origin Solution \ 1 . This vector field points radially outward from the origin in the same direction as n Thus' since ii, is a unit vector, E · t.A' =E · n t.A = IIE 11 t.A. 2 R On the sphere, IIE II = q/ , so Figure 19.11: Flux of a vector field through the closed surfaces whose cross-sections are shown in the xy-plane z z x -y ii r E . dA = Js - ii y x r- lsE Figure 19.12: The closed cube and closed cylinder, both oriented outward (a) Since the vector field appears to be parallel to the faces of the cube which are perpendicular to they- and z-axes, we expect the flux through these faces to be zero. The fluxes through the two faces perpendicular to the x-axis appear to be equal in magnitude and opposite in sign, so we expect the net flux to be zero. (b) Since the top and bottom of the cylinder are parallel to the flow, the flux through them is zero. On the curved sU1face of the cylinder, iJ and t.A appear to be everywhere parallel and in the same direction, so we expect each term iJ · t.A to be positive, and therefore the flux integral fs iJ ·dA to be positive. (c) As in part (b), the flux through the top and bottom of the cylinder is zero. In this case iJ and t.A are not parallel on the round surface of the cylinder, but since the fluid appears to be flowing inward as well as swirling, we expect each term iJ · t.A to be negative, and therefore the flux integral to be negative. Calculating Flux Integrals Using dA = rt d A n For a small patch of surface t.S with unit normal and area t.A, the area vector is t.A =11, t.A. The next example shows how we can use this relationship to compute a flux integral. Examples An electric charge q is placed at the origin in 3-space. The resulting electric field E (r) at the point with position vector r is given by - E (r)= q r 3 llrll ' Find the flux of E out of the sphere of radius R centered at the origin. (See Figure 19.13.) l�m ll6A 11-+0 LE . t.A = lim � _J_2 t.A= _J_ lim � t.A. 6A-+0 L..., R R2 6A-+0 L..., The last sum approximates the surface area of the sphere. In the limit as the subdivisions get finer we have Jim � t.A =Surface area of sphere. 6A-+OL..., Thus, the flux is given by � Solution 1011 q q . ·dA- = 2}r� Lt.A= R 2 ·(Surface area of sphere)= ;2 (41rR2 )=41rq. R 0 This result is known as Gauss's law. To compute a flux with an integral instead of Riemann sums, we often write dA = ii, dA as in the next example. Example6 Solution Suppose S is the surface of the cube �ounded by the six planes x = ±1, y = ±1, and z = ±1. Compute the flux of the electric field E of the previous example outward through S. It is enough to compute the flux of E through a single face, say_the top face S1 defined by z =1, where -1 S x S 1 and -1 S y S 1. By symmetry, the flux of E through the other five faces of S must be the same. On the top face, S1, we have dA = dA = k dx dy and n xi+ yJ + k E (x y 1) -- q-,--,--__::_::_ __ ·,' (x 2 +y2 +1)3/2 · The corresponding flux integral is given by f Js 1 E . dA = q ff 1 -1 1 _1 ff 1 xi + yJ + f . f d·x dy = q 2 2 3/ 2 _1 (x +y +1) Computing this integral numerically shows that Flux through top face= Thus, Total flux of E out of cube= 1 _1 1 d .d (x 2 +y2 +1)3/2 x Y· f E . dA � 2.0944q. ls1 {E ls · dA � 6(2.0944q) =12.5664q. 1012 19.1 THE IDEA OF A FLUX INTEGRAL Chapter Nineteen FLUX INTEGRALS AND DIVERGENCE Example 5 on page 1010 showed that the flux of E through a sphere of radius R centered at the origin is 41rq. Since 41r � 12.5664, Example 6 suggests that z (a) z (b) y x Total flux of E out of cube= 41rq. By computing the flux integral in Example 6 exactly, it is possible to verify that the flux of E through the cube and the sphere are exactly equal. When we encounter the Divergence Theorem in Chapter 20 we will see why this is so. 19. The triangular plate with vertices (1, 0, 0), (0, 1, O), (0, 0, 1), oriented away from the origin. In Exercises 20-22, find the flux of H tlu·ough the surface S. Notes on Orientation 20. z (c) x (d) z Two difficulties can occur in choosing an orientation. The first is that if the smface is not smooth, it may not have a normal vector at every point. For example, a cube does not have a normal vector along its edges. When we have a surface, such as a cube, which is made of a finite number of smooth pieces, we choose an orientation for each piece separately. The best way to do this is usually clear. For example, on the cube we choose the outward orientation on each face. (See Figure 19.14.) -Y x (e) S is the cylinder x2 + y2 = 1, closed at the ends by the planes z 22. S is the disk of radius 1 in the plane x + y + z oriented in upward. 23. I F' = 3i + 2; + k 25. H J)-y = -zi + xk. 26. r) = r. In Exercises 12-15, compute the flux of iJ = i + 2J - 3k through the rectangular region with the orientation shown. 12. The second difficulty is that there are some surfaces which cannot be oriented at all, such as the Mobius strip in Figure 19.15. ln Exercises 1---4, find the area vector of the oriented flat sur­ face. 1. The triangle with vertices (0, 0, 0), (0, 2, 0), (0, 0, 3) ori­ ented in the negative x direction. 2. Circular disc of radius 5 in the xy-plane, oriented up­ ward. 3. y = 10, 0 S x S 5, 0 S z S3, oriented away f om the xz-plane. r 4. y = -10, 0 S x S 5, 0 S z S 3, oriented away f om the xz-plane. r 5. Find an oriented flat surface with area vector 150J. 6. Let S be the diskof radius 3 perpendicular the the y-axis, centered at (0, 6, 0) and oriented away from the origin. Is (xi+ yJ) ·dA a vector or a scalar? (0, 2, 2) (2, 2, 2) 7. Compute 1 14. ( 4i + 5k ) · dA, where S is the square '::. 2, 0) 8. Compute 1 (b) Away from the origin. I (0, 0, 4) x / (2,2,0) (0, 2, 0) ( , 0, 3) 0 (O, 2' 3) � y (2, o, 0) Toward the origin. (b) Away from the origin. 9. Let F' (x, y, z) = zi. For each of the surfaces in (a)-(e), say whether the flux of F' through the surface is posi­ tive, negative, or zero. The orientation of the surface is indicated by a normal vector. f5 (sinx i+(y2 +z2 )J +y2 k)·dA whereSis a diskof radius 1r in the plane x = 31r /2, oriented in the positive x-direction. J5(5i +5J +5k)·dAwhereSis a diskof radius 3 in the plane x + y + z = 1,-oriented upward. In Exercises 30-52, calculate the flux of the vector field through the surface. 30. z 31. F' = 2f + 3.'f through the square of side 1r in the xyplane, oriented upward. F' = 2f + 3.'f through the unit disk in the yz-plane, centered at the origin and oriented in the positive x­ direction. 32. F' = xi + yJ + zk through the square of side 1.6 cen� Y tered at (2, 5, 8), parallel to the xz-plane and oriented (2, 2, 0) x away from the origin. (2i + 3k) · dA, where S is the disk of ra­ dius 4 perpendicular to the x-axis, centered at (5, 0, 0) and oriented (a) 15. / of side length 3 perpendicular to the z-axis, centered at (0, 0, -2) and oriented (a) Toward the origin. ( z (2, 0, 4) 28. 29. y (xi + 4J) · dA where Sis the disk of radius 5 per­ r· � x Exercises c = xi dA where S is the sphere of radius 3 centered at 27. fs the origin. (3, 2, 4) z : Exercises and Problems for Section 19.1 1 13. z (0, o, 2) 1 = zxk 24. pendicular to the x-axis, centered at (3, 0, 0) and oriented toward the origin. 11. Repeat Exercise 9 with the vector field F' ( Figure 19.14: The orientation vector field 11 on the cube surface determined by the choice of unit normal vector at the point P 1 In Exercises 26--29, calculate the flux integral. 10. Repeat Exercise 9 with F' (x, y, z) Figure 19.15: The Mobius strip is an example of a non-orientable surface = 0 and z = 1 and oriented outward. = 2 oriented in Find the flux of the vector fields in Exercises 23-25 out of the closed box OSx S1, 0 Sy S2, 0 S z S3. z x = 2i + 3J + 5k 21. Sis the disk of radius 1 in the plane x the positive x-direction. 1--=ry I 1013 For Exercises 16--19 find the flux of the constant vector field iJ = i - J + 3k through the given surface. 16. A diskof radius 2 in the xy-plane oriented upward. 17. A triangular plate of area 4 in the yz-plane oriented in the positive x-direction. 18. A square plate of area 4 in the yz-plane oriented in the positive x-direction. JI4 33. in a horizontal F' = zk through a square of side plane 2 units below the xy-plane and oriented downward. 34. F' = -yi + and S is the square plate in the yz­ plane with corners at (0, 1, 1), (0, -1, 1), (0, 1, -1), and (0, -1, -1), oriented in the positive x-direction. 35. 36. xJ F' = 7i + 6J + 5k andSis a diskof radius 2 in the yz­ plane, centered at the origin and oriented in the positive x-direction. F' = xi + 2yJ + 3zk and S is a square of side 2 in the plane y = 3, oriented in the positive y-direction. 1014 37. F = 7i+6J +5k and Sis a sphere of radius 1r centered at the origin. 38. F = -5i'' through the sphere of radius 2 centered at the origin. 39. F = xi+Y.7 + (z2 + 3)k and Sis the rectangle z = 4, 0 :::; x :::; 2, 0 S y :::; 3, oriented in the positive z­ direction. 40. 19.1 THE IDEA OF A FLUX INTEGRAL Chapter Nineteen FLUX INTEGRALS AND DIVERGENCE F = 6i + 7J through triangle of area IO in the plane x + y = 5, oriented in the positive x-direction. 45. 46. 47. 48. F 2zi + xJ + xk through the rectangle x = O S y :::; 2, 0 S direction. 4, z S 3, oriented in the positive x­ F = i + 2J tlu-ough a square of side 2 lying in the plane x + y + z = 1, oriented away from the 01igin. F = (x2 + y2 )k through the disk of radius 3 in the xy-plane and centered at the origin and oriented upward. F = cos(x2 + y2 )k through the disk x 2 oriented upward in the plane z = 1. + y2 S 9 41. F = 6i + x 2J - k, through the square of side 4 in the 49. F- = e'Y2+ • 2-i through the disk of radius 2 in the yzplane y = 3, centered on the y-axis, with sides parallel to the x and z axes, and oriented in the positive y-direction. 42. 43. F = (x + 3)i + (y + 5)J + (z + 7)k through the rectangle x = 4, 0 S y :::; 2, 0 S z :::; 3, oriented in the positive x-direction. F = 7r through the sphere of radius 3 centered at the origin. 44. F = -3r through the sphere of radius 2 centered at the origin. plane, centered at the origin and oriented in the positive x-direction. 50. F = -yi + xJ through the disk in the xy-plane with radius 2, oriented upward and centered at the origin. 51. F = r through the disk of radius 2 parallel to the xy­ plane oriented upward and centered at (0, 0, 2). 52. F = (2 - x)i through the cube whose vertices include the points (0, 0, 0), (3, 0, 0), (0, 3, 0), (0, 0, 3), and ori­ ented outward. Problems 53. Let B be the surface of a box centered at the origin, with edges parallel to the axes and in the planes x = ±1, y = ±1, z = ±1, and let S be the sphere of radius I centered at origin. (a) Indicate whether the following flux integrals are pos­ itive, negative, or zero. No reasons needed. (a) Ia xi · dA (c) Is lx li · dA (b) (d) 56. (a) What do you think will be the electric flux tlu-ough the cylindrical surface that is placed as shown in the constant electric field in Figure 19.16? Why? (b) What if the cylinder is placed upright, as shown in Figure 19. 17? Explain. Ia yi · dA Is(Y - x)i · dA (b) Explain with reasons how you know which flux in­ tegral is greater: 1 xi · dA. or l xi · dA? 54. Suppose that E is a uniform electric field on 3-space, so E (x, y, z) = at+ bj +ck, for all points (x, y, z), where a, b, c are constants. Show, with the aid of sym­ metry, that the flux of E through each of the following closed surfaces S is zero: Figure 19.16 (a) S is the cube bounded by the planes x = ±1, y = ±1, and z = ±1. (b) Sis the sphere x2 + y2 + z2 = 1. (c) S is the cylinder bounded by x2 + y2 = 1, z = 0, and z = 2. 55. Water is flowing down a cylindrical pipe of radius 2 cm; its speed is (3- (3/4)r2 ) cm/sec at a distance r cm from the center of the pipe. Find the flux through the circu­ lar cross-section of the pipe, oriented so that the flux is positive. 57. Let S be part of a cylinder centered on the y-axis. Ex­ plain why the three vectors fields F, G, and ii have the same flux through S. Do not compute the flux. F =xi+ 2yzk G = :r:i + y sin xJ + 2yzk ii =xi+ cos(x2 + z)J + 2yzk r (a) For what value of pis the flux a maximum? (b) What is that maximum value? 