What is torque acting on a plane coil of ''n'' turns carrying a current ''i'' and having an area A, when placed in a constant magnetic field B ?

Explanation

For Rectangular LOOP PQRS :
Length PR = QS = l
breadth PQ = RS = b
Current = i
Magnetic field Induction = B
Angle made by normal to plane of coil with B `= theta`
Forces on conductor PR and SQ, F `= Bil sin theta`
Forces on conductor PQ and RS, F = 0
Torque on a rectangular coil, `T = F xx _|_^(r)` distance (b) `rArr tau = Bil sin theta (b)`
`:. tau = BiA sin theta [because A = l xx b]`
If the loop has n turns , then `tau = B` in A sin `theta`.