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

Options

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

Answer

A

Explanation

The point charge moves is circle as shown in figure. The magnetic field vectors at a point `P` on 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 the figure.

Hence as the particle rotates in circle, only magnitude of magnetic field remains constant at `P` but its direction changes.
Alternate solution: The magnetic field at a point on the axis due to charge moving along a circular path is given by
`(mu_(0))/(4pi) (qvec(v)xxvec(r))/(r^(3))`
It can be seen that the magnitude of the magnetic field at a point on the axis remains constant. But direction of the field keeps on changing.