61. Let S be the cube with side length 2, faces parallel to the coordinate planes, and centered at the origin. (a) Calculate the total flux of the constant vector field = -i + 2J + k out of S by computing the flux through each face separately. (b) Calculate the flux out of S for any constant vector field = ai + bJ + ck. (c) Explain why the answers to parts (a) and (b) make sense. - . E (x, y, z) v 62. Let S be the tetrahedron with vertices at the origin and at (1, 0, 0), (0, 1, 0) and (0, 0, 1). (a) Calculate the total flux of the constant vector field = -i + 2J + k out of S by computing the flux through each face separately. (b) Calculate the flux out of S in part (a) for any con­ stant vector field . (c) Explain why the answers to parts (a) and (b) make sense. v ·v 63. Let P(x, y, z) be the pressure at the point (x, y, z) in a fluid. Let F (x, y, z) = P(x, y, z)k. Let S be the sur­ face of a body submerged in the fluid. If S is oriented inward, show that Is F · dA is the buoyant force on the body, that is, the force upward on the body due to the pressure of the fluid surrounding it. [Hint: F · dA = P(x, y, z)k · dA = (P(x, y, z) dA) · k.] 64. A region of 3-space has a temperature which varies from point to point. Let T(x, y, z) be the temperature at a point (x, y, z). Newton's law of cooling says that grad T is proportional to the heat flow vector field, F , where F points in the direction in which heat is flowing and has magnitude equal to the rate of flow of heat. Rate of heat loss from l,V =k 1 (gradT)·dA. s E , in the xy-plane, given + yj = 2>- xi x2 +y2 . (b) Compute the flux of E outward through the cylinder x 2 + y2 = R2 , for O S z S h. 66. An infinitely long straight wire lying along the z-axis carries an electric current I flowing in the k direction. Ampere's Law in magnetostatics says that the current gives rise to a magnetic field B given by I -yi + xJ B- (x, Y, z) = - 2 21r x + y2 v (a) Suppose F = k grad T for some constant k. What is the sign of k? (b) Explain why this form of Newton's Jaw of cooling makes sense. (c) Let W be a region of space bounded by the surface S. Explain why Figure 19.17 65. The z-ax:is carries a constant electric charge density of>­ units of charge per unit length, with A > 0. The resulting electric field is E . (a) Sketch the electric field, 58. Find the flux of F = r /llr 11 3 out of the sphere of radius R centered at the origin. 2 59. Find the flux of F = i'' / I lrl1 out of the sphere of radius R centered at the origin. 60. Consider the flux of the vector field F = /I Ir I Ip for p ::". 0 out of the sphere of radius 2 centered at the origin.· 1015 (a) Sketch the field B in the xy-plane. (b) Suppose S1 is a disk with center at (0, 0, h), radius a, and parallel to the xy-plane, oriented in the k di­ rection. What is the flux of B through S1 ? Is your answer reasonable? (c) Suppose S2 is the rectangle given by x = 0, a :::; y :::; b, 0 :::; z :::; h, and oriented in the -i direc­ tion. What is the flux of B through S2 ? Does your answer seem reasonable? 67. An ideal electric dipole in electrostatics is characterized by its position in 3-space and its dipole moment vector p. The electric field D , at the point with position vector of an ideal electric dipole located at the origin with dipole moment p is given by r, fJ (r) = 3 (r · r)r 5 llf 11 iI . llr 113 Assume p = pk, so the dipole points in the k direction and has magnitude p. (a) What is the flux of D through a sphere S with center at the origin and radius a > O? (b) The field D is a useful approximation to the elec­ tric field E produced by two "equal and opposite" charges, q at 2 and -q at 1, where the distance llr 2 - r 1 II is small. The dipole moment of this con­ figuration of charges is defined to be q(r 2 - r 1). Gauss's Law in electrostatics says that the flux of E through S is equal to 41r times the total charge enclosed by S. What is the flux of E through S if the charges at 1 and 2 are enclosed by S? How does this compare with your answer for the flux of fJ through s if iI ·= q(r 2 - r 1)? r r r r 1016 Chapter Nineteen FLUX INTEGRALS AND DIVERGENCE 19.2 FLUX INTEGRALS FOH GRAPHS, CYLINDERS, AND SPHERES How do we divide S into small pieces? One way is to use the cross-sections off with x or y constant and take the patches in a wire frame representation of the surface. So we must calculate the area vector of one of these patches, which is approximately a parallelogram. Strengthen Your Understanding In Problems 68-69, explain what is wrong with the statement. 68. For a certain vector field F and oriented surface S, we have Is F · dA = 2i - 3] + k. 69. If S is a region in the xy-plane oriented upwards then Isft. dA > o. 81. If Is F · dA > Is G points on the surface S. F 1 = 2T - 3J - 4k F2 = T - 2J + 1f F3 = -1T +sJ +6k F'4 = -11i +4J - 5k F's= -5i + 3] + 5k 70. A nonzero vector field F such that Is F · dA = 0, where S is the triangular surface with corners (1, 0, 0), (0, 1, 0), (0, 0, 1), oriented away from the origin. 71. A nonconstant vector field F (x, y, z) and an oriented surface S such that Is F · dA = 1. Are the statements in Problems 72-81 true or false? Give rea­ sons for your answer. z (a) 72. The value of a flux integral is a scalar. 73. The area vector A of a flat, oriented smface is parallel to the surface. 74. If S is the unit sphere centered at the origin, oriented outward and the flux integral Is F · dA is zero, then ft = o. 75. The flux of the vector field F = i through the plane x = O, with O :::; y :::; 1, 0 :::; z :::; 1, oriented in the i direction is positive. 76. If Sis the unit sphere centered at the origin, oriented out­ ward and F = xi+ yJ + zk = r, then the flux integral Is F · dA is positive. 77. If Sis the cube bounded by the six planes x = ±1, y = ±1, z = ±1, oriented outward, and F = k, then Isft . (c) (b) L / x I 19.2 · dA = 2 Is G · dA. )­ -Y J)- [l6A 11-tO L F . 6.A. y. +f(x, y)k. ar - + fxk' y Suppose Sis the graph of the differentiable function z = f(x, y), oriented upward, and that F is a smooth vector field. In Section 19.1 we subdivided the surface into small pieces with area vector 6.A and defined the flux of F through S as follows: l�m vx x v To find iJ x and iJ y , notice that a point on the surface has position vector i =xi+ yJ Thus, a cross-section of S with y constant has tangent vector Flux of a Vector Field Through the Graph of z = f ( x, y) = Figure 19.19: Parallelogram-shaped patch in the tangent plane to the surface Consider the patch of surface above the rectangular region with sides 6.x and 6.y in the xy ­ plane shown in Figure 19.18. We approximate the area vector, 6.A, of this patch by the area vector of the corresponding patch on the tangent plane to the surface. See Figure 19.19, This patch is the parallelogram determined by the vectors iJ x and iJ y, so its area vector is given by 6.A :::::: In Section 19.1 we computed flux integrals in certain simple cases. In this section we see how to compute flux through surfaces that are graphs of functions, through cylinders, and through spheres. r F . dA fx Figure 19.18: Surface showing coordinate patch and tangent vectors r x and r y z FLUX INTEGRALS FOR GRAPHS, CYLINDERS, AND SPHERES ls Coordinate patch x x x w. ·iJ y )- F . dA = - Ls F . dA. 2 rY z I 79. If S1 is a rectangle with area I and S2 is a rectangle with area 2, then 2Is 1 F · dA = Is F · dA. According to the geometric definition of the cross product on page 745, the vector iJ x w has magnitude equal to the area of the parallelogram formed by iJ and tu and direction perpendicular to this parallelogram and determined by the right-hand rule. Thus, we have Area vector of parallelogram = A = iJ x z y (e) dA = o. 80. If F = 2G, then Is F The Area Vector of a Coordinate Patch z (d) z 78. If S is an oriented surface in 3-space, and -S is the same surface, but with the opposite orientation, then Is · dA then IIF II > IIG II at all 82. For each of the surfaces in (a)-(e), pick the vector field F 1, F2, F 3, F4, F's, with the largest flux through the surface. The surfaces are all squares of the same size. Note that the orientation is shown. In Problems 70-71, give an example of: 1017 rx =- = i ax and a cross-section with x constant has tangent vector ar = - r r y = ay J - +fy k . The vectors x and iJ x are parallel because they are both tangent to the surface and parallel to the xz-plane. Since the x-component of r x is i and the x-component of iJ x is (6.x)i, we have iJ x = (6.x)r x· Similarly, we have iJ y (6.y)r y· So the upward-pointing area vector of the parallelogram is This is our approximation for the area vector 6.A on the surface. Replacing 6.A, 6.x, and 6.y by dA, dx and dy, we write dA = (-fx i -fy ] + dxdy . f) 1018 19.2 FLUX INTEGRALS FOR GRAPHS, CYLINDERS, AND SPHERES Chapter Nineteen FLUX INTEGRALS AND DIVERGENCE The Flux of F Through a Surface Given by a Graph of z z = f(x, y) F · dA = l F(x, y,.f(x,y)) · (-fxi - fy ] Coordinate patch R Suppose the surface Sis the part of the graph of z= J(x,y) above a region R in the xy-plane, and suppose S is oriented upward. The flux of F through S is ls + k) dxdy. 1019 z f= y Example1 Compute fs F · dA where F(x,y, z) = zk and Sis the rectangular plate with corners (0, 0, 0), (1, 0, 0), (0, 1, 3), (1, 1, 3), oriented upward. See Figure 19.20. z (0,1,3) ·x y Figure 19.21: Outward-oriented cylinder Figure 19.22: Coordinate patch with area b.A on surface of a cylinder Replacing 6A, D.z, and 66 by dA, clz, and dB , we write dA= (cos B i + sineJ) RdzdB . This gives the following result: The Flux of a Vector Field Through a Cylinder The flux of F through the cylindrical surface S, of radius R and oriented away from the z-axis, is given by x Figure 19.20: The vectrn:: field F = zk on the rectangular surface S Solution ls We find the equation for the plane S in the form z = f(x, y). Since f is linear, with x-slope equal to O and y-slope equal to 3, and f(0, 0)= 0, we have clA=(-fx i - fy ] + k)dxdy=(Oi - 3J +k)dxdy = (-3J + k)dxdy. The flux integral is therefore ls F · dA = 11 11 3yk · (-3J + k) dxdy= 11 11 h F(R,B,i) · (cos Bi +sineJ) RdzdB,. where Tis the Bz-region corresponding to S. z= J(x,y) = o+ox+3y=3y. Thus, we have F ·dA= Example2 Compute fs F · dA where F(x, y, z) = yJ and Sis the part of the cylinder of radius 2 centered on the z-axis with x � 0, y � 0, and O :::; z :::; 3. The surface is oriented toward the z-axis. z 3ydx dy=1.5. Flux of a Vector Field Through a Cylindrical Surface .x Consider the cylinder of radius R centered on the z-axis illustrated in Figure 19.21 and oriented away from the z-axis. The coordinate patch in Figure 19.22 has surface area given by ./ 6A ,:::o R666z. The outward unit normal n == n points in the direction of xi +yJ, so xi+yj llxi + yJII Rcos Bi +R sinBJ . 6..,. =cos 6..,.i + sm J. R Therefore, the area vector of the coordinate patch is approximated by 6A =n6A,:::o (cos Bi +sineJ)R6z66. Figure 19.23: The vector field F Solution = yJ on the surface S In cylindrical coordinates, we have R =2 and F = yJ =2sineJ. Since the orientation of S is toward the z-axis, the flux through S is given by r- - rr - - - JsF ·dA =- J 2sin6j ·(cos Bi +sin6j)2dzd6=-4 r ro 3 sin Bdzd6=-31r. }0 /2 . J 2 1020 19.2 FLUX INTEGRALS FOR GRAPHS, CYLINDERS, AND SPHERES Chapter Nineteen FLUX INTEGRALS AND DIVERGENCE Flux of a Vector Field Through a Spherical Surface Consider the piece of the sphere of radius R centered at the origin, oriented outward, as illustrated in Figure 19.24. The coordinate patch in Figure 19.24 has surface area given by �A� R sin¢�¢�8. Example3 Solution 2 The outward unit normal n points in the direction of 1' Find the flux of F outward. ls ls F -, _, �A �n �A= ; �A= (sin¢cos8i +sin¢sin8] +cos¢k)R sin¢�¢�8. ll II Example4 2 �e by d.A, d¢, and dB, we write 27r r/2 8sin¢cos2 ¢d¢d8 Thus, we obtain the following result: y I I \ The flux of F through the spherical surface S, with radius R and oriented away from the origin, is given by = f. 1 1:: 11 ; \ I ,� ' \ dA I I , I I I I F(R,8,¢). (sin¢cos8i +sin¢sin8] +cos¢k) R sin¢d¢d8, 2 . I I \ t I I -� -�-, ......,,,... " " ,' ... � " \ t ,, .,, I I I where Tis the 8¢-region corresponding to S. 3. r) The Flux of a Vector Field Through a Sphere ls lls = l61r The magnetic field iJ due to an ideal magnetic dipole,µ, located at the origin is defined to be - _ fJ, 3(µ . r B (r) = + 3 5 . ll f 11 ll f 11 I = - cos3 ¢ 1r/2 ) 1 ) 3 </>=D = 21r (8 ( Figure 19.25 shows a sketch of iJ in the plane z = 0 for the dipoleµ = i. Notice that iJ is similar to the magnetic field of a bar magnet with its north pole at the tip of the vector i and its south pole at the tail of the vector i. Compute the flux of iJ outward through the sphere S with center at the origin and radius R. d.A = _!