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GATE 2020 PH – Question 44

Electromagnetic theory · Solutions of electrostatic and magnetostatic problems including boundary value problems · 2 marks · Multiple choice

A charge q moving at uniform speed enters a cylindrical region in free space at t = 0 and leaves at $t=\tau$, as shown. Which graph represents total electric flux $\Phi(t)$ through the entire closed cylindrical surface?

Diagram for GATE 2020 PH question 44
  1. Graph A
  2. Graph B
  3. Graph C
  4. Graph D

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Show answer and explanation

Correct answer: (D) Graph D

Explanation

Gauss’s law states that flux through any closed surface is enclosed charge divided by $\epsilon_0$. While the charge lies inside the cylinder, the flux is $q/\epsilon_0$, independent of its position. Before entry and after exit the enclosed charge is zero and so is the flux. The rectangular pulse from 0 to τ is graph D; its edge values at the instant of crossing do not affect this choice.

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