Physics · Electromagnetic Waves

NEET (UG) 2025 — Question 24

A parallel plate capacitor made of circular plates is being charged such that the surface charge density on its plates is increasing at a constant rate with time. The magnetic field arising due to displacement current is :

Option A:
zero at all places
Option B:
constant between the plates and zero outside the plates
Option C:
non-zero everywhere with maximum at the imaginary cylindrical surface connecting peripheries of the plates
Correct
Option D:
zero between the plates and non-zero outside

Answer: C

Step-by-step solution

$\mathrm{I}_{\mathrm{d}}=\epsilon_{0} \frac{\mathrm{~d} \phi_{e}}{\mathrm{dt}}=\epsilon_{0} \frac{\mathrm{~d}}{\mathrm{dt}}\left(\mathrm{EA} \cos 0^{\circ}\right)$

$\mathrm{I}_{\mathrm{d}}=\epsilon_{0} \mathrm{~A} \frac{\mathrm{dE}}{\mathrm{dt}}$

$\because \mathrm{E}=\frac{\sigma}{\epsilon_{0}} ; \mathrm{I}_{\mathrm{d}}=\epsilon_{0} \mathrm{~A} \frac{\mathrm{~d}}{\mathrm{dt}}\left(\frac{\sigma}{\epsilon_{0}}\right)$

$\Rightarrow \mathrm{I}_{\mathrm{d}}=\mathrm{A}\left(\frac{\mathrm{d} \sigma}{\mathrm{dt}}\right)=$ constant

Also $B$ due to $I_{d}$ is $\max ^{m}$ at surface of imaginary cylinder

Answer key and solution verified before publishing.

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Exam: NEET (UG) 2025
Paper: 2025 paper
Subject: Physics
Chapter: Electromagnetic Waves
Topic: Displacement Current and Equation of EM Waves