Question

The source of electromagnetic wave can be a charge.

• A. Moving with a constant velocity
• B. Moving in a circular orbit
• C. At rest
• D. Moving parallel to the magnetic field

Answer 1

The Correct Option is B

Electromagnetic waves are generated by accelerating charges or changing electric or magnetic fields. Let's analyze each option:

1. Moving with a constant velocity:

A charge moving with a constant velocity constitutes a steady current. While a steady current produces a magnetic field, it does not produce electromagnetic waves. Electromagnetic waves are generated when there is a *change* in the electric or magnetic field with time. A charge moving with constant velocity does not have a changing acceleration, and therefore does not radiate electromagnetic waves.

2. Moving in a circular orbit:

A charge moving in a circular orbit is constantly changing its direction of motion. This change in direction implies that the velocity is changing, and hence the charge is accelerating. Specifically, it experiences centripetal acceleration, which is directed towards the center of the circular path. An accelerating charge is a source of electromagnetic waves. Think of synchrotron radiation, which is produced by charged particles moving in circular paths in magnetic fields.

3. At rest:

A charge at rest produces a static electric field around it. However, since it is not moving or accelerating, the electric field is constant in time. A static charge does not generate electromagnetic waves.

4. Moving parallel to the magnetic field:

If a charge is moving parallel to a constant magnetic field, the magnetic force on the charge is given by $\vec{F} = q(\vec{v} \times \vec{B})$. If $\vec{v}$ is parallel to $\vec{B}$, then $\vec{v} \times \vec{B} = 0$, so the magnetic force is zero. If there are no other forces causing acceleration, the charge will continue to move with constant velocity (or remain at rest if initially at rest, but the option suggests "moving"). As discussed in point 1, a charge moving with constant velocity does not emit electromagnetic waves.

Conclusion:

Out of the given options, only a charge moving in a circular orbit (option 2) is necessarily accelerating and hence can serve as a source of electromagnetic waves. The other options describe charges that are either at rest or moving with constant velocity, neither of which generates electromagnetic waves.

Final Answer: The final answer is Moving in a circular orbit

Answer 2

(A) Moving with a constant velocity: This will produce an electric as well as a magnetic field, but not electromagnetic radiation.

(B) Moving in a circular orbit: This is the correct answer. An accelerating charge, such as one moving in a circular orbit, emits electromagnetic radiation.

(C) At rest: A stationary charge produces only a static electric field, not electromagnetic waves.

(D) Moving parallel to the magnetic field: This produces an electric and magnetic field, not EM radiation

The correct answer is (B): Moving in a circular orbit