Which of the following physical quantities has zero average value in a plane electromagnetic wave?
A. Electric energy
B. Magnetic field
C. Magnetic energy
D. Electric field
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → B and D only
In a plane electromagnetic wave the fields vary sinusoidally, \( E = E_0\sin(kx - \omega t) \) and \( B = B_0\sin(kx - \omega t) \), and a sine function is positive for half a cycle and negative for the other half. The energy densities depend on the squares of the fields, which are never negative.
(A) Electric energy — Incorrect. \( u_E = \frac{1}{2}\varepsilon_0 E^{2} \) is never negative; its average is \( \frac{1}{4}\varepsilon_0 E_0^{2} \), not zero.
(B) Magnetic field — Correct. B takes equal positive and negative values over a cycle, so its average is zero.
(C) Magnetic energy — Incorrect. \( u_B = B^{2}/2\mu_0 \) is never negative; its average is \( B_0^{2}/4\mu_0 \), equal to the average electric energy density.
(D) Electric field — Correct. Like B, the electric field averages to zero over a cycle.
Note: A similar question was also asked in CUET (UG) 2025 on 02-06-2025 (Slot 1).