4.2.2 Motors

Question 1

Two parallel wires carry current. Suggest why there is a force on each wire.

Answer 1

When a current flows through a wire, a magnetic field is produced. The fields of the two wires interact therefore the wires experience a force. The force is repulsive if currents in wires flow in the same direction and attractive if they flow in opposite directions.

Question 2

The motor effect

Answer 2

When a current carrying wire in a magnetic field experiences a force.
F = magnetic flux density x current x length

Question 3

Electric motor - What is it? What causes the coil to rotate?

Answer 3

An electric motor uses the motor effect to transfer electrical energy to kinetic energy. An electric motor consists of a loop of wire, with a current, in a magnetic field. The wire will experience a force, given by F =BIL. As current flows in opposite directions in the opposite sides of the coil, forces act in opposite directions on opposite sides of the coil, causing the coil to rotate.

Question 4

What happens every half turn of the coil?

Answer 4

Every half turn of the coil, the two sides switch positions so if the current continues to run in the same direction the forces would act in the same way. The coil would then turn anti-clockwise and not clockwise. Therefore the current direction must be reversed, every half turn of the coil, to ensure the coil continues to turn in the same direction.

Question 5

What is a split ring commutator used for?

Answer 5

A split ring commutator is used to reverse the direction of the current in the coil every half turn of the coil. This reverses the direction of the forces on the sides of the coil every half turn, ensuring the coil continues to rotate in the same direction.
In order to increase the speed of the motor:
- increase the magnitude of the current flowing in the coil
- increase the strength of the magnetic field
- increase the number of coils of wire will
- increase the length of the coil