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Problem 1.4 A wave traveling along a string is given by y(x,t) = 2 sin(4π t + 10π x) (cm), where x is the distance along the string in meters and y is the vertical displacement. Determine: (a) the direction of wave travel, (b) the reference phase φ0 , (c) the frequency, (d) the wavelength, and (e) the phase velocity. Solution: (a) We start by converting the given expression into a cosine function of the form given by (1.17): ³ π´ y(x,t) = 2 cos 4π t + 10π x − (cm). 2 Since the coefficients of t and x both have the same sign, the wave is traveling in the negative x-direction. (b) From the cosine expression, φ0 = −π /2. (c) ω = 2π f = 4π , f = 4π /2π = 2 Hz. (d) 2π /λ = 10π , λ = 2π /10π = 0.2 m. (e) up = f λ = 2 × 0.2 = 0.4 (m/s). Problem 1.8 Two waves on a string are given by the following functions: y1 (x,t) = 4 cos(20t − 30x) (cm) y2 (x,t) = −4 cos(20t + 30x) (cm) where x is in centimeters. The waves are said to interfere constructively when their superposition |ys | = |y1 + y2 | is a maximum, and they interfere destructively when |ys | is a minimum. (a) What are the directions of propagation of waves y1 (x,t) and y2 (x,t)? (b) At t = (π /50) s, at what location x do the two waves interfere constructively, and what is the corresponding value of |ys |? (c) At t = (π /50) s, at what location x do the two waves interfere destructively, and what is the corresponding value of |ys |? Solution: (a) y1 (x,t) is traveling in positive x-direction. y2 (x,t) is traveling in negative x-direction. (b) At t = (π /50) s, ys = y1 + y2 = 4[cos(0.4π − 30x) − cos(0.4π + 3x)]. Using the formulas from Appendix C, 2 sin x sin y = cos(x − y) − (cos x + y), we have ys = 8 sin(0.4π ) sin 30x = 7.61 sin 30x. Hence, |ys |max = 7.61 µ ¶ π π 2nπ cm, where and it occurs when sin 30x = 1, or 30x = + 2nπ , or x = + 2 60 30 n = 0, 1, 2, . . . . nπ cm. (c) |ys |min = 0 and it occurs when 30x = nπ , or x = 30 Problem 1.13 The voltage of an electromagnetic wave traveling on a transmission line is given by υ (z,t) = 5e−α z sin(4π × 109t − 20π z) (V), where z is the distance in meters from the generator. (a) Find the frequency, wavelength, and phase velocity of the wave. (b) At z = 2 m, the amplitude of the wave was measured to be 2 V. Find α . Solution: (a) This equation is similar to that of Eq. (1.28) with ω = 4π × 109 rad/s and β = 20π rad/m. From Eq. (1.29a), f = ω /2π = 2 × 109 Hz = 2 GHz; from Eq. (1.29b), λ = 2π /β = 0.1 m. From Eq. (1.30), up = ω /β = 2 × 108 m/s. (b) Using just the amplitude of the wave, −α 2 2 = 5e , µ ¶ −1 2 α= ln = 0.46 Np/m. 2m 5 Problem 1.26 Find the phasors of the following time functions: (a) υ (t) = 9 cos(ω t − π /3) (V) (b) υ (t) = 12 sin(ω t + π /4) (V) (c) i(x,t) = 5e−3x sin(ω t + π /6) (A) (d) i(t) = −2 cos(ω t + 3π /4) (A) (e) i(t) = 4 sin(ω t + π /3) + 3 cos(ω t − π /6) (A) Solution: (a) Ve = 9e− jπ /3 V. (b) v(t) = 12 sin (ω t + π /4) = 12 cos (π /2 − (ω t + π /4)) = 12 cos (ω t − π /4) V, e V = 12e− jπ /4 V. (c) i(t) = 5e−3x sin (ω t + π /6) A = 5e−3x cos[π /2 − (ω t + π /6)] A = 5e−3x cos (ω t − π /3) A, (d) (e) Ie = 5e−3x e− jπ /3 A. i(t) = −2 cos(ω t + 3π /4), Ie = −2e j3π /4 = 2e− jπ e j3π /4 = 2e− jπ /4 A. i(t) = 4 sin(ω t + π /3) + 3 cos(ω t − π /6) = 4 cos[π /2 − (ω t + π /3)] + 3 cos(ω t − π /6) = 4 cos(−ω t + π /6) + 3 cos(ω t − π /6) = 4 cos(ω t − π /6) + 3 cos(ω t − π /6) = 7 cos(ω t − π /6), Ie = 7e− jπ /6 A. Problem 1.27 Find the instantaneous time sinusoidal functions corresponding to the following phasors: (a) Ve = −5e jπ /3 (V) (b) Ve = j6e− jπ /4 (V) (c) Ie = (6 + j8) (A) (d) I˜ = −3 + j2 (A) (e) I˜ = j (A) (f) I˜ = 2e jπ /6 (A) Solution: (a) Ve = −5e jπ /3 V = 5e