____ dA = (sin¢cos8i + sin¢sin8] +cos¢k) R2 sin¢d¢d8. llrll f ·dA e and ¢, with O ::; e ::; 21r and O ::; 2cos¢k · ( sin¢cos8i +sin¢sin8] +cos¢k)4sin¢d¢d8 lo lo Therefore, the area vector of the coordinate patch is approximated by Replacing �A, �¢. and ·dA = = ! 7°1 n = - = sin¢cos8i + sin¢sin8j +cos<pk. llrll -I = zk through S, the upper hemisphere of radius 2 centered at the origin, oriented The hemisphere S is parameterized by spherical coordinates ¢ ::; 1r /2. Since R = 2 and F = zk = 2 cos ¢k, the flux is = xi +y] +zk, so 1021 . ' , I ,, x \ I I ,, . _, I I , I I \ " ... I I I \ I I . . - Figure 19.25: The magnetic field of a dipole, i, at the ongm: Solution ls ls· ls � ls Since i -i B = Iii ll3 ls (-�;I�: · 1' = x and llrII =Ron the sphere of radius R, we have iJ .d A = = 3( (- ; 3 + �; 11 1 1 1�·11: dA = r ;I � ).I 1 1;� 4 dA = ls dA = �4 + + 3(i . r)r 3 (i Iii ll5 rll i ;iJ� 1 2 ) dA x dA. But the sphere S is centered at the origin. Thus, the contribution to the integral from each positive x-value is canceled by the contribution from the coITesponding negative x-value; so fs x dA = 0. Therefore, B · dA = 4 1- R21s xdA = 0. s Exercises and Problems for Section 19.2 x Figure 19.24: Coordinate patch with area t.A on surface of a sphere Exercises In Exercises 1-4, find the area vector dA for the surface z = f(x,y), oriented upward. 1. f(x,y) = 3x - 5y 2. f(x, y) = 8x + 7y 3. f(x,y) = 2x2 - 3y2 4. f(x,y) =xy+y2 In Exercises 5-8, write an iterated integral for the flux of F through the surface S, which is the part of the graph of z = f(x,y) cmTesponding to the region R, oriented upward. Do not evaluate the integral. = lOi + 20J + 30k f(x,y) = 2x - 3y 5. F(x,y,z) R: -2 S xS 3, 0 Sy S 5 6. F(x,y,z)=zi+xJ +yk f(x, y) = 50 - 4x + lOy R: 0 S x S 4, 0 S y S 8 7. F(x,y,z)=yzi+xyJ +xyk f(x, y) = cos x + sin 2y R: Triangle with vertices (0, 0), (0, 5), (5, 0) 8. F(x,y,z) = cos(x + 2y)J = xe 3Y R: Quarter disk of radius 5 centered at the origin, in quadrant I In Exercises 9-12, compute the flux of F through the surface S, which is the part of the graph of z = f(x, y) corresponding to region R, oriented upward. 9. F(x,y,z) = 3i - 2J + 6k f(x, y) = 4x - 2y R: 0 S x S 5, 0 S y S 10 = i - 2J + zk f(x, y) = xy 10. F(x, y,z) R: 0 S x S 10, 0 S y S 10 11. F(x,y,z)=cosyi+zJ +k f(x,y) = x 2 + 2y R: O S x S 1, 0 S y S 1 12. F(x,y,z)=xi+zk = zJ + 6xk R: Triangle with vertices (-1, 0), (1, 0), (0, 1) In Exercises 13-16, write an iterated integral for the flux of F through the cylindrical surface S centered on the z-axis, ori­ ented away from the z-axis. Do not evaluate the integral. In Problems 30-45 compute the flux of the vector field through the surface S. 19. F(x,y,z)=:ryzJ +:re z k 30. F = zk and S is the portion of the plane x + y + z = 1 S: radius 10, x 2 0, y 2 0, 0 S z S 3 S: radius 2, 0 S y S x, 0 S z S 10 = xyi + 2zJ S: radius 1, x 2 0, 0 Sy S 1/2, 0 S z S 2 In Exercises 21-24, write an iterated integral for the flux of F through the spherical surface S centered at the origin, oriented away from the origin. Do not evaluate the integral. F (x, y, z) = z i 2 In Exercises 25-27, compute the flux of F through the spher­ ical surface S centered at the origin, oriented away from the origin. - 16. F (x,y,z) -y 35. 36. S: radius 20, x 2 0, y 2 0, z 2 0 + zk 38. In Exercises 28-29, compute the flux of = zk through the rectangular region with the orientation shown. (2,0,4) - = x2 yzJ--, +z3-k S: radius 2, between the xy-plane and the paraboloid z = x2 + y2 In Exercises 17-20, compute the flux of F tlu·ough the cylin­ drical surface S centered on the z-axis, oriented away from the z-axis. I (0,0,3) 39. (0,0,4) t� 40. (0, 2, 0) y 2,3 1 (2,0,0) Figure 19.26 In Problems 46-47, compute the flux of F through the cylin­ drical surface in Figure 19.27, oriented away from the z-axis. F = ln(x2 )i + ex J + cos(l - z)k and Sis the part of the surface z = -y + 1 above the square O S x S 1, O S y S 1, oriented upward. z F = 5i + 7J + zk andSis a closed cylinder of radius 3 centered on the z-axis, with -2 S z S 2, and oriented outward. s F = xi + yJ + zk andSis a closed cylinder of radius 2 centered on the y-axis, with -3 S y S 3, and oriented outward. � y (2,2,0) x F = xi + yJ and S is the part of the surface z = 25 - (x 2 + y2 ) above the disk of radius 5 centered at the origin, oriented upward. 41. (2,2,0) 29. F = -yJ + zk and S is the part of the surface z = y2 + 5 over the rectangle -2 S x S 1, 0 Sy S 1, oriented upward. triangle R with vertices (0, 0), (0, 2), (1, 0). S: radius 1, above the cone cf>= 1r/4. z x 1, = 3xi + yJ + zk and Sis the part of the surface z = -2x - 4y + 1, oriented upward, with (x, y) in the 27_. F(x,y,z) =xi+ Y1 28. = 2xJ + yk and S is the part of the surface z + 1 above the square O S x S 1, 0 S y S y Figure 19.27 37. F S: radius 4, entire sphere 14. F(x,y,z) = xi + 2yJ + 3zk S: radius 10, 0 S z S 5 - 32. F 25. F(x,y,z) = zi 26. F(x,y,z) = yi - xJ = (x-y)i+zJ +3xk andSis the part of the plane = x + y above the rectangle OS x S 2, 0 S y S 3, oriented upward. 34. S: radius 2, x 2 0 '" 24. f ( x, y, z) = e k S: radius 3, y 2 0, z S O S: radius 10, x 2 0, y 2 0, 0 S z S 5 - z S: radius 5, entire sphere 23. y 31. F 33. + 3zk z 1 that lies in the first octant, oriented upward. oriented upward. 21. F (x,y,z) = i + 2J + 3k S: radius 10, z 2 0 F (x, y, z) = i + 2J + 3k 2x 15. F (x,y,z) = z i + e _j + k S: radius 6, inside sphere of radius 10 F 18. F(x,y,z)=yi+xzk 20. F(x,y,z) 1023 Problems S: radius 5, y 2 0, 0 S z S 20 v f(x,y)=x+y+2 13. 17. F (x,y,z) 22. F(x,y,z)=xi+ 2yJ f(x, y) ··�·11111, 19.2 FLUX INTEGRALS FOR GRAPHS, CYLINDERS, AND SPHERES Chapter Nineteen FLUX INTEGRALS AND DIVERGENCE 1022 F = cos(x2 + y2 )k andSis as in Exercise 38. 47. F = xzi + yzJ + z3 k In Problems 48-51, compute the flux of F through the spher­ ical surface, S. 48. F = zk and S is the upper hemisphere of radius 2 cen­ tered at the origin, oriented outward. above the rectangle O S x S 2, 0 S y S 3, oriented downward. 49. F = r and S is the part of the surface z = 50. and S is the spherical cap given by + z 2 = 1, z 2 0, oriented upward. F = z 2k and S is the upper hemisphere of the sphere x2 + y 2 + z2 = 25, oriented away from the origin. F = xi + yJ + zk and S is the surface of the sphere x 2 + y2 + z2 = a2, oriented outward. F = r and S is the part of the plane x + y + z = 1 above the disk x2 + 42. F 43. F y2 x2 S 1, oriented downward. + yk and S is the = 9, z 2 0, oriented upward. xzi x2 + y2 + z2 +y 2 hemisphere = 2 x2 52. Compute the flux of F = xi+yJ +zk over the quarter + k through the sur­ face S given by x = sin y sin z, with O S y S 1r/2, O S z S 1r/2, oriented in the direction of increasing x. 53. Compute the flux of F =xi+ J 44. F = yi + J - xzk and Sis the surface y = +z , with x2 + z2 S 1, oriented in the positive y-direction. F = x2i + y2J + z2k 51. F = yi - xJ + zk x2 + y 2 cylinder S given by x 2+y 2 = 1, 0 S x S 1, 0 S y S 1, OS z S 1, oriented outward. -xzi - yzJ + z k and S is the cone z x2 + y2 for O S z S 6, oriented upward. J 45. 46. f =xi+ y_j 2 and Sis the oriented triangular surface shown in Figure 19.26. J 54. Compute the flux of F = (x + z)i + + zk through the surface S given by y = x2 + z2, with O S y S 1, x 2 0, z 2 0, oriented toward the xz-plane. 1024 sphere of radius p. In addition, E s atisfie� Gauss's L aw for any simple closed surface S enclosmg a vol­ ume Hf: 55. Let ft = (xzeYz)i + xzJ + (5 + x 2 + y2 )k. C alcu­ late the flux of ft through the disk x2 + y2 :S 1 m the xy-plane, oriented upward. 56. Let fi = (e y + 3z + 5)i + (1:_ + 5z + 3)J + (3z + e"'Y)k. C alculate the flux of H through the square of side 2 with one vertex at the origin, one edg e along the positive y- axis, one edge in_the 3z-plane with x > 0, z > 0, and the normal ii = i - k . x j. E "'Y s p :Sa p > a. (a) Describ e the charge distribution in words. _ (b) Find the electric field E due too. Assume th!t E can be written in spherical coordin ates as E E(p)ep, where eP is the unit outward normal to the Strengthen Your Understanding with th In Problems 59-60, explain what is wrong e HIIJID statement. cylindrical co­ 59. Flux outward through the cone, given in the formula using d e ut comp ordinates by z = r, can b e dB. Rdz BJ) dA = (cosBi +sin ard, the formula 60. For the surface z = f(x, y) oriented upw dA gives ii . dA = k J, odV, w k a constant. e with density (in 58. Electric charge is distributed in spac 3 s by coulomb/m ) given in cylindrical coordinate 57. Electric charge is distributed in space with density (in coulomb/m3 ) given in spherical coordinates by _ 80 (a constant) o(p, rp, B) - { 0 1025 19.3 THE DIVERGENCE OF A VECTOR FIELD Chapter Nineteen FLUX INTEGRALS AND DIVERGENCE = ii dA = (- fxi - Jy] + k) dxdy = - f,,i - fy] + k and dA = dxdy. In Problems 61-62, give an ex ample of: o (r' () ' z) = { 80 (a constant) 0 if r >a THE DIVERGENCE OF A VECTOR FIELD Imagine that the vector fields in Figures 19.29 and 19.30 are velocity vector fields describing the flow of a fluid. 2 Figure 19.29 suggests outflow from the origin; for example, it could represent the expanding cloud of matter in the big-bang theory of the origin of the universe. We say that the origin is a source. Figure 19.30 suggests flow into the origin; in this case we say that the origin is a sink. In this section we use the flux out of a closed surface surrounding a point to measure the outflow per unit volume there, also called the divergence, or flux density. y y nds only o� z; 66. The vector field, ft, in Figu r� 19.28 depe an mcreasmg is that is, it is of the form g(:,)k, �here g . s the flux of esent r p e r function. The integral fs F dA ft through this rectangle, S, oriented upward.e?In each of the following c ases, how does the flux chang x-direction, (a} The rectangle is twice as wide in the 0 0, (2, ), (2,1, 3), n, origi with new corners at the . (0,1,3). . ar.e at corne1s (b) The rectangle is moved so that its . (1, 0,0), (2, 0,0), (2, 1,3), (1, 1, 3) rd: (c} The orientation is changed to downwa t its new cor­ a th so , e siz in d e tripl (d} The rectangle is , (0, 3, 9). , ( , 9) 0 3, 0, , 3 ) ( 3 origin, e th at ners are \ ""\ "-.." II/ .---,,,.- ' ,..__ \ (a) Describe the charge distribution in words. (b} Find the electric field E due to o. Assume th'.1_t E can b e written in cylindrical coordinates as E E(r)e,., where e,. is the unit m�ward vector to t�e cylinder of radius r, and that E satisfies Gauss s Law (see Problem 57). �ce z_= f (x, y) 61. A function J(x, y) such that, for the_surf + k )dxdy · j + i oriented upwards, we have dA = ( � radius l O cen­ 62. An oriented surface S on the cyli�1der = 600, where dA · F s f t a th tered on the z-axis such --- ' ' x / // I /II Figure 19.29: Vector field showing a source /// / \ \ ,,,.-.