The figure below shows the north and south poles of permanent magnet in which n turn coil of area of cross-section `A` is resting, such that for a current I passed through the coil, the plane of the coil makes an angle with respect to the direction of magnetic field `B`. If the plane of the magnetic field and the coil are horizontal and vertical respectively, the torque on the coil will be

Options

A. `tau=niAB costheta`
B. `tau=niAB sintheta`
C. `tau=niAB`
D. None of the above, since the magnetic field is radial

Answer

A

Explanation

Plane of coil is having angle theta with the magnetic field.
`:. Tau=MB sin (90-theta)` or `tau=niAB cos theta`
[As M=niA]