Question

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:

• (A) only
• (D) only
• (C) and (E) only
• (B) and (D) only

Hint:

{Electromagnetic Fields}: Time-varying electric fields and accelerating charges are fundamental in generating electromagnetic waves, which consist of oscillating electric and magnetic fields. Understanding the sources of these fields is essential in electromagnetism and applications like wireless communication.

Answer 1

The Correct Option is B

A time-varying magnetic field can be produced by sources that involve changing electric fields or accelerating charges. Let's analyze each option:

1. A permanent magnet: A permanent magnet produces a static (unchanging) magnetic field. Therefore, it does not produce a time-varying magnetic field.
2. An electric field changing linearly with time: A changing electric field can induce a magnetic field according to Maxwell's equations. However, if the electric field changes linearly with time (i.e., has a constant rate of change), the induced magnetic field is steady, not varying with time.
3. Direct current: Direct current (DC) produces a constant magnetic field. It does not produce a time-varying magnetic field.
4. A decelerating charge particle: An accelerating (or decelerating) charge particle produces electromagnetic waves (EMW), which have both time-varying electric and magnetic fields.
5. An antenna fed with a digital signal: An antenna fed with a digital signal transmits electromagnetic waves, which consist of time-varying electric and magnetic fields.

Analysis of Options:

• Option (1): (B) and (D) only
  • (B) An electric field changing linearly with time: Produces a steady induced magnetic field.
  • (D) A decelerating charge particle: Produces time-varying magnetic fields.
  • Partial: Only (D) is correct.
• Option (2): (C) and (E) only
  • (C) Direct current: Produces a constant magnetic field.
  • (E) An antenna fed with a digital signal: Produces time-varying magnetic fields.
  • Partial: Only (E) is correct.
• Option (3): (D) only
  • (D) A decelerating charge particle: Correct.
• Option (4): (A) only
  • (A) A permanent magnet: Incorrect.

Conclusion:

Only option (D) results in a time-varying magnetic field. Therefore, the correct answer is (3) (D) only.