--- x ""-.. \"" Figure 19.30: Vector field showing a sink Definition of Divergence To measure the outflow per unit volume of a vector field at a point, we calculate the flux out 6f a small sphere centered at the point, divide by the volume enclosed by the sphere, then take the limit of this flux-to-volume ratio as the sphere contracts around the point. Geometric Definition of Divergence F The divergence, or flux density, of a smooth vector field , written div ff , is a scalar-valued function defined by J8 ff. dA divF(x,y,z)= lim Volume-+0 Volume of S Here Sis a sphere centered at (x, y, z), oriented outward, that contracts down to (x, y, z) in the limit. The limit can be computed using other shapes as well, such as the cubes in Example 2. Cartesian Coordinate Definition of Divergence ft =xi+ yJ. If false ? Give rea­ Are the statements in Problems 63-65 true or sons for your answer. F = F1i + F2J + F3k, then 8F1 div.F = -OX above a '.S 63. If S is the part of the graph of f lying ce area surfa has S then x :S b, c ::; y ::; d, J; + J3 + 1dxdy. fed J: -J (x, Y) oriented 64. If A (x, y) is the area vector for z = f = - f (x, Y) upward and B (x, y) is !h e area vecto� for z y). (x, -B = oriented upward, then A (x, y) 2 2 2 ented outward 65. If S is the sphere x + y _+ z = 1 ori erpend1cular to p is z) y, (x, F n e th 0, = and fs ft . dA xi+ yJ + zk at every point of S. if r :Sa 19.3 8F2 8F3 +- +--. 8y oz The dot product formula gives an easy way to remember the Cartesian coordinate definition, 888and suggests another common notation for divF, namely 'v ·F . Using 'v = - i - j - k, 0x 8y 0z we can write + Figure 19.28 + div ff= 'v· ff 2 Although not all vector fields represent physically realistic fluid flows, it is useful to think of them in this way. ----- '-¥--­ . 1026 Chapter Nineteen FLUX INTEGRALS AND DIVERGENCE Example1 Solution 19.3 THE DIVERGENCE OF A VECTOR FIELD T herefore, the flux t hrough the cube is given by 1 v. Calc ulate the divergence of F(f)= f at th e origin (a) Using the geometric definition. (b) Using the Cartesian coordinate definition. (a) Using the m ethod of Example 5 on page 1010, y ou ca n calculate the flux of F out of the sphere of radius a, centered at the origin; it is 47ra3. So we have (b) In Cartesian coordinates, F (x, y,z) =xi+ YJ + zk, so 8 8 8 div F = �(x) + �(y) + �(z) = 1 + 1 + 1 = 3. uz uy ux dA = r . dIVV (2,2,0) = a 8 8 . d. 1vv = � (-x) + �(O) + -(0) = -1 + O + O= -1 ux uy oz -- - . -- - . -- - . -- -- - !/ -- - . -- - . -- - . v v (a) (i) T he vector fie ld = -xi is parall el to the x-axis and is shown in the xy-plane in Fig­ ure 19.31. To comput e the flux density at (0, 0, 0), we use a cube S1, centered at the origin with ed ges paralle l to the axes, of length 2c. Then the flux through the faces perpendicular to the y- and z-axes is zer o (because the vector field is parallel to these faces). On the faces perpendicu lar to the x-axis, the vector fie ld and the outward normal are parallel but point in opposite directions. On the face at x= c, where v = -ci and .6.A. = IIA. Iii, we have (0, 0, 0) On the face at x= -c, where v= ci and D.A. = -IIA Iii, the dot product is still negative: r . r . v dA v dA + }Face x=c }Face x=-c 3 2 = -c · Area of one face + (-c) · A re a of other face =-2c(2c) = -8c . dA = Thus, v· dA J (-sc3) s =lim --3 =-1. lim Volume--,O Volume of cube c--,O (2c) Since the vector field points inward toward the yz-plane, it makes sense t hat the divergence is negative at the origin. (ii) Take S2 to be a cube as before, but centered this time at th e point (2,2,0). See Figure 19.31. As before, the flux through the faces perpendicular to the y- and z-axes is zero. On the face divv(0,0,0)= at x = 2 + c, v . .6.A = -(2 + c) 11.6.A 11· On the face at x= 2 - c with outward normal, the dot p roduct is positive, and v . .6.A = (2- c) 11.6.AII· + + + + + + --• s .... -.... -.... -.... -.... -- ( 2, 2, 0 ) .... � x Figure 19.31: Vector field v = -xi in the xy-plane v . .6.A = -cll.6.AII· r v. y 81 v . .6.A = -c 11.6.A11· }S1 . 3 fs v dA = lim -8c 1·Im - -3 ) = -1· · ( Yolurne--,O Volume of cube c--,O (2c) .!P (a) Using the geomet ric definition, find the divergence of = -xi at : (i) (0, 0,0) (ii) (2,2,0). (b) Con firm that the co ordinate definition gives the same results. T herefore, the flux through the cube is given by . �! though '.he vect r field is flow_ing away from the point (2,2,O) on the left, this outflow � is mailer. ma mtud_t; than the mflow on the right, so the net outflow is negative. � . � (b) Smee v = -xi + 0.7 +Ok, t he formula gives T he next example shows that the divergence can be negative if there is net inflow to a point. Solution r . v dA + v dA }Face x=2 -c }Face x=2+c = (2 - c) · Area of one face - (2 + c) · Area of other face = -2c(2c)2 = -Sc3 Then, as before , S2 47ra3 Flux lim 43 = lim 3= 3. div F (0,0,0) = lim --- = a--,O a--,O a--,O Volume 37ra Example2 1027 Why Do the Two Definitions of Divergence Give the Same Result? T_h� g�ometric defini tion d�fin s di: F as the flux density of F . To see wh y the coordinate defi­ � m 10n is also t?e flux density, 1magme computing the fl ux out of a small box-shaped s urface at � (xo,Yo, zo), wit? si_ des of leng th D.x, Doy, and Doz par allel to the axes. On S 1 (the back face of the _ I:,1gure 19.32, �here x= xo), the outward normal is in the negative x-direction so bo! shown 111 . F is approximately consta nt on S , we have dA = -dydz i . Assummg 1 s 1s, F. dA = r F. ls, (-i)dydz � -Fi(xo,Yo,zo) s2 F . dA = dydz = -Fi (xo,Yo, zo) · A rea of S1 = -F1(x0, y0, zo) Doy Doz. Oi:i, S 2, the f ce where x = xo + D.x, th e � dA = dydz i . T herefore, 1 r ls, r ls2 outward normal points in F . idydz � Fi (Xo + D.x,Yo, Zo) r Js2 the dydz positive x-direction, so = Fi (xo + D.x,Yo, zo) · A rea of S 2 = Fi (x0 + D.x,y0, zo) Doy Doz. 1028 Chapter Nineteen FLUX INTEGRALS AND DIVERGENCE 19.3 THE DIVERGENCE OF A VECTOR FIELD ""' \ 55 (Bottom) x Thus, Figure 19.32: Box used to find div F at (xo, yo, zo) r ls, F . dA + r ls2 F . dA � Fi(x o + 6.x, Yo, z o)6.y6.z - Fi(x o, Yo, zo)6.y6. z _ A (x o+ 6.x, Yo, z o) F1 (x o, Yo, zo) 6.x6.y6.z 6.x oFi � -6.x6.y6.z . OX / Solution // // / // Thus, adding these contributions, we have x \ ""' (a) The components of E are E = o ( o ( oz Since the vol ume of the box is 6.x 6.y 6.z, the flux density is 0F3 0F2 oF 1 6.x 6.y6.z + -6.x6.y6.z + �6.x6.y6.z uz . oy OX � 6.x6.y6.z oA 0F3 oFi =-+-+. oz oy ox Example3 is - E i" = fif\lP i" = xi + yJ + zk , i" i O · (a) Find a formula for div E. _ (b) Is there a val ue of p for which E is divergence-free? If so, find it. + (x2+ y2+ z2)p/2 i (x2+ y2+ z2)p/2 J+ (x2+ y2+ z2)p/2 k. x .,. y -; z 1 pz 2 (x2 + y2+ z2)p/2 ) = (x2 + y2+ z2)p/2 - (x2+ y2+ z2)(p/2)+1 · z . div E = 2+ z2) 3 p(x2 + y__ -"----'---____:: ...:....__ (x2+ y2+ z2)p/2 (x2+ y2+ z2)(p/2)+1 3-p 3-p (x2+ y2+ z2)p/2 lir IIP. Magnetic Fields An important class of div ergence-free vector fields is the magn etic fields. One of Maxwell's Laws of Electromagnetism is that the magnetic field B satisfie s ' F = 0 everywhere that F is said to be dive,gence free or sol.eno1'da,l 1'f dIV F igure 19.33 shows, for three values of the constant p, the vector field p=3 (b) The divergence is zero when p = 3, so F (i") = i" /llf"ll 3 is a divergence-free vector field. Notice that the div ergence is zero even though the v ectors point outward from the origin. _ Divergence-Free Vector Fields F // / I p l = Figure 19.33: The vector field E (1°") = r /1'77 ff P for p = 0, 1, and 3 in the xy-plane p=O So 0F3 0F2 oFi Total flux through S � - 6.x 6.y 6.z+ � 6.x 6.y 6.z+ !Cl 6.x 6.Y 6.z . uz uy ox A vector field defi ne d. I / x 1 px2 2 2 ox (x + y + z2) p/2 ) = (x2+ y2+ z2)p/2 - (x2+ y2+ z2)(p/2)+1 o y 2 1 PY ( 2 2 oy (x + y + z2) p/2 ) = (x2+ y2+ z2)p/2 - (x2+ 2+ z2)(p/2)+1 y 0F3 A - 6.x 6.y LJ.Z. oz s y // We compute the partial derivatives By an analogous argument, the contribution to the flux from S3 and S4 (the surfaces perpendicular to the y-axis) is approximately 0F2 - 6.x 6.y 6. z , oy and the contr ibution to the flux from Ss and S5 is approximately Total flux through Volume of box y 1029 div B Example4 = 0. An infi nitesimal current loop, similar to that shown in F ig ure 19.34, is called a magnetic dipole. Its magnitude is described by a constant vector fl,, called the dipole moment. Th e magnetic field due to a magnetic dipole with moment fl, is - fl, B = - lif 11 3+ 3(µ. r)i" lif 115 ' r i o. •· 1030 Chapter Nineteen FLUX INTEGRALS AND DIVERGENCE Show that div B (a) Area= a Y (b) "'-\\II!/ "' '/1 // ���·--�x = To show that div B 0 we can use the following version of the product rule for the divergence: if g is a scalar function and F is a vector field, then div(g.F) = (gradg) · F + g div F. Problems . div __ r iv µ . r l 5 =d lf1 1 . ((- -)-) ( - ) µ ·r r 5 llf11 r __ . r = g ad(µ r). llf Jl5 - . __ . v r) di ple 3 on page 1028, we have From Problems 67 and 68 on page 796 and Exam grad (1) 3 llfll -3r grad(µ . r) =llfll 5 ' = µ, div (-) r + (1,i ll 5 f11 B µ+3gra d(µ . r) . 3µ r 6µ . r +----- - rad ( ; 3) . = g 11 1 11 . . fl, 3r = -5 llrll = 0. llf11 5 11 : 5 11 -2 . 2. div((2sin(xy)+tanz)i+(tany)J +(e +Y )k) 3. Which of the following two vector fields, sketched in the xy-plane, appears to have the greater divergence at the origin? The scales are the same on each. I I 1 I I 1. J e I J centered at the point (1, 2, 3). 19. Let F be a vector field with div F Estimate Js F · dA where Sis (a) y ���� '--'-' \ \ .........._,,, \ .,......../// I ///// H10 (II) I//// I///.,,/ \ ,, x . . . , , ,. :::::!::::: '' ' I I \ \ ''-'-- 0��) ��\� I I • / I I I I ,t' ;' I I I I / / / ..................... .. ' ' I = (-x+y)i + (y+z)J + (-z+x)k 7. F(x,y,z) 8. F(x,y) = (x 2 - y2 )i + 2xyJ = 3x2 i - sin(xz)(i + k) 9. F (x, y, z) 10. F = (ln(x 2+l)i +(cosy) J +(xye z ) k) =a - 12. F (x,y) 13. f (r) = - x i" -yi + xJ = ---"-------"---­ x2 + 2 y i" - 1"' 0 - llf - r a II ' 1"' -1-. 1"' a ' (b) Use !he geometric definition and part (a) to find div F at �e point (a, b, c). (c) Find div F using partial derivatives. S1 (Back) z S4 (Back) \ y (i) A sphere of radius 0.1 centered at (2, 0, O). ...... \ \ '\.. x (a) div F at (4, 5, 2) (b) The flux' of F out of a sphere of radius 0.2 centered at (4, 5, 2). = 2xi - 3yJ + 5zk through a box with four of its corners at the points (a, b, c), (a+w, b, c), (a, b+w, c), (a, b, c+w) and edge length w. See Figure 19.35. 21. (a) Find the flux of F Ss (Bottom) x x2 + y 2 _ z. 20. The flux of F out of a small sphere of radius 0.1 centered at (4, 5, 2), is 0.0125. Estimate: y '' \ = -yi + xJ 6. F(x, y) =-xi+ yJ 11. f (r) (ii) A box of side 0.2 with edges parallel to the axes and centered at (0, 0, 10). (b) The P0nt (2, 0, 0) is called a source for the vector field F; the point (0, 0, 10) is called a sink. Ex­ plain the reason for these names using your answer to part (a). (I) 5. F (x,y) 18. A smooth vector �eld F has div F (1, 2, 3) = 5. Esti­ mate the flux of F out of a small sphere of radius 0.01 + 3(µ . r) div ( 11 :115 ) Exercises 2 .:::.:::.:::J.:::.:::.::: r 17. A small sphere of radius 0.1 surrounds the point (2, 3, -1). The flux of a ve_<:tor field G into this sphere 1s 0.000041r. Estimate div G at the point (2, 3, -1). Exercises and Problems for Section 19.3 x2 ... ... In Exercises �-13, _find t�e divergence of the vector field. (Note: = xi +yj +zk .) 16. Draw two vector fields that have zero divergence every­ where. l lf115 Are the quantities in Exercises 1-2 vectors or scalars? Calcu­ late them. 2 2 2 z 1. div((x + y)i + (xye )j - ln(x + y )k ) - 15. Draw two vector fields that have negative divergence ev­ erywhere. . Putting these results together gives div •- 14. Draw two vector fields that have positive divergence ev­ erywhere. 5 r ) =llf 11 ( llrll 5 y - ____________....__ x i i ilti· I I I 0, we have (See Problem 36 on page 1032.) Thus, since divµ = and ... /Ill\\"'(c) y - ::;;1::: Figure 19.34: A cun-ent loop Solution �I�t� ��� 4. For each of the following vector fields, sketched in the xy-plane, decide if the divergence is positive, zero, or negative at the indicated point. = 0. 1031 19.3 THE DIVERGENCE OF A VECTOR FIELD Figure 19.35 = (3x + 2)i + 4xJ + (5x + l)k. Use the method of Exercise 21 to find div F at the point (a, b, c) by two different methods. 