The motor effect - CCEA (Higher tier only)

• A current-carrying conductor (e.g. a wire) in a magnetic field will experience a force.

• The force is perpendicular to the direction of both the current and the magnetic field.

• Fleming’s Left Hand Rule can be used to determine the direction of the force, current or magnetic field.

• This force forms the basis of the electric motor.

The motor effect turns electrical energy into kinetic energy, so electricity can be used to create movement.

The motor effect is due to a force on a current carrying conductor (i.e. a wire) when it is in a magnetic field.

A current carrying conductor creates its own magnetic field around the conductor.

A magnet also has a magnetic field around it.

If these two magnetic fields interact then they will push or pull on each other.

The result is that the current carrying conductor is forced to move.

When a current carrying conductor is placed within a magnetic field (i.e. between the poles of a magnet) it will experience a force.

It is possible to determine the direction of this force using Fleming’s left hand rule.

Key points

• 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 size of the force is increased if:

• The current in the wire increases.

• The strength of the magnetic field increases.

• The length of wire inside the magnetic field is increased

The force is always greatest when the direction of the current is 90° to the direction of the magnetic field.

The direction of the force is always at 90° (or perpendicular) to both the direction of the current and 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 pointed in the direction of the magnetic field lines - from north to south.

• The second finger is pointed in the direction the current flows.

• The thumb shows the direction of the force on the conductor carrying the current.

Note: it is important to point the second finger in the direction of conventional current flow, i.e. from positive to negative.

In which direction will this wire feel a force?

With forefinger (magnetic field) pointing from N to S (i.e. 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.

Electric motors are found in many household devices e.g. tumble dryers, washing machines, vacuum cleaners, electric knives, food mixers, hair dryers and electric toothbrushes.

A coil of wire carrying a current in a magnetic field experiences a force that tends to make it rotate.

This effect can be utilised in an electric motor.

The diagram shows a simple motor using direct current (DC).

The rectangular loop of wire lies between the poles of a magnet.

The current flows in opposite directions along the two sides of the loop.

Using Fleming’s left-hand rule on this diagram shows that the right hand side of the loop is pushed up and the left hand side is pushed down.

This causes a turning effect on the loop.

If the number of loops is increased to form a coil, the turning effect is greatly increased.

This is the principle involved in electric motors.

Starting from the position shown in the diagram of the dc motor:

• Current in the left-hand side of the coil causes a downward force, and current in the right-hand side of the coil causes an upward force.

• The coil rotates anticlockwise because the forces are in opposite directions.

• Each side of the coil is now near the opposite magnetic pole.

The direction of rotation of the coil can be reversed by:

• Reversing the direction of the current, or

• reversing the direction of the magnetic field (changing over the north and south poles).

The speed of rotation of the coil can be increased by:

• Increasing the size of the current.

• Using a stronger magnet.

• Increasing the number of turns of wire in the coil.

• Reducing friction between the coil and the axel it rotates on.

Question

1. A wire carrying a current at right angles to a magnetic field experiences a force. List 3 ways of increasing the size of the force.

2. An electric current flows in a loop of wire in the direction shown in the diagram below. The loop is between the poles of a magnet.

Complete the table below to show which sections of the loop experience a force.

3. What electrical device is based on this arrangement of coil and magnet?

4. What would happen if the north and south poles were reversed?

Answer

1. Three ways of increasing the size of the force are:

   • Increasing the size of the current.

   • Using a stronger magnet.

   • Increasing the length of wire inside the magnetic field.

2. Completed table:

3. An electric motor.

4. The coil would rotate in the opposite direction but at the same speed.