Magnetism is due to the magnetic fields around magnets. The fields can be investigated by looking at the effects of the forces they exert on other magnets and magnetic materials.

A wire carrying a current creates a magnetic field. This can interact with another magnetic field, causing a force that pushes the wire at right angles. This is called the motor effect.

The force on a given length of wire in a magnetic field increases when:

- the current in the wire increases
- the strength of the magnetic field increases
- the length of conductor in the field is increased

For any given combination of current and magnetic field strength, the force is greatest when the direction of the current is 90° to the direction of the magnetic field. There is no motor effect force if the current and magnetic field are parallel to each other.

The direction of a motor effect force can be found using Fleming's left hand rule.

Hold your thumb, forefinger and second finger at right angles to each other:

- the forefinger is lined up with magnetic field lines pointing from north to south
- the second finger is lined up with the current pointing from positive to negative
- the thumb shows the direction of the motor effect force on the conductor carrying the current

In which direction will this wire feel a force?

With forefinger (magnetic field) pointing left to right, and second finger (current) pointing down, your left thumb (force) will point towards you. This is the direction in which the force acts.

Note that the direction of the force can be changed by changing either the direction of the current or the field.

To calculate the force on a wire carrying a current at right angles to a magnetic field, use the equation:

force = magnetic flux density × current × length

This is when:

- is force in newtons (N)
- is magnetic flux density (magnetic field strength) in tesla (T)
- is current in amperes – also referred to as amps – (A)
- is length in metres (m)

2 A flows through a 50 cm wire. Calculate the force acting on the wire when it is placed at right angles in a 0.4 T magnetic field.

50 cm = 50 ÷ 100 = 0.5 m

= 0.4 × 2 × 0.5

force = 0.4 N

- Question
A 5.0 cm wire carries a current of 0.75 A. Calculate the force acting on the wire when it is placed at right angles in a 0.60 T magnetic field.

5.0 cm = 5 ÷ 100 = 0.05 m

= 0.60 × 0.75 × 0.05

force = 0.0225 N