22. Suppose F v 23. Use the geometric definition of divergence to find div = -2r. Check that you get the at the origin, where same result using the definition in Cartesian coordinates. v 24. (a) Letf(x,y)=axy+ax2 y+y3 .Finddivgradf. (b) If possible, choose a so that div grad f = O for all x,y. 25. Let F = (9a2 x + 10ay 2 )i + (10z3 - 6ay)J 2 2 (3z + 10x + 10y )k. Find the value(s) of a making div F WO W A�ci�m - 1032 19.3 THE DIVERGENCE OF A VECTOR FIELD Chapter Nineteen FLUX INTEGRALS AND DIVERGENCE r 26. The vector field F (1"'') = /111"113 is not defined at the origin. Nevertheless, we can attempt to use the flux defi­ nition to compute div F at the origin. What is the result? 27. The divergence of a magnetic vector field B must be zero everywhere. Which of the following vector fields cannot be a magnetic vector field? 32. Due to roadwork ahead, the traffic on a highway slows linearly from 55 miles/hour to 15 miles/hour over a 2000-foot stretch of road, then crawls along at 15 miles/hour for 5000 feet, then speeds back up linearly to 55 miles/hour in the next 1000 feet, after which it moves steadily at 55 miles/hour. (a) Sketch a velocity vector field for the traffic flow. (b) Write a formula for the velocity vector field (miles/hour) as a function of the distance x feet from the initial point of slowdown. (Take the direction of motion to be i and consider the various sections of the road separately.) (c) Compute div at x = 1000, 5000, 7500, 10,000. Be sure to include the proper units. v (a) B(x, y, z) =-yi+xJ+ (x+y)k (b) B(x,y,z) (c) B(x,y,z) = -zi+yJ+xk = (x2 - y 2 - x)i + (y - 2xy)J 28. Let F(x,y,z) = zk. (a) Calculate div F . (b) Sketch F. Does it appear to be diverging? Does this agree with your answer to part (a)? v v · 33. The velocity field in Problem 32 does not give a com­ plete description of the traffic flow, for it takes no account of the spacing between vehicles. Let p be the density (a) Calculate div F . (cars/mile) of highway, where we assume that p depends (b) Sketch F. Does it appear to be diverging? Does this only on x. agree with your answer to part (a)? 29. Let F (?) = r /llr 11 3 (in 3-space), r =I- 5. (a) Using your highway experience, arrange in ascend­ ing order: p(O), p (lOOO), p(5000). (b) What are the units and interpretation of the vector field pv? (c) Would you expect pv to be constant? Why? What does this mean for div(pv )? (d) Determine p(x) if p(O) = 75 cars/mile and pv is constant. (e) If the highway has two lanes, find the approximate number of feet between cars at x = 0, 1000, and 5000. Problems 30-31 involve electric fields. Electric charge pro­ duces a vector field E , called the electric field, which rep­ resents the force on a unit positive charge placed at the point. Two positive or two negative charges repel one another, whereas two charges of opposite sign attract one another. The divergence of E is proportional to the density of the electric charge (that is, the charge per unit volume), with a positive constant of proportionality. UIIH�II 30. A certain distribution of electric charge produces the electric field shown in Figure 19.36. Where are the charges that produced this electric field concentrated? Which concentrations are positive and which are nega­ tive? r 34. Let =xi+ Y] + zk and c = C1i + C2 ) + C3k, a constant vector; let S be a sphere of radius R centered at the origin. Find (b) f (r x c) . dA (a) div(r x c) 8 y 35. Show that if a is a constant vector and f(x, y, z) is a function, then div(f = (grad !) . a) a. 36. Show that if g(x,y, z) is a scalar-valued function and F (x, y, z) is a vector field, then -4 -3 -2 -1 1 2 x div(gF)=(gradg)·F +g div F. 37. If f(x, y,z) and g(x, y,z) are functions with continuous second partial derivatives, show that div(grad f x grad g) = 0. Figure 19.36 31. The electric field at the point r as a result of a point charge at the origin is E (?) = /llf 11 3 . kr (a) Calculate div E for r =f. 0. (b) Calculate the limit suggested by the geometric defi­ nition of div E at the point (0, 0, 0). (c) Explain what your answers mean in terms of charge density. In Problems 38-40 use Problems 36 and 37 to find the diver­ gence of the vecto/ field. The vectors and b are constant. a - 38. F = 40. 1 l llf lP - a - xr c = (b . na x .,: 39. B 41. Let F(x, y) = u(x,y)i + v(x,y)J be a 2--dimensional vector field. Let F(x, y) be the magnitude of F and let e(x, y) be the angle of F with the positive x-axis at the point (x, y), so that u = F cos (J and v = F sine. Let T be the unit vector in the direction of F, and let fJ be the unit vector in the direction of k x F, perpendicular to F . Show that This problem shows that the divergence of the vector field F is the sum of two terms. The first term, F(JfJ , is due to changes in the direction of F perpendicular to the flow lines, so it reflects the extent to which flow lines of F fail to be parallel. The second term, Fi' , is due to changes in the magnitude of F along flow lines of F . 42. In Problem 64 on page 1015 it was shown that the rate of heat loss from a volume Vin a region of non-uniform temperature equals k f8(grad T) · dA, where k is a con­ stant, Sis the surface bounding V, and T(x, y, z) is the temperature at the point (x, y, z) in space. By taking the limit as V contracts to a point, show that, at that point, �� = Bdiv gradT where B 'is a constant with respect to x, y, z, but may depend on time, t. 1033 43. A vector field, v, in the plane is a point source at the ori­ gin if its direction is away from the origin at every point, its magnitude depends only on the distance from the ori­ gin, and its divergence. is zero away from the origin. (a) Explain why a point source at the origin must be of - for some the form v- = [f(x 2 +y2)] (xi- + yj) positive function f. (b) Show that v =K(x 2 +y 2)-1 (xi + yJ) is a point source at the origin if K > 0. (c) What is the magnitude llv II of the source in part (b) as a function of the distance from its center? (d) Sketch the vector fieldv = (x 2 +y2)-1 (xi +yJ). 1 (e) Show that rp = tog(x2 + y 2 ) is a potential func­ tion for the source in part (b). 45. div(2xi) = 2i. 46. ForF(x,y,z) = (x 2+y)i+(2y+z)J-z2 k we have divF = 2xi +2J - 2zk. 47. The divergence of f(x, y, z) div f(x, y, z) = 2x + z + y. x2 + yz is given by In Problems 48-50, give an example of: 48. A vector field F (x, y, z) whose divergence is a nonzero constant. 49. A nonzero vector field zero. F (x, y, z) whose divergence is v, (a) Explain why a point sink at the origin must be of the form v = [f(x 2 +y 2 )] (xi + yJ) for some negative function f. (b) Show thatv =K(x 2 +y2)-1 (xi +yJ) is a point sink at the origin if K < 0. (c) Determine the magnitude llv II of the sink in part (b) as a function of the distance from its center. 1 2 (d) Sketch v = -(x 2 +y )- (xi + yJ). 2 (e) Show that rp = log(x + y2) is a potential func­ tion for the sink in part (b). 1 55. A constant vector field divergence. F ai + bJ + ck 56. If a vector field F in 3-space has zero divergence then F = ai +bJ +ck where a, band care constants. 57. If F is a vector field in 3-space, and f is a scalar func­ tion, then div(! F) = fdivF. 58. If F_js a vector field in 3-space, and F = grad f, then divF = 0. 59. If F is a vector field in 3-space, then grad(div F) = O. Are the statements in Problems 51-63 true or false? Give rea­ sons for your answer. 61. If f(x, y, z) is any given continuous scalar function, then there is at least one vector field F such that divF = f. ·G) =F(div G)+(div F)G 53. div F is a scalar whose value can vary from point to point. I� has zero 60. The field F ( r) = 51. div(F + G) =div F + div G ., 54. If F is a vector field in 3-space, then divF is also a vec­ tor field. 50. A vector field that is not divergence free. 52. grad(F ,,. ,,, 44. A vector field, in the plane is a point sink at the ori­ gin if its direction is toward the origin at every point, its magnitude depends only on the distance from the origin, and its divergence is zero away from the origin. Strengthen Your Understanding In Problems 45-47, explain what is wrong with the statement. JI r is divergence free. 62. If F and G are vector fields satisfying divF = divG then F = G. 63. There exist a scalar function f and a vector field F sat­ isfying div(grad !) =grad(div F ). I� 1034 Chapter Nineteen FLUX INTEGRALS AND DIVERGENCE 19.4 THE DIVERGENCE THEOREM 64. For r =xi+ yJ + zk, an arbitrary function J(x, y, z) and an arbitrary vector field, F (x, y, z), which of the following is a vector field, and which is a constant vector field? 19.4 (a) gradf (b) (d) (div i)F (e) grad(div F) (divF)i (c) (divr)i Flux out of vV Flux out of W = ( lw div F dV. We have calculated the flux in two ways, as a flux integral and as a volume integral. Therefore, these two integrals must be equal. This result holds even if vV is not a rectangular solid. Thus, we have the following result.3 The Divergence Theorem is a multivariable analogue of the Fundamental Theorem of Calculus; it says that the integral of the flux density over a solid region equals the flux integral through the boundary of the region. Theorem 19.1: The Divergence Theorem The Boundary of a Solid Region A solid region is an open region in 3-space. The boundary of a solid region may be thought of as the slcin between the interior of the region and the space around it. For example, the boundary of a solid ball is a spherical surface, the boundary of a solid cube is its six faces, and the boundary of a solid cylinder is a tube sealed at both ends by disks. (See Figure 19.37). A surface which is the boundary of a solid region is called a closed s111face. If HI is a solid region whose boundary Sis a piecewise smooth surface, and if f is a smooth vector field on an open region containing HI and S, then r ls Hf = Solid cube W =Solid Cylinder W=Ball S = Sphere f . dA = where S is given the outward orientation. S = 6 square faces S = Tube and two disks r lw div f dV, Example1 Use the Divergence Theorem to calculate the flux of the vector field f (r) = r through the sphere of radius a centered at the origin. Solution In Example 5 on page 1010 we computed the flux using the definition of a flux integral, giving dA = 41ra 3 r Now we use div F Figure 19.37: Several solid regions and their boundaries ls r ( r · dA = ( Consider a solid reoion HI in 3-space whose boundary is the closed surface S. There are two ways to find the total flux of a vector field F out of Hf. One is to calculate the flux of F through S: · = div(xi + yJ + zk) = 3 and the Divergence Theorem: ls Calculating the Flux from the Flux Density b L Flux out of small boxes � L div f !:::,. V. We have approximated the flux by a Riemann sum. As the subdivision gets finer, the sum approaches an integral, so THE DIVERGENCE THEOREM HIIM = 1035 lw div F' dV = ( lw 3 dV = 3 · i 1ra3 = 41ra3.. 3 - - Example2 Flux out of W = ls F · dA. f = x2 _ (x, y, z) ( through the cube in Figure 19.40. Another way is to use div f, which gives the flux density at any point in vV. We subdivide VV into small boxes, as shown in Figure 19.38. Then, for a small box of volume!:::,. V, Flux out of box � Flux density · Volume Use the Divergence Theorem to calculate the flux of the vector field = div F !:::,. V. + y2 )i + (y2 + z2 )J + (x2 + z2 )k z What happens when we add the fluxes out of all the boxes? Consider two adjacent boxes, as shown in Figure 19.39. The flux through the shared wall is counted twice, once out of the box on each side. When we add the fluxes, these two contributions cancel, so we get the flux out of the solid region formed by joining the two boxes. Continuing in this way, we find that Fluxes through inner wall cancel x Figure 19.40 Solution Figure 19.38: Subdivision of region into small boxes Figure 19.39: Adding the flux out of adjacent boxes The divergence of F is div F 2x + 2y + 2z. Since div f is positive everywhere in the first quadrant, the flux through Sis positive. By the Divergence Theorem, 3 A proof of the Divergence Theorem using the coordinate definition of the divergence. can be found in the online supple­ ment at www.wiley.com/college/hughes-hallett. 