Electric motors

A coil of wire carrying a current in a magnetic field experiences a force that tends to make it rotate. This effect can be used to make an electric motor.

Explaining an electric motor

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

A simple electric motor. A metal coil sits in a magnetic field and is connected to a cell via a commutator (also called a split ring). When current is applied the coil rotates.Fleming’s left-hand rule can be used to explain why the coil turns

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

  • current in the left hand part of the coil causes a downward force, and current in the right hand part of the coil causes an upward force
  • the coil rotates anti-clockwise because of the forces described above

When the coil is vertical, it moves parallel to the magnetic field, producing no force. This would tend to make the motor come to a stop, but two features allow the coil to continue rotating:

  • the momentum of the motor carries it on round a little
  • a split ring commutator changes the current direction every half turn

Once the conducting brushes reconnect with the commutator after a half turn:

  • current flows in the opposite direction through the wire in the coil
  • each side of the coil is now near the opposite magnetic pole

This means that the motor effect forces continue to cause anti-clockwise rotation of the coil.

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