The motor effect

When a wire carrying a current is placed in a magnetic field, the magnet and the wire exert a force on each other. This is the motor effect.

The force is biggest when the wire is at right angles to the field, and there is no force when the wire is parallel to it.

Fleming's left-hand rule gives the direction of the force. Hold your thumb and first two fingers of your left hand at right angles to each other:

• First finger: Field (north to south)
• Second finger: Current (positive to negative)
• Thumb: Motion (the direction of the force)

Reversing the current or the field reverses the force.

For a wire at right angles to a magnetic field, this equation is on the equation sheet:

force = magnetic flux density × current × length, or F = BIl

Magnetic flux density is a measure of the strength of the field.

Example: a 5 cm length of wire carries 2 A at right angles to a field of 0.3 T.

1. Convert: 5 cm = 0.05 m
2. F = BIl = 0.3 × 2 × 0.05
3. F = 0.03 N

A simple dc motor has a coil of wire on an axle between the poles of a magnet.

1. Current flows one way along one side of the coil and the other way along the opposite side.
2. So the forces on the two sides act in opposite directions: one side is pushed up, the other down.
3. This makes the coil rotate.
4. A split-ring commutator reverses the current in the coil every half turn, so the coil keeps turning the same way.

To make the motor turn faster, increase the current, use a stronger magnet, or add more turns to the coil.

Loudspeakers and headphones use the motor effect to turn changes in current into changes in air pressure: sound waves.

1. A coil of wire, fixed to a paper cone, sits around a permanent magnet.
2. An alternating current flows through the coil.
3. The force on the coil keeps changing direction, so the coil and cone move in and out.
4. The cone makes the air vibrate, producing sound waves.

The sound has the same frequency as the alternating current.