1036 Chapter Nineteen FLUX INTEGRALS AND DIVERGENCE 1 F. 8 dA = 111 1 1 0 1 2(x+y+z)dxdydz = 1 19.4 THE DIVERGENCE THEOREM 1 1 2 l\x +2x(y+z))\ dydz Example4 O - }, \ +2(y+ z)dydr [(y+:': 2yz{ dz � !,' = i\2 + 2z) dz= (2z + z 2 { = 3. Solution Calculate 0= 3 ' Is F . dA, using the Divergence Theorem if possible, for the following surface s: and thus 1 defined. Notice that the flux is not zero, although div F is zero eveEy where it is re in T,V, whe very (b) Suppose Hi is the solid region enclosed by S2 . Since div F = 0 e the Divergence Theorem in this case, giving r F. ls2 dA = r lw div F dV r F. ls2 dA. div ff dV = o. { F . dA = { F . dA - h r 1� F . dA = r ls, 1�{ F . dA, F . dA. In Example 3 we showed that Is, F · dA = 41r, so Is2 F · dA = 41r also. Note that it would have been more difficult to compute the integral over the ellipsoid directly. Electric Fields The electric field produced by a positive point charge q placed at the origin is a � � s1 s, = lw ls2 defined every where inside (a) We cannot use th e Divergence Theorem directly Eecause F is not ery where on S1, the flux ev ard w the sphere (it is not defined at the origin). Since F points out out of S1 is positive. On S1 , a dA, F · dA = IIF lldA = 3 a so 2 dA = 2._(Area of S1) = \41ra = 41r. F · dA = 2_ 1 dA fi-0, 3 ' 16, But S consists of S2 oriented outward and S1 oriented inward, so (a) S1 is the sphere of radius a centered at the origin. (b) S2 is the sphere of radius a centered at the point (2a, 0, 0). Solution llfll f ft . dA = f f llfll f = The ellipsoid contains the sphere ; let Hi be the solid region between the m. Since W does not contain the origin, div F is defined and e qual to zero e very where in VV. Thus, if Sis the boundary of 1'V, then ls In Example 3 on page 1028 we saw that the following vector field is divergence free : F (f ) = r F. ls, An important application of the Diverge nce Theorem is the study of divergence-free vector fields. - F (f ) = show that The Divergence Theorem and Divergence-Free Vector Fields Example3 Le t S1 be the sphere of radius 1 centered at the origin and let S2 be the ellipsoid x2 +y2 +4z2 both oriented outward. For 1037 = r lw OdV we can use Using Example 3, w e see that the flux of the electric field through any sphere centered at the origin is 41rq. In fact, using the idea of Example 4, we can show that the flux of E through any closed surface containing the origin is 41rq. See Problems 36 and 37 on page 1039. This is a special case of Gauss's Law, which state s that the flux of an electric field through any closed surface is proportional to the total charge enclosed by the surface . Carl Friedrich Gauss (1777-1855) also discovered the Divergence Theorem, which is sometimes called Gauss's Theorem. = o. y S, even in case s The Diveroence Theorem applie s to any solid re gion VV and its boundar region ?et ween solid e th is Hi if , e exampl For e s. where the boundary consists of two or more surfac pomt, then ame s e h t at red e nt e c h bot 2, the sphere 31 of radius 1 and the sphere S2 of radius . outward he t requues m e eor h T e nc e rg e Div the boundary of Hi consists of both S and S2 . The (See center. e th ard w to points S on and 1 orientation, which on 32 points away from the center 1 Figure 19.41.) Exercises and Problems for Section 19.4 Exercises J� For Exercises 1-5, compute the flux integral F ·dA in two ways, ifpossible, directly and using the Divergence Theorem. In each case, S is dosed and oriented outward. 1. F (f) = f and S is the cube enclosing the volume 0 :::; x :::; 2, 0 :::; y :::; 2, and O :::; z :::; 2. Figure 19.41: Cut-away view of the region W between two spheres, showing orientation vectors 2. F = Y] and Sis a closed vertical cylinder of height 2, with its base a circle ofradius 1 on the xy-plane, centered at the origin. 3. F =x 2 i+2y2 J +3z2 k andSis the surface ofthe box with faces x = 1, x = 2, y = 0, y = 1, z = 0, z = 1. 4. F = (z2 + x)i + (x 2 + y)J + (y 2 + z)k andSis the closed cylinder x2 + z2 = 1, with O :::; y :::; 1, oriented outward. 5. F = -zi + xk and Sis a square pyramid with height 3 and base on the xy-plane ofside length 1. Find the flux of the vector fields in Exercises 6-12 out of the closed box O :::; x :::; 2, O :::; y :::; 3, O :::; z :::; 4. 6· F 7. G = 4i +7J -k = yi + zk 1038 Chapter Nineteen FLUX INTEGRALS AND DIVERGENCE 8. fi = xyi + zJ + yk 9. J = xy 2J +xk 10. N = ez i + sin(xy)k 19.4 THE DIVERGENCE THEOREM 11. 111 = ((3x + 4y)i + ( 4y + 5z).7 + (5z + 3x)k 12. 111 where div M = xy + 5 Problems (c) Use your answers to parts (a) and (b) to calculate the flux out of a box of side 10 centered at the origin and with sides parallel to the coordinate planes. (The box is also oriented outward.) 13. Find the flux of F = zi + y.7 + xk out of a sphere of radius 3 centered at the origin. 14. Find the flux of F = xyi + yz.7 + zxk out of a sphere of radius 1 centered at the origin. 15. Find the flux of F = x 3 i + y 3. 7 + z3 k through the closed surface bounding the solid region x 2 + y 2 S 4, OSzS5, oriented outward. 2 2 2 16. The region W lies between the spheres x + y + z = 4 2 2 2 and x2+y 2 + z = 9 and within the cone z = x + Y S, ori> O; its boundary is the closed7surface, with z 33 -:' 3 ented outward. Find the flux of F = x i + y J + z k out of S. 17. Find the flux of F through the closed cylinder of ra­ dius 2, centered on the z-axis, with 3 S z S 7, if F = (x + 3e yz )i + (ln(x2 z2 + 1) + y).7 + zk. e Y z i + (tan( 0.00l x 2 z2 ) + 2 2 2 y )J + (ln(l + x y ) + z2 )k out of the closed box OSxS5, 0 S y S 4, 0SzS3. 2 2 (b) Can you use the Divergence Theorem to compute 2 the flux of /I \r 11 out of a closed cylinder of ra­ dius 1, length 2, centered at the origin, and with its axis along the z-axis? (c) Compute the flux of /I \r 112 out of the cylinder in part (b). 2 (d) Find the flux of i;' /I \r 11 out of a closed cylinder of radius 2, length 2, centered at the origin, and with its axis along the z-axis. r r 26. Let S be the cube in the first quadrant with side 2, one corner at the origin and edges parallel to the axes. Let 2 2 -:' 2 k­ 2 2 F- 1 = (xy + 3xz )i- + (3x y + 2yz )J + 3zy - 19. Find the flux of F = x i + z.7 + yk out of the closed cone x = y 2 + z2 , withOSx S 1. F 2 = (xy 2+5e yz)i +(yz2 +7sin(xz)).J +(x2 z+cos (xy))k z3 y 3 -:' 2 x3 7 2 2 F 3 = (xz + 3 ) i + (yz + 3 ) J + (zy + 3 ) k . 2 J 20. Suppose F is a vector field with div F = 10. Find the flux of F out of a cyUnder of height a and radius a, cen­ tered on the z-axis and with base in the xy-plane. 21. A cone has its tip at the point (0, 0, 5) and its base the disk D, x2 + y 2 S 1, in the xy -plane. The surface of the cone is the curved and slanted face, S, oriented upward, and the flat base, D, oriented downward. The flux of the constant vector field F = ai + b. 7 + ck through S is given by 1 _ Arrange the flux integrals of F1, F2 , F3 out of Sin increasing order. 27. LetdivF =2(6-x) andOSa,b,cSlO. (a) Find the flux of F out of the rectangular solidO S xSa, 0Sy Sb, 0SzSc. (b) For what values of a, b, c is the flux largest? What is that largest flux? f . dA = 3.22. Is it possible to calculate ID F · dA? If so, give the answer. If not, explain what additional information you would need to be able to make this calculation. 28. Assumer # r· 23. A vector field F satisfies div F = 0 everywhere. Show that Is F · dA = 0 for every closed smface S . r 3 l\r 11 F be the vector field . 2 (a) Calculate the flux of F out of the unit sphere x + 2 2 y + z = 1 oriented outward. (b) Calculatediv F. Show your work and simplify your answer completely. 6. Let F = r /1\r 11 3 . (a) Calculate the flux of F out of the unit sphere x 2 + y 2 + z2 = 1 oriented outward. (b) Calculate div F. Simplify your answer completely. (c) Use your answer to parts (a) and (b) to calculate the flux out of a box of side LO centered at the origin and with sides parallel to the coordinate planes. (The box is also oriented outward.) 22. If V is a volume surrounded by a closed surface S, show dA = V. that � Is 24. Let = xi + yj + zk and let given by 1 2 25. (a) Finddiv(r/l\rll ) wherer = xi+y.7forr#O. J 18. .Find the .flux of F 31. (a) Let div F = x2 +y 2+z2+3. Calculate Is F ·dA 1 where S1 is the sphere of radius I centered at the ori­ gin. (b) Let 82 be the sphere of ra dius 2 centered at the ori­ gin; let 83 be the sphere of radius 3 centered at the origin; let 84 be the box of side 6 centered at the origin with edges pai:allel to the axes. Without cal­ culating them, arrange the following integrals in in­ creasing order: r In Problems 29-30, find the flux of F = /l\f 11 3 through the surface. [Hint: Use the method of Problem 28.] 29. S is the ellipsoid x2 + 2y 2 + 3z2 = 6. 30. Sis the closed cylinder y 2 + z2 = 4, -2 SxS2. s. f · dA. 32. Suppose div F = xyz 2 . (a) Find div F at the point (1, 2, 1). [Note: You are given div F, not F .] (b) Using your answer to part (a), but no other informa­ tion about the vector field F, estimate the flux out of a small box of side 0.2 centered at the point (1, 2, 1) and with edges parallel to the axes. (c) Without computing the vector field F, calculate the exact flux out of the box. 33. Supposediv F = x +y +3. Find a surface S such that Is F · dA is negative, or explain why no such surface exists. 2 35. If a surface S is submerged in an incompressible fluid, a force F is exerted on one side of the surface by the pres­ sure in the fluid. If the z-axis is vertical, with the positive direction upward and the fluid level at z = 0, then the component of force in the direction of a unit vector i1 is given by the following: F . 11 (a) What is the value of div F? Include units. (b) Assume the heat flows outward symmetrically. Ver­ ify that F = ai;' , where = xi + y.7 + zk and a is a suitable constant, satisfies the given conditions. Find a:. (c) Let T(x, y, z) denote the temperature inside the earth. Heat flows according to the equation F = -k grad T, where k is a constant. Explain why this makes sense physically. (d) If T is in °C, then k = 30,000 watts/km 0 C. Assum­ ing the earth is a sphere with radius 6400 km and surface temperature 20° C, what is the temperature at the center? r =- 1 zo9a . dA, where 8 is the density of the fluid (mass/�olume), g is the acceleration due to gravity, and the smface is oriented away from the side on which the force is exerted . In this problem we consider a totally submerged closed surface enclosing a volume V. We are interested in the force of the liquid on the external smface, so Sis oriented inward. Use the Divergence Theorem to show that: (a) The force in the i and J directions is zero. (b) The force in the k direction is 8g V, the weight of the volume of fluid with the same volume as V. This is Archimedes' Principle. 36. According to Coulomb's Law, the electrostatic field E at the point due to a charge q at the origin is given by r - (r) = 2 34. As a result of radioactive decay, heat is generated uni­ formly throughout the interior of the earth at a rate of 30 watts per cubic kilometer. (A watt is a rate of heat pro­ duction.) The heat then flows to the earth's surface where it is lost to space. Let F(x, y, z) denote the rate of flow of heat measured in watts per square kilometer. By def­ inition, the flux of F across a surface is the quantity of heat flowing through the surface per unit of time. 1039 E q r l\r ll3· (a) Compute div E . (b) Let Sa be the sphere of radius a centered at the ori­ gin and oriented outward. Show that the flux of E tlu·ough Sa is 41rq. (c) Could you have used the Divergence Theorem in part (b)? Explain why or why not. (d) Let S be an arbitrary, closed, outward-oriented sur­ face surrounding the origin. Show that the ·flux of E through S is again 41rq. [Hint: Apply the Diver­ gence Theorem to the solid region lying between a small sphere Sa and the surface S.] 37. According to Coulomb's Law, the electric field E at the point due to a charge q at the point O is given by r r E- (r) =q (f-fo) . l\r -r o ll3 Suppose S is a closed, outward-oriented surface and that r o does not lie on S. Use Problem 36 to show that 1E s , . dA = { 41rq if q lies inside S O if q lies outside S. Strengthen Your Understanding In Problems 38-39, explain what is wrong with the statement. 