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Magnetics question

2023 · 13 Apr · Shift 1 · Q52
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  5. /2023 · 13 Apr · Shift 1 · Q52

Magnetics question

2023 · 13 Apr · Shift 1 · Q52

JEE MainPhysicsMagneticsMCQ+4 / −1
The source of time varying magnetic field may be (A) a permanent magnet (B) an electric field changing linearly with time (C) direct current (D) a decelerating charge particle (E) an antenna fed with a digital signal Choose the correct answer from the options given below:
  1. A
    (D) only
  2. B
    (A) only
  3. C
    (B) and (D) only
  4. D
    (C) and (E) only
View written solutionFree

Correct answer: QUESTION/OPTIONS APPEAR FAULTY. THE CORRECT SET SHOULD BE (B), (D), AND (E).

  1. We need to identify which among (A)–(E) can be a source of a time-varying magnetic field.

A time-varying magnetic field can arise from:

  • time-varying currents,
  • moving/accelerating charges,
  • a time-varying electric field (Maxwell's displacement current term).

From Maxwell–Ampère law:

∇×B=μ0J+μ0ε0∂E∂t\nabla \times \mathbf{B} = \mu_0 \mathbf{J} + \mu_0\varepsilon_0 \frac{\partial \mathbf{E}}{\partial t}∇×B=μ0​J+μ0​ε0​∂t∂E​

So magnetic field can be produced by current density J\mathbf{J}J and also by changing electric field ∂E∂t\dfrac{\partial \mathbf{E}}{\partial t}∂t∂E​.

  1. Check each statement.

(A) A permanent magnet

A permanent magnet produces a magnetic field that is essentially static in time. So it is not a source of a time-varying magnetic field.

Hence, (A) is incorrect.

(B) An electric field changing linearly with time

If electric field changes with time, then

∂E∂t≠0\frac{\partial \mathbf{E}}{\partial t} \neq 0∂t∂E​=0

Therefore, by Maxwell’s correction, it can generate magnetic field. Since the electric field is changing linearly with time, the associated magnetic field source is time-dependent.

Hence, (B) is correct.

(C) Direct current

A steady direct current produces a steady magnetic field, not a time-varying one.

Hence, (C) is incorrect.

(D) A decelerating charge particle

A decelerating charge is an accelerated charge. Accelerated charges produce time-dependent electromagnetic fields. Thus it can be a source of a time-varying magnetic field.

Hence, (D) is correct.

(E) An antenna fed with a digital signal

A digital signal is time-varying, so the current in the antenna changes with time. A time-varying current produces a time-varying magnetic field.

Hence, (E) is correct.

  1. So the correct set from physics is:
(B), (D), (E)(B),\ (D),\ (E)(B), (D), (E)

Now compare with the given options:

  • Option A: (D)(D)(D) only — incomplete
  • Option B: (A)(A)(A) only — wrong
  • Option C: (B)(B)(B) and (D)(D)(D) only — misses (E)(E)(E)
  • Option D: (C)(C)(C) and (E)(E)(E) only — includes wrong (C)(C)(C)

Thus none of the provided options is fully correct.

  1. Comparison with stored correct answer.

Stored correct answer is A, i.e. (D)(D)(D) only. But from Maxwell’s equations and electromagnetic radiation theory:

  • (B)(B)(B) is definitely correct,
  • (D)(D)(D) is correct,
  • (E)(E)(E) is also correct.

So the stored answer does not agree with the physics.

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