The torque of a uniform magnetic field on a current-carrying frame
The two opposite sides of a current-carrying frame in a magnetic field feel Ampère forces (F = I·B·l) that are equal in size and opposite in direction, so the frame is acted on by a couple. This couple gives a torque about the axis of rotation: M = I·B·S·sinα for one turn and M = N·I·B·S·sinα for a coil of N turns, where α is the angle between B and the normal to the frame. The torque is greatest (Mmax = N·I·B·S) when the plane of the frame is parallel to the field (α = 90°); when the plane is perpendicular to the field (α = 0°) the torque is zero and the frame is in equilibrium. If the current is reversed, the torque reverses and the frame turns the other way. In moving-coil ammeters the frame is held by springs, so the deflection angle is proportional to the current; the operation of DC motors rests on the same principle.
At the desk, without a power source: look at a ready-made teaching model (a moving-coil meter or toy motor that the teacher shows) and find the permanent magnet, the frame and the spring. Write in your notebook what job each part does. Only the teacher runs live experiments, with a low-voltage battery; never experiment with household mains.