38. The flux integral Is F · dA can be evaluated using the Divergence Theorem, where F = 2xi - 3.7 and S is the triangular surface with corners (1, 0, 0), ( 0, 1, 0), (0, 0, 1) oriented away from the origin. 39. If Sis the boundary of a solid region W, where Sis ori­ ented outward, and F is a vector field, then 1 s div F 1 dA = f dV. ' w '" 1040 Chapter Nineteen FLUX INTEGRALS AND DIVERGENCE In Problems 40--41, give an example of: of a solid region such 40. A surface - is the boundary - S that that fs F · dA = 0 if F (x, y, z) = yi +xz.i + y2 k". 41. A vector field F such that the flux of of radius 1 centered at the origin is 3. F out of a sphere Are the statements in Problems 42-46 true or false? The smooth vector field F is defined everywhere in 3-space and has constant divergence equal to 4. 42. The field F has a net inflow per unit volume at the point (-3,4,0). 43. The vector field F could be the field (y-5x)k. F = xi +(3y)J + 44. The vector field F could be a constant field. 45. The flux of F through a circle of radius 5 lying anywhere on the xy-plane and oriented upward is 4(1r52 ). 46. The flux of F through a closed cylinder of radius 1 cen­ tered along the y-axis, 0 S yS 3 and oriented outward is 4(31r). Are the statements in Problems 47-55 true or false? Give rea­ sons for your answer. 47. fs F · dA = div F. 48. If F is a vector field in 3-space, and W is a solid re­ gion with boundary surface S , then fs div F · dA = fw F d\!. vector field in 3-space, and 49. If F is a diveroence-free 0 - S is a closed surface oriented inward, then fs F · dA = 0. 50. If F is a vector field in 3-space satisfying div F = 1, and S is a closed surface oriented outward, then fs F dA is equal to the volume enclosed by S. 51. Let vV be the solid region between the sphere S1 of ra­ dius 1 and S2 of radius 2, both centered at the origin. If F is a vector field in 3-space, then fw div F d\! = F . dA - j'S1 F · dA, where both S1 and S2 are S2 oriented outward. I 52. Let S1 be the squareOS x S 1,0 S yS 1,z = 0 ori­ ented downward and let S2 be the squareO S x S 1,0S y S 1, z:: 1 oriented ipwar�. If F is� vect�r field, then F · dA + F · dA , where W r div F d\! = Jw S1 S2 is the solid cubeOS x S 1, 0 S yS 1,0 S z S 1. J • Calculating flux integrals over surfaces Graphs: dA = (-fx i - jy ) +k) dx dy Cylinders: dA = (cos Bi +sin BJ )R dz dB Spheres: dA = (sin¢ cos Bi +sin¢ sin BJ + cos ¢k)R2 d¢ dB (3i + 4]) · dA a vector or a scalar? Calculate it. yi - xJ 2. Is div ( 2 a vector or a scalar? Calculate it. x +y2 ) J-=r 55. Let Sh be the surface consisting of a cylinder of height 2 2 h, closed at the top. The curved sides are x + y = 1, 2 2 forOS z S h, and the top x +y S 1, for z = _h, ort ented outward. If F is divergence free, then fs F · dA is independent of the height h. h 56. Let F = (5x + 7y)i + (7y + 9z)J + (9z +llx)k, and let Q.i be the flux of F tlU"ough the surfaces S;. for i =1-4. Arrange Q; in ascending order, where (a) S1 is the sphere of radius 2 centered at the origin (b) S2 is the cube of side 2 centered at the origin and with sides parallel to the axes (c) S 3 is the sphere of radius 1 centered at the origin (d) S4 is a pyramid with all four corners lying on S3 • Divergence Geometric and coordinate definition of divergence, cal­ culating divergence, interpretation in terms of outflow per unit volume. • The Divergence Theorem Stiitement of the theorem, divergence-free fields, har­ monic functions. 3. Let F (x, y, z) i + 2] + k. Each of the surfaces in (a)-(e) are squares of side length 4, with orientation given by the normal vector shown. Compute the flux of F through these surfaces. y x I x )- 21. y In Exercises 4-7, are the flux integrals positive, negative, or zero? Let S be a disk of radius 3 in the plane x = 7, centered on the x-axis and oriented toward the origin. 4.1 (xJ +yk). dA 6. 1 (yi 5. 1 (xi + yk) . dA + xk) . dA 1.1((x - lO)i + (x + 10)]). dA In Exercises 8-23, find the flux of the vector field through the surface S. 8. 9. 10. 11. 12. 13. 14. 15. 19. 20. z (e) 0, then div F = 0 at all points inside S. 18. y I 54. If Sis a sphere of radius 1, centered at the origin, oriented outward, and F is a vector field satisfying fs F · dA = Exercises 1 x 17. z (d) 1041 16. F = (x2 + y2 )i + xyJ and S is the square in the xy-plane with corners at (1,1,0), (-1,1, 0), (1, -1,O), (-1, -1,0), and oriented upward. y y J _ _______ REVIEW EXERCISES AND PROBLEMS FOR CHAPTER NIN_ET_EEN 1. Let S be the disk of radius 5 perpendicular to the y-axis, centered at (0, 7,0) and oriented toward the origin. Is (c) z (b) z J CHAPTER SUMMARY (see also Ready Reference at the end of the book) • Flux Integrals Oriented surfaces, definition of flux through a smface, definition of flux integral as a limit of a Riemann sum x 53. Let S1 be the square OS x S 1,0 S yS 1, z = 0 ori­ ented downward and let S2 be the squareO S x S 1, 0 S y S 1, z:: 1 oriented �pwar�. If F =_ cos(:i_yz)k, then r div F d\! = o F · dA + S1 F · dA , where W Jw S is the solid cubeO S-x S 1,0 Sy S 1,0 S zS 1. J tl "GJ z (a) REVIEW EXERCISES AND PROBLEMS FOR CHAPTER NINETEEN F = 2i + 3] through a disk of radius 5 in the plane y = 2 oriented in the direction of increasing y. F = i + 2] through a sphere of radius 3 at the origin. F = yJ through the square of side 4 in the plane y = 5. The square is centered on the y-axis, has sides parallel to the axes, and is oriented in the positive y-direction. F = xi through a square of side 3 in the plane x = -5. The square is centered on the x-axis, has sides parallel to the axes, and is oriented in the positive x-direction. F = (y+3)] through a square of side 2 in the xz-plane, oriented in the negative y-direction. 22. F = zi + yJ + 2xk and S is the rectangle z = 4, O S x S 2, 0 S y S 3, oriented in the positive z­ direction. = (x + cos z)i +yJ + 2xk and S is the rectangle y S 3, 0S z S 4, oriented in the positive x-direction. F x = 2, 0 S = x2 i + (x + e Y )J - k, and S is the rectangle y = -1, 0 S xS 2, 0 S zS 4, oriented in the negative y-direction. F F = (5 +xy)i +zJ +yzk and Sis the 2 x 2 square plate in the yz-plane centered at the origin, oriented in the positive x-direction. F = xi + Y.1 and S is the surface of a closed cylinder of radius 2 and height 3 centered on the z-axis with its base in the xy-plane. F = -yi + xJ + zk and S is the surface of a closed cylinder of radius 1 centered on the z-axis with base in the plane z = -1 and top in the plane z = 1. 23. F = x i + y j +zk and S is the cone z = V� x2 + y 2 , oriented upward with x2 +y2 S 1, x 2 0, y 2 0. - 2- 2- - In Problems 24-27, give conditions on one or more of the con­ stants a, b, c to ensure that the flux integral fs F · dA has the given sign. 24. Positive for F = ai +b] +ck arid Sis a disk perpen­ dicular to the y-axis, through (0, 5, 0) and oriented away from the origin. 25. Negative for F = ai + b] + ck and S is the upper half of the unit sphere centered at the origin and oriented downward. 26. Positive for F = axi +ay] + azk and S is a sphere centered at the origin. 27. Positive for F = ai + b] + ck and S is the triangle cut off by x + y +z = 1 in the first quadrant, oriented upward. 28. Calculate the flux of F = xyi + yzJ + zxk out of the closed boxO S x S 1, 0 S y S 1,0 S zS 1 (a) Directly (b) Using the Divergence Theorem. 29. Compute the flux integral fs(x3 i +2y] + 3k) . dA, where Sis the 2 x 2 x 2 rectangular surface centered at the origin, oriented outward. Do this in two ways: (a) Directly (b) Using the Divergence Theorem F = xk through the square O S x S 3, 0 S yS 3 in the xy-plane, with sides parallel to the axes, and oriented upward. In Exercises 30-35, use the Divergence Theorem to calculate the flux of the vector field out of the surface. = 6i + x2 ] - k, through a square of side 2 in the plane z = 3, oriented upward. 31. F = i - J - k through a cube of side 2 with sides parallel to the axes. F 30. F = -xi +2y] +(3+2z)k out of the sphere of radius 2 centered at (1,2,3). F = (2x + y)i +(3y + z)J + (4z +x)k and Sis the sphere of radius 5 centered at the origin. 1042 32. 33. Chapter Nineteen FLUX INTEGRALS AND DIVERGENCE F = x2i + y2J + e"'Y k and S is the cube of side 3 in the first octant with one corner at the origin, and sides parallel to the axes. F = ( e 2 + x) i + (2y + sin x)J + (e Sis the sphere of radius 1 centered at (2, 1, 0). "'2 z) k and 34. 35. F = x3 i + y3J + (x2 + y2 ) k cylinder x2 not included) x2 outward. (a) (c) J • 8 1 is a horizontal square of side 1 with one corner at (0, 0, 2), above the first quadrant of the xy-plane, oriented upward. • 8 2 is a horizontal square of side 1 with one corner at (0,0, 3), above the third quadrant of the xy-plane, oriented upward. • 83 is. a square of side J2 in the xz-plane with one corner at the origin, one edge along the positive x­ axis, one along the negative z-axis, oriented in the negative y- direction. • 84 is a square of side J2 with one corner at the ori­ gin, one edge along the positive y-axis, one corner at (1, 0, 1), oriented upward. 37. Let f(x, y, z) = xy + exyz . Find (a) grad f (b) Ic grad f · di", where C i.s the line from the point (1,1,1) to (2, 3,4). (c) Is grad f · d.A, where S is the quarter disk in the xy-plane x2 + y2 ::::; 4,x 2 0,y 2 0, oriented up­ ward. 38. The flux of the constant vector field ai +bJ +ck through the square of side 2 in the plane x = 5, oriented in the positive x-direction, is 24. Which of the constants a, b, c can be determined from the information given? Give the vaJue(s). 39. (a) Let F = (x2 + 4)i + yJ. Which of the following flux integrals is the largest? Explain. Isi F · d.A, where 81 is the disk of radius I in the plane x = 2 centered on the x-axis and oriented away from the origin. Is2 F · d.A, where 82 is the disk of radius 1 in the plane y = 4 centered on the y-axis and oriented away from the origin. Is3 F · d.A, where 83 is the disk of radius 1 in the plane x = -3 centered on the x-axis and ori­ ented toward the origin. (b) Calculate the integral you chose in part (a). 40. Figure 19.42 shows a cross-section of the earth's mag­ netic field. Assume that the earth's magnetic and geo­ graphic poles coincide. Is the magnetic flux through a horizontal plate, oriented skyward, positive, negative, or zero if the plate is + y2 = 4 with O ::::; and S is the closed z ::::; 5. F = xi + yJ + zk and Sis the open cylinder (ends Problems 36. Arrange the following flux integrals, Is; F · d.A, with i = 1, 2, 3,4, in ascending order if F = -i - + k and if the Si are the following surfaces: REVIEW EXERCISES AND PROBLEMS FOR CHAPTER NINETEEN + y2 ::::; 1, with O ::::; z ::::; 1, oriented At the north pole? On the equator? (b) .� .. North pole .� � i� .. � South pole At the south pole? • . Figure 19.42 41. (a) Let div(F) = x 2 + y 2 - z2 . Estimate the flux out of a small sphere of radius 0.1 centered at each of the following points (ii) (0,0,1) (i) (2,1,1) (b) What do the signs of your answers to part (a) tell you about the vector field near each of these two points? For Problems 42--44, (a) Find the flux of the given vector field out of a cube in the first octant with edge length c, one corner at the origin and edges along the axes. (b) Use your answer to part (a) to find div F at the origin using the geometric definition. (c) ComputedivF at the origin using partial derivatives. 42. 43. F = xi F = 2i + Y] + 3k In Problems 45--49, calculate the flux of F through the cylin­ der x 2 + y2 = 2, -3 ::::; z ::::; 3 and its base, oriented outward. The cylinder is open at the top. + x2J + 5k 45. F 47. F = zi +xJ +yk F = y 2i + x 2J + 7zk ft = x3i + y3J + f 46. F 48. 