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The electric field of an electromagnetic wave in free space is given by E=10cos(107t+kx)j^\vec{E} = 10 \cos(10^7 t + kx) \hat{j} V/m, where tt and xx are in seconds and meters respectively. It can be inferred that: (1) The wavelength λ\lambda is 188.4 m188.4 \text{ m}. (2) The wave number kk is 0.33 rad/m0.33 \text{ rad/m}. (3) The wave amplitude is 10 V/m10 \text{ V/m}. (4) The wave is propagating along +x+x direction. Which one of the following pairs of statements is correct?

A

(3) and (4)

B

(1) and (2)

C

(2) and (3)

D

(1) and (3)

Step-by-Step Solution

  1. Analyze Amplitude: The standard wave equation is E=E0cos(ωt±kx)E = E_0 \cos(\omega t \pm kx). Comparing with the given equation, the amplitude E0=10 V/mE_0 = 10 \text{ V/m}. Thus, statement (3) is correct.
  2. Analyze Direction: The argument of the cosine function is (107t+kx)(10^7 t + kx). The positive sign between the time and position terms (++) indicates that the wave is propagating in the negative xx-direction . Thus, statement (4) is incorrect.
  3. Analyze Wavelength: The angular frequency is ω=107 rad/s\omega = 10^7 \text{ rad/s}. The speed of light in vacuum is c=3×108 m/sc = 3 \times 10^8 \text{ m/s}. The wavelength λ\lambda is given by λ=2πcω\lambda = \frac{2\pi c}{\omega}. λ=2π×3×108107=60π60×3.1416=188.5 m\lambda = \frac{2\pi \times 3 \times 10^8}{10^7} = 60\pi \approx 60 \times 3.1416 = 188.5 \text{ m} Thus, statement (1) is correct (188.4 m is a reasonable approximation).
  4. Analyze Wave Number: The wave number k=ωc=1073×108=1300.033 rad/mk = \frac{\omega}{c} = \frac{10^7}{3 \times 10^8} = \frac{1}{30} \approx 0.033 \text{ rad/m}. Statement (2) claims it is 0.33 rad/m0.33 \text{ rad/m}, which is incorrect by a factor of 10.

Therefore, only statements (1) and (3) are correct.

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