Question

A moving charge produces
A. Only electric field
B. Only magnetic field
C. Both electric and magnetic field
D. Neither the electric field nor the magnetic field

Answer 1

Hint: In this question we have been asked to state whether a moving charge produces an electric field, magnetic field, both the field or neither of them. Therefore, to answer this question, we shall discuss how the electric and magnetic field is produced. We know that a stationary charge only produces electric fields. Therefore, we shall only consider the moving particle.

Complete answer:
The special theory of relativity when combined with electrostatics can give us the answer as to what a moving charge produced. We know that statically charged particles produce electric fields. Therefore, let us assume a wire having an infinite charge observed through the initial frame. Now, let the positive charges be flowing to the right and an equal number of negative charges be flowing to the left. Now, the net electric field will be zero as the total charge at any point is zero. Therefore, if an electron is placed next to this wire, it will experience no force. Now assume that we are moving past the wire with constant acceleration to the right on a rocket. Now, the relative motion of the charge and the rocket causes the negative charge to be length contracted and the positive charge be length expanded. Therefore, at any given point in the accelerating rocket frame, the negative charge will be more than the positive charge. These charges will now repel each other. However, the special theory of relativity states that both frames are valid and should agree that the electron does not accelerate towards or away from the line. This contradiction can be overcome by assuming a new force is generated when the electron is moving. Therefore, we can say that the moving charge produces both electric and magnetic fields.
Therefore, the correct answer is option C.

Note: The moving electron alone does not produce the magnetic field. As the electron moves relative to ions, there is a relativistic charge per unit volume difference between the positive and negative charges. The magnetic field can be assumed to be the other side of the coin. It is the same as the electric field when it is viewed from another frame of reference.