Identify the correct statements among the following:
A. In the propagation of electromagnetic waves, the electric and magnetic field vectors do not have constant phase difference.
B. When a capacitor is discharging, the charge decreases on it and the direction of displacement current is in the opposite direction to the current due to the electric field.
C. The velocity of electromagnetic waves is the same in every medium.
D. An electric charge oscillating harmonically produces electromagnetic waves.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → B and D only
The fields of an electromagnetic wave oscillate in phase, and accelerated charges radiate the waves.
(A) In the propagation of electromagnetic waves, the electric and magnetic field vectors do not have constant phase difference. — Incorrect. Their phase difference is constant (zero).
(B) When a capacitor is discharging, the charge decreases on it and the direction of displacement current is in the opposite direction to the current due to the electric field. — Correct. \( i_d = \varepsilon_0\,d\Phi_E/dt \); as the field between the plates decreases, the displacement current points opposite to the field \( \vec{E} \).
(C) The velocity of electromagnetic waves is the same in every medium. — Incorrect. The speed \( 1/\sqrt{\mu\varepsilon} \) is c only in vacuum.
(D) An electric charge oscillating harmonically produces electromagnetic waves. — Correct. An oscillating charge is accelerated, so it radiates.
Note: Statement (B) is loosely worded. It is read here as “the displacement current is opposite to the field between the plates”; read otherwise it would be false, and no option gives “(D) only”.
NCERT (2026-27):
Displacement Current – “… The current, given by Eq. (8.4), is a new term, and is due to changing electric field (or electric displacement, an old term still used sometimes). It is, therefore, called displacement current or Maxwell’s displacement current.”
Nature of Electromagnetic Waves – “… We can write Ex and By as follows:
\( \displaystyle E_x = E_0 \sin (kz - \omega t) \qquad [8.7(a)] \)
\( \displaystyle B_y = B_0 \sin (kz - \omega t) \qquad [8.7(b)] \)”
Nature of Electromagnetic Waves – “… Thus, the velocity of light depends on electric and magnetic properties of the medium.”
Sources of Electromagnetic Waves – “… It is an important result of Maxwell’s theory that accelerated charges radiate electromagnetic waves. … Consider a charge oscillating with some frequency. (An oscillating charge is an example of accelerating charge.)”