If a current flows around a loop of wire in a magnetic field, with the loop as shown, the loop will start to rotate clockwise below because oppositely directed forces on left and right hand wires produce a torque, or tuning force.
The torque will be zero when the loop of wire is vertical, because the left hand force will be vertically up and the right and force will be vertically down, so both forces will act along the same line and produce no net torque.
The loop will continue to rotate though, but now the force will reverse direction because the current – clockwise above – will flow anticlockwise. The loop will slow down and perhaps stop.
In order for rotation to be continuous, the current needs to change direction every half turn, when the loop reaches the vertical. This is done by using a commutator and brushes as shown below.