j(π /3−π ) V = 5e− j2π /3 V, v(t) = 5 cos (ω t − 2π /3) V. (b) Ve = j6e− jπ /4 V = 6e j(−π /4+π /2) V = 6e jπ /4 V, v(t) = 6 cos (ω t + π /4) V. (c) ◦ Ie = (6 + j8) A = 10e j53.1 A, i(t) = 10 cos (ω t + 53.1◦ ) A. (d) ◦ Ie = −3 + j2 = 3.61 e j146.31 , i(t) = Re{3.61 e j146.31 e jω t } = 3.61 cos(ω t + 146.31◦ ) A. ◦ (e) Ie = j = e jπ /2 , i(t) = Re{e jπ /2 e jω t } = cos(ω t + π /2) = − sin ω t A. (f) Ie = 2e jπ /6 , i(t) = Re{2e jπ /6 e jω t } = 2 cos(ω t + π /6) A. Problem 1.9 Give expressions for y(x,t) for a sinusoidal wave traveling along a string in the negative x-direction, given that ymax = 40 cm, λ = 30 cm, f = 10 Hz, and (a) y(x, 0) = 0 at x = 0, (b) y(x, 0) = 0 at x = 3.75 cm. Solution: For a wave traveling in the negative x-direction, we use Eq. (1.17) with ω = 2π f = 20π (rad/s), β = 2π /λ = 2π /0.3 = 20π /3 (rad/s), A = 40 cm, and x assigned a positive sign: µ ¶ 20π y(x,t) = 40 cos 20π t + (cm), x + φ0 3 with x in meters. (a) y(0, 0) = 0 = 40 cos φ0 . Hence, φ0 = ±π /2, and ¶ µ 20π π x± y(x,t) = 40 cos 20π t + 3 2 ¢ ¡ ½ 20π −40 sin ¡ 20π t + 3 ¢x (cm), if φ0 = π /2, = 40 sin 20π t + 203π x (cm), if φ0 = −π /2. (b) At x = 3.75 cm = 3.75 × 10−2 m, y = 0 = 40 cos(π /4 + φ0 ). Hence, φ0 = π /4 or 5π /4, and ¢ ¡ ½ 40 cos ¡20π t + 203π x + π4 ¢(cm), if φ0 = π /4, y(x,t) = 40 cos 20π t + 203π x + 54π (cm), if φ0 = 5π /4. Problem 1.15 A laser beam traveling through fog was observed to have an intensity of 1 (µ W/m2 ) at a distance of 2 m from the laser gun and an intensity of 0.2 (µ W/m2 ) at a distance of 3 m. Given that the intensity of an electromagnetic wave is proportional to the square of its electric-field amplitude, find the attenuation constant α of fog. Solution: If the electric field is of the form E(x,t) = E0 e−α x cos(ω t − β x), then the intensity must have a form I(x,t) ≈ [E0 e−α x cos(ω t − β x)]2 ≈ E02 e−2α x cos2 (ω t − β x) or I(x,t) = I0 e−2α x cos2 (ω t − β x) where we define I0 ≈ E02 . We observe that the magnitude of the intensity varies as I0 e−2α x . Hence, at x = 2 m, at x = 3 m, I0 e−4α = 1 × 10−6 (W/m2 ), I0 e−6α = 0.2 × 10−6 I0 e−4α 10−6 =5 = I0 e−6α 0.2 × 10−6 e−4α · e6α = e2α = 5 α = 0.8 (NP/m). (W/m2 ). Problem 1.29 The voltage source of the circuit shown in Fig. P1.29 is given by vs (t) = 25 cos(4 × 104t − 45◦ ) (V). Obtain an expression for iL (t), the current flowing through the inductor. R1 i A iL iR 2 + vs(t) L R2 R1 = 20 Ω, R2 = 30 Ω, L = 0.4 mH Figure P1.29: Solution: Based on the given voltage expression, the phasor source voltage is ◦ Ves = 25e− j45 (V). (9) The voltage equation for the left-hand side loop is R1 i + R2 iR2 = vs (10) diL , dt (11) For the right-hand loop, R2 iR2 = L and at node A, i = iR2 + iL . (12) Next, we convert Eqs. (2)–(4) into phasor form: R1 Ie+ R2 IeR2 = Ves R2 IeR2 = jω LIeL Ie = IeR2 + IeL e we have: Upon combining (6) and (7) to solve for IeR2 in terms of I, IeR2 = jω L I. R2 + j ω L (13) (14) (15) (16) Substituting (8) in (5) and then solving for Ie leads to: jR2 ω L e e R1 Ie+ I = Vs R2 + j ω L µ ¶ jR ω L 2 Ie R1 + = Ves R2 + j ω L µ ¶ R1 R2 + jR1 ω L + jR2 ω L e I = Ves R2 + j ω L ¶ µ ω L R + j 2 Ves . Ie = R1 R2 + jω L(R1 + R2 ) (17) Combining (6) and (7) to solve for IeL in terms of Ie gives IeL = R2 e I. R2 + j ω L (18) Combining (9) and (10) leads to ¶µ ¶ µ R2 + j ω L R2 e Ves IL = R2 + j ω L R1 R2 + jω L(R1 + R2 ) R2 Ves . = R1 R2 + + jω L(R1 + R2 ) Using (1) for Ves and replacing R1 , R2 , L and ω with their numerical values, we have IeL = 30 j4 × 104 × 0.4 × 10−3 (20 + 30) 20 × 30 + ◦ 30 × 25 = e− j45 600 + j800 ◦ 7.5 − j45◦ 7.5e− j45 − j98.1◦ e = = ◦ = 0.75e j53.1 6 + j8 10e 25e− j45 Finally, iL (t) = Re[IeL e jω t ] = 0.75 cos(4 × 104t − 98.1◦ ) (A). (A). ◦