49. = y2i + z2J 52. (a) Let F be a smooth vector field defined throughout 3-space. What must be true of F if the flux of F through any closed surface is zero? (b) What value of a ensures that the flux of F = a(e"' +y2 -x)i +12y(l-e"' )J through any closed surface is zero? a = a1i + a2J + a3k be a constant vector and let r = xi + yJ + zk . 53. Let (a) Calculatediv(r x a). (b) Calculate the flux of i" x a out of a cube of side 5 centered at the origin with edges parallel to the axes. 54. Find the flux of F out of the closed surface S given by x2 + y2 + z2 = 100. You are given that F is contin­ uous, with div F = 3 inside the cube -2 ::::; x ::::; 2, -2 ::::; y ::::; 2, -2 ::::; z ::::; 2 and div F = 5 outside the cube. 55. The closed surface S consists of 81 , the cone x Jy 2 + z2 for O ::::; x ::::; 2, and a disk 82. Let F 3xi + (x + y )k 2 2 + 4yJ + 5zk. (a) In what plane does the disk 82 lie? How is it ori­ ented? (b) Find the flux of F through (ii) 81 (i) 82 56. Find the flux integral, using i" (a) r r. = xi + yJ + zk. cl.A' where 81 is the dis)< x Js 1 oriented away from the origin. = 5, y2 + z 2 r. ::::; 7, (b) r d.A' where 82 is the closed cylinder y2 Js2 z2 = 7, 0 ::::; x ::::; 5. (c) 44. ft= xi +yJ = z2i 50. Find the constant vector field F parallel to i + k and giving a flux of --7 through S, the cone z = r, with 0 ::::; z ::::; 4, oriented upward. 2 51. (a) Finddiv(r/llf 11 ) wherer = xi +yJ forr =!= 6. (b) Where in 3-space isdiv(f' /I ff 11 2 ) undefined? (c) Find the flux of i" /I Ir fl 2 through the closed cylin­ der of radius 2, length 3, centered at (5, 0, 0), with its axis parallel to the z-axis. (d) Find the flux of i" /I Ii" 11 2 through the curved sides of the open cylinder in part (c). r + i" . cl.A, where 83 is the curved side of the open Js3 cylinder y2 + z 2 = 7, 0 ::::; x ::::; 5, oriented away from the x-axis. 57. Let F (x, y, z) = Ji (x, y, z)i + h(x, y, z)J + k be a vector field with the property that div F = 5 every­ where. Let S be the hemisphere z = -.J9 - x2 - y2 , with its boundary in the xy-plane and oriented downward. Find 1 F 58. Let F · = r/1'17113 . d.A. (a) Calculate div F . Where is div F undefined? 1043 (b) Find ]"5 F · d.A where S is a sphere of radius IO centered at the origin. (c) Find 8 F · d.A where B1 is a box of side 1 cen­ 1 tered at the point (3, 0,0) with sides parallel to the axes. (d) Find 8 F · d.A where B2 is a box of side l cen­ 2 tered at the origin with sides parallel to the axes. (e) Using your results to parts (c) and (d), explain how, with no further calculation, you can find the flux of this vector field through any closed surface, provided the origin does not lie on the surface. I I 59. The gravitational field, origin is given by F , of a planet of mass m at the F = -Gm fc: lfr 113 . Use the Divergence Theorem to show that the flux of the gravitational field through the sphere of radius a is inde­ pendent of a. [Hint: Consider the region bounded by two concentric spheres.] 60. A basic property of the electric field E is that its diver­ gence is zero at points where there is no charge. Suppose that the only charge is along the z-axis, and that th� elec­ tric field E points radially out from the z-axis and. its magnitude depends only on the distance r from the z­ axis. Use the Divergence Theorem to show that the mag­ nitude of the field is proportional to 1/r. [Hint: Consider a solid region consisting of a cylinder of finite length whose axis is the z-axis, and with a smaller concentric cylinder removed.] 61. A fluid is flowing along a cylindrical pipe of radius a in the i direction. The velocity of the fluid at a radial dis­ tance r from the center of the pipe is = u(l-r2 /a 2 )i. v (a) What is the significance of the constant u? (b) What is the velocity of the fluid at the wall of the pipe? (c) Find the flux through a circular cross-section of the pipe. 62. A closed surface S encloses a volume 1,v. The function p(x, y, z) gives the electrical charge density at points in space. The vector field J (x, y, z) gives the electric cur­ rent density at any point in space and is defined so that the current through a small area d.A is given by Current through small area � J · d.A. (a) What do the following integrals represent, in terms of electricity? (i) !v p dV (ii) 1 J . d.A (b) Using the fact that an electric current through a surface is the rate at which electric charge passes through the surface per unit time, explain why ( J · d.A � ls -� at (1 w p dV) . 1 1044 PROJECTS FOR CHAPTER NINETEEN Chapter Nineteen FLUX INTEGRALS AND DIVERGENCE if its direction is awa.y from the origin at every point, its magnitude depends only on the distance from the origin, and its divergence is zero except at the origin. (Such a vector field might be used to model the photon flow out of a star or the neutrino flow out of a supernova.) (a) Show that v = K(x2 +y2 +z 2 )-312(xi +yJ +zk) 63. (a) A river flows across the xy-plane in the positive x-direction and aronnd a circular rock of radius 1 centered at the origin. The velocity of the river can be modeled using the potential function ¢ = x + (x/(x2 + y2 )). Compute the velocity vector field, v = grad¢. (b) Show that div v = 0. (c) Show that the flow of v is tangent to the circle x2 + y2 = 1. This means that no water crosses the circle. The water on the outside must therefore all flow around the circle. (d) Use a computer to sketch the vector field v in the region outside the unit circle. 64. A vector field is a point source at the origin in 3-space is a point source at the origin if I( > 0. (b) Determine the magnitude llv II of the source in part (a) as a function of the distance from its center. (c) Compute the flux of v through a sphere of radius r centered at the origin. (d) Compute the flux of v through a closed surface that does not contain the origin. CAS Challenge Problems 65. Let S be the part of the ellipsoid x 2 + y2 + 2z2 = 1 ly­ ing above the rectangle -1/2 S x S 1/2, -1/2 S y S 1/2, oriented upward. For each vector field (a)-(c), say whether you expect Is F · dA to be positive, negative, or zero. Then evaluate the integral exactly using a com­ puter algebra system and find numerical approximations for..your answers. Describe and explain what you notice. (a) Cb) Cc) 66. Let F = (z + 4)k, and let S be the surface with normal pointing in the direction of the negative y-axis parame­ terized, for O S s. S 2, 0 S t S 21r, by i"(s,t)=s2 costi +sJ +s2 sintk. (a) Sketch S and, without evaluating the integral, say whether Is F · dA is positive, negative, or zero. Give a geometric explanation for your answer. (b) Evaluate the flux integral. Does it agree with your answer to part (a)? F = xi ff = (x + l)i ff = yJ PROJECTS FOR CHAPTER NINETEEN - = xi+ yJ + zk (x2 + y2 + z2)3/2 and let oriented surfaces S1 and S2 be as in Figure 19.43. The surface S2 is on the unit sphere centered at the origin. Lines from the origin to the boundary of S1 intersect the unit sphere at the boundary of S2 . Both surfaces are oriented away from the origin. Show that (a) The divergence of F is zero. Cb) Is, ft · dA = Is ft . dA Is, F · di" the origin. = A vector field is spherically symmetric about the origin if, on every sphere centered at the origin, it has constant magnitude and points either away from or toward the origin. A vector field that is spherically :rymmetric about the origin can be written in terms of the spherical coordinate p = 117"'11 as F = f(p)e P where f is a function of the distance p from the origin, f (0) = 0, and Pis a unit vector pointing away from the origin. (a) Show that e div ft = ;2 : P (p2 J(p)), Pi= o. (b) Use part (a) to show that if F is a spherically symmetric vector field such that div F away from the origin then, for some constant k, = O (c) Use part (a) to confirm the Divergence Theorem for the flux of a spherically symmetric vector field through a sphere centered at the origin. (d) A form of Gauss's Law for an electric field E states that div E (r) = c5(r), where c5 is a scalar-valued function giving the density of electric charge at every point. Use Gauss's Law and part (a) to find the electric field when the charge density is oW) = { �o for some nonnegative a. Assume that where. E llrll :=::: a llrll > a, is spherically symmetric and continuous every­ Gauss's Law states that the flux of an electric field through a closed surface; S, is proportional to the quantity of charge, q, enclosed within S. That is, F (x, y) (c) 2. Divergence of Spherically Symmetric Vector Fields 3. Gauss's Law Applied to a Charged Wire and a Charged Sheet 1. Solid Angle Let 1045 2 D, the area of S2 . The value Dis called the solid angle subtended by S1 at Figure 19.43 hi. dA = kq. In this project we use Gauss's Law to calculate the electric field of a uniformly charged wire in part (a) and a flat sheet of charge in part (b). (a) Consider the-electric field due to an infinitely long, straight, uniformly charged wire. (There is no current running through the wire-all charges are fixed.) Assuming that the wire is infinitely long means that we can assume that the electric field is perpendicular to any cylinder that has the wire as an axis and that the magnitude of the field is constant on any such cylinder. Denote by Er the magnitude of the electric field due to the wire on a cylinder of radius r. (See Figure 19.44.) Imagine a closed surface S made up of two cylinders, one of radius a and one of larger radius b, both coaxial with the wire, and the two washers that cap the ends. (See Figure 19.45.) The outward orientation of S means that a normal on the outer cylinder points away from the wire and a normal on the inner cylinder points toward the wire. (i) Explain why the flux of E, the electric field, through the washers is 0. (ii) Explain why Gauss's Law implies that the flux through the inner cylinder is the same as the flux through the outer cylinder. [Hint: The charge on the wire is not inside the surface SJ. (iii) Use part (ii) to show that Eb/ Ea = a/b. (iv) Explain why part (iii) shows that the strength of the fi�ld due to an infinitely long uniformly charged wire is proportional to l/r. 1046 Chapter Nineteen FLUX INTEGRALS AND DIVERGENCE t Wire Figure 19.44 ]ia b Wire Figure 19.45 Figure 19.46 (b) Now consider an infinite flat sheet uniformly covered with charge. As in part (a), symmetry shows that the electric field E is perpendicular to the sheet and has the same magnitude at all points that are the same distance from the sheet. Use Gauss's Law to explain why, on any one side of the sheet, the electric field is the same at all points in space off the sheet. [Hint: Consider the flux through the box with sides parallel to the sheet shown in Figure 19.46.] 4. Flux Across a Cylinder: Obtaining Gauss's Law from Coulomb's Law An electric charge q is placed at the origin in 3-space. The induced electric field point with position vector f is given by Coulomb's Law, which says - E (f) = q r 3 llf 11 ' E (r) at the r =1 o. In this project, Gauss's Law is obtained for a cylinder enclosing a point charge by direct calcu­ lation from Coulomb's Law. (a) Let S be the open cylinder of height 2H and radius R given by x 2 + y 2 = R 2 , H, oriented outward. (i) Show that the flux of E the electric field, through S is given by , 1s (ii) What are the limits of the flux - E · dA = 47rq -H :::; z :::; H )H2 + R2 J� E · dA if • H ---+ 0 or H ---+ oo when R is fixed? • R ---+ 0 or R ---+ oo when His fixed? (b) Let T be the outward-oriented, closed cylinder of height 2H and radius R whose curved side is given by x2 +y2 = R 2 , -H :::; z :::; H, whose top is given by z = H, x 2 +y 2 :::; R2 , and bottom by z = -H, x2 + y 2 :::; R 2 . Use part (a) to show that the flux of the electric field, E through T is given by , lE . dA = 47rq. Notice that this is Gauss's Law. In particular, the flux is independent of both the height, H, and radius, R, of the cylinder.