Chapter 29
8. (a) Recalling the straight sections discussion in Sample Problem — “Magnetic field at the
center of a circular arc of current,” we see that the current in segments AH and JD do not
contribute to the field at point C. Using Eq. 29-9 (with φ = π) and the right-hand rule, we find
that the current in the semicircular arc H J contributes μ 0i 4 R1 (into the page) to the field at C.
Also, arc D A contributes μ 0i 4 R2 (out of the page) to the field there. Thus, the net field at C is
B=
μ 0i ⎛ 1
⎜
4 ⎝ R1
−
1 ⎞ (4p ×10 -7 T ⋅ m A)(0.281A) ⎛
1
1
⎞
-6
−
⎟ =
⎜
⎟ = 1.67 ×10 T.
R2 ⎠
4
⎝ 0.0315m 0.0780m ⎠
10. (a) Recalling the straight sections discussion in Sample Problem — “Magnetic field at the
center of a circular arc of current,” we see that the current in the straight segments collinear with
C do not contribute to the field at that point.
Equation 29-9 (with φ = π) indicates that the current in the semicircular arc contributes μ 0i 4 R
to the field at C. Thus, the magnitude of the magnetic field is
μ 0i
(4p ×10 -7 T ⋅ m A)(0.0348A)
B=
=
= 1.18 ×10 -7 T.
4R
4(0.0926m)
(b) The right-hand rule shows that this field is into the page.
11. (a) BP1 = μ0i1 / 2π r1 where i1 = 6.5 A and r1 = d1 + d2 = 0.75 cm + 1.5 cm = 2.25 cm, and
BP2 = μ0i2 / 2π r2 where r2 = d2 = 1.5 cm. From BP1 = BP2 we get
⎛r ⎞
⎛ 1.5 cm ⎞
i2 = i1 ⎜ 2 ⎟ = ( 6.5 A ) ⎜
⎟ = 4.3A.
⎝ 2.25 cm ⎠
⎝ r1 ⎠
(b) Using the right-hand rule, we see that the current i2 carried by wire 2 must be out of the page.
12. (a) Since they carry current in the same direction, then (by the right-hand rule) the only
region in which their fields might cancel is between them. Thus, if the point at which we are
evaluating their field is r away from the wire carrying current i and is d – r away from the wire
carrying current 3.00i, then the canceling of their fields leads to
μ 0i
μ0 (3i )
=
⇒
2π r 2π (d − r )
r=
d 16.0 cm
=
= 4.0 cm.
4
4
(b) Doubling the currents does not change the location where the magnetic field is zero.
29. Each wire produces a field with magnitude given by B = μ0i/2πr, where r is the distance from
the corner of the square to the center. According to the Pythagorean theorem, the diagonal of the
square has length 2a , so r = a 2 and B = μ 0i 2pa . The fields due to the wires at the
upper left and lower right corners both point toward the upper right corner of the square. The
fields due to the wires at the upper right and lower left corners both point toward the upper left
corner. The horizontal components cancel and the vertical components sum to
μi
2μ i 2 ( 4π ×10 T ⋅ m A ) ( 20 A )
Bnet = 4 0 cos 45° = 0 =
= 8.0 ×10−5 T.
πa
π ( 0.20 m )
2π a
−7
In the calculation cos 45° was replaced with 1 2 . The total field points upward, or in the +y
r
ˆ In the figure below, we show the contributions from the
direction. Thus, Bnet = (8.0 × 10−5 T)j.
individual wires. The directions of the fields are deduced using the right-hand rule.
35. Equation 29-13 gives the magnitude of the force between the wires, and finding the xcomponent of it amounts to multiplying that magnitude by cosφ =
d2
. Therefore, the xd12 + d22
component of the force per unit length is
Fx
μ0i1i2 d 2
(4π× 10−7 T ⋅ m/A)(4.00 ×10−3 A)(6.80 × 10−3 A)(0.050 m)
=
=
2π [(0.0240 m) 2 + (0.050 m) 2 ]
L 2π (d12 + d 22 )
.
= 8.84 ×10−11 N/m.
45. (a) Two of the currents are out of the page and one is into the page, so the net current
enclosed by the path is 2.0 A, out of the page. Since the path is traversed in the clockwise sense,
a current into the page is positive and a current out of the page is negative, as indicated by the
right-hand rule associated with Ampere’s law. Thus,
r
r
∫ B ⋅ ds = −μ i = −(4π ×10
0
−7
T ⋅ m/A) ( 2.0 A ) = −2.5 ×10−6 T ⋅ m.
z
(b) The net current enclosed by the path is zero (two currents are out of the page and two are into
r r
the page), so B ⋅ ds = μ 0ienc = 0 .
50. It is possible (though tedious) to use Eq. 29-26 and evaluate the contributions (with the intent
to sum them) of all 1200 loops to the field at, say, the center of the solenoid. This would make
use of all the information given in the problem statement, but this is not the method that the
student is expected to use here. Instead, Eq. 29-23 for the ideal solenoid (which does not make
use of the coil radius) is the preferred method:
B = μ 0in = μ 0i
FG N IJ
H lK
where i = 3.60 A, l = 0.950 m, and N = 1200. This yields B = 0.00571 T.