Text lesson

Unit 2 — Electricity and Magnetism

Science — Third Preparatory

3.4 The simple electric motor (DC motor)

If we put a coil instead of a straight wire between the poles of the magnet, the coil turns. Parts of a simple DC motor:

a coil of insulated copper wire, free to rotate between the poles of a strong U-shaped permanent magnet;
a commutator (split ring) a metal cylinder split into two halves insulated from each other, connected to the coil ends;
two graphite brushes that touch the commutator and carry current into and out of the coil; they are the contacts through which current enters and leaves;
a direct-current source (battery).
How it works.

When current flows in the coil, a magnetic force acts on each of the two long sides (AB and CD).
Because the currents in the two sides are in opposite directions, the forces are equal in magnitude but opposite in direction and act along different lines of action.
Two such forces form a couple, producing a turning effect called torque, which rotates the coil.
(An everyday example of a couple turning a steering wheel with both hands.) The split-ring commutator reverses the current in the coil every half turn, so that the torque always turns the coil the same way and the rotation continues.
Factors affecting the torque (and so the speed) of a motor:

increasing the current through the coil;
increasing the number of turns of the coil;
increasing the area of the coil;
increasing the magnetic field strength, by using stronger magnets or more magnets.

Direction of rotation can be reversed by (a) reversing the current direction (swap the cell connections) or (b) reversing the magnet’s poles (turn the magnet round).
Doing both together leaves the direction unchanged.
Energy conversion an electric motor converts electrical energy into mechanical (kinetic) energy.
Applications:

fans, blenders, washing machines, electric cars, the metro.
A drill/screwdriver has a speed control low speed for screws (accuracy and no damage to the screw head), high speed for drilling into walls (more power to penetrate hard material).

Unit 2 — Electricity and Magnetism · 18 / 25

3.4 The simple electric motor (DC motor)