Question

A magnetic field can be produced by:
$
  (a){\text{ a moving charge}} \\
  {\text{(b) a changing electric field}} \\
  {\text{(c) none of (a) and (b)}} \\
  {\text{(d) both of (a) and (b)}} \\
$

Answer 1

Hint – In this question use the term that a moving charge gives rise to current as current by definition is the movement of electrons, so that this current gives rise to magnetic field so that a moving charge has an indirect relation to magnetic field output, moreover a shifting electric field also plays a vital role in magnetic field development, using this term to get the correct response.

Complete step-by-step answer:
Magnetic field due to moving charges

All charged particles on the move generate magnetic fields. Moving point charges, like electrons, generate complicated but well-known magnetic fields that depend on the particle charge, velocity and acceleration.
In concentric circles, magnetic field lines form around a cylindrical current-carrying conductor, like a wire length. One can evaluate the position of such a magnetic field by using the "right hand law" (see figure).
With distance from the wire the magnetic field strength decreases. (For a wire of infinite length the force is inversely proportional to the size)
The magnetic field B for an infinitely long wire is given-

$B = \dfrac{{{\mu _o}I}}{{4\pi R}}$

Magnetic field due to changing electric field
The law of Faraday can be loosely defined as 'a shifting magnetic field produces an electrical field.' This is the concept behind the generator of electricity.
The Law of Ampere notes approximately that 'a shifting electrical field produces a magnetic field.' This rule can therefore be extended to create a magnetic field and drive an electric motor.
So that's the response they need.
Therefore choice (D) is the appropriate answer.

Note – There is a very significant property between a changing magnetic field and a changing electric field, both of which often oscillate together and are perpendicular to one another. Therefore, the electromagnetic waves form also perpendicular to this plane which contains electric and magnetic fields. So in short if the electric field is along the x-axis and magnetic field is along y-axis then the electromagnetic waves thus produced will be along z-axis.