7.2.3 Electric motors (HT)
The electric motor
Definition
Electric motor
An electric motor is a device that uses the force on a current-carrying conductor in a magnetic field to produce rotation.
- A simple motor has a coil of wire between the poles of a magnet, with the field running from the north pole to the south pole across the coil.
- When current flows, the field exerts a force on each side of the coil.
- The two opposite sides carry current in opposite directions, so the forces on them are in opposite directions: one side is pushed up and the other down.
- Because the forces act at different positions and in opposite directions, they produce a turning effect that makes the coil rotate.
Key Idea
The coil rotates because the forces on its opposite sides act in opposite directions, forming a turning effect rather than pushing the whole coil one way.
Keeping the coil rotating
- A direct-current motor uses a split-ring commutator to reverse the current through the coil every half-turn.
- Brushes keep electrical contact between the power supply and the rotating split ring.
- Reversing the current also reverses the forces on the two sides, so the turning effect keeps acting the same way and the coil keeps rotating in the same direction instead of turning back.
Common Mistake
- Do not say only that the magnet pulls the coil around; the field exerts forces on the current-carrying sides of the coil.
- The two forces act in opposite directions at different positions, which is what produces the turning effect.
- Reversing the current every half-turn does not reverse the overall rotation; the commutator reverses it at the right time so rotation continues the same way.
Exam technique
- Give a linked chain: current in a coil, forces on opposite sides, forces in opposite directions, turning effect, rotation.
- For continuous rotation, state that the split-ring commutator reverses the current every half-turn, rather than just writing the motor effect makes it spin.
Self review
- What happens to a current-carrying coil placed in a magnetic field?
- Why are the forces on opposite sides of the coil in opposite directions?
- How do these forces produce a turning effect?
- What is the purpose of the split-ring commutator?
- How do the brushes help the rotating coil stay connected to the power supply?
