A flat coil of `n` turns, area `A` and carrying a current `I` is placed in a uniform magnetic field of magnitude `B`. The plane of the coil makes an angle `theta` with the direction of the field. The torque acting on the coil is

Options

A. `B I n A sin theta`
B. `(n A I)/(B) sin theta`
C. `B I n A cos theta`
D. `B I n^(2) A cos theta`

Answer

C

Explanation

`vec(tau) = vec(M) xx vec(B)`
`tau = MB sin theta, theta`: angle between `vec(M)` and `vec(B)`
Here angle between `vec(M)` and `vec(B)` is `(90 - theta)`
`tau = nIAB sin (90 - theta) = nIAB cos theta`