Question

A point charge is moving in clockwise direction in a circle with constant speed. Consider the magnetic field produced by the charge at a fixed point $P$ (not at the center of circle) on the axis of the circle. Then,

• A. it is constant in magnitude only
• B. it is constant in direction only
• C. it is constant both in direction and magnitude
• D. it is constant neither in magnitude nor in direction

Answer 1

Answer: (A)

The point charge moves in circle as shown in figure. The magnetic field vectors at a point $P$ on the axis of circle are ${\vec{B}}_A$ and ${\vec{B}}_C$ at the instant the point charge is at $A$ and $C$, respectively, as shown in figure.
Hence, as the particle rotates in circle, only magnitude of magnetic field remains constant at $P$ but its direction changes.