Physics 3 - Electromagnetism - The motor effect

When a current is passed through a wire in a magnetic field, a force may be exerted on the wire. This effect is known as the motor effect. 

The force can be increased by:

- Increasing the current

- using a stronger magnet

The force depends on the angle between the wire and the magnetic field lines. The force is:

- Greatest when the wire is perpendicular to the magnetic field

- Zero when the wire is parallel to the magnetic field lines.

The direction of the force is always at right angles to the wire and the field lines. Also, the direction of the force is reversed if the direction of the current or the magnetic field is reversed.

An electric motor is designed to use the motor effect. We control the speed of an electric motor by changing the current. Also, we can reverse its direction by reversing the current.

A loudspeaker is designed to make a diaphragm attached to a coil vibrate when alternating current passes through the coil.

- When a current passes through the coil, a force due to the motor effect makes the coil move.

- Each time the current changes its direction, the force reverses its direction. So the coil is repeatedly forced backwards and forwards. This motion makes the diaphragm vibrate so sound waves are created.