phy Qscillation

Resolve the magnetic field B in directions perpendicular and parallel to the plane of the coil.

The ocmponent pararllelto the plane of the coil = B.sin(theta)
This component interacts with the current flowing on top and bottom of the coil, the magnetic force F = [B.sin(theta)].L.i
where L is the length at the top and bottom of the coil
The force pulls the top of the coil to the right and the bottom to the left, as found by using Fleming's Left Hand Rule.

Toroque T = F.L', where L' is the length of the sides of the coil
i.e. T = [B.sin(theta)].L.i.L'

But (L.L') is the area of the coil A
hence, T = A.Bi.sin(theta)

SInce there are N turns of the coil, the total torque
= NABi.sin(theta)