Electricity and magnetism are closely linked. A current makes a magnetic field, and a changing magnetic field can make a current. These effects run motors, generators and transformers.
Electromagnetic induction
When a magnet moves near a coil (or a wire cuts magnetic field lines), an EMF is induced. The induced EMF is larger with a stronger magnet, more turns on the coil, and faster movement. This is how a generator produces electricity.
Key idea
Lenz's law: the induced current always opposes the change that causes it. An induced EMF needs a changing magnetic flux.
The motor effect
A current-carrying wire in a magnetic field feels a force. Fleming's left-hand rule gives the direction: thumb = force (motion), first finger = field, second finger = current. Reversing the current or the field reverses the force.
Transformers
A transformer changes an alternating voltage using two coils on an iron core. It works only with AC, because a steady DC current gives no changing flux.
Worked example
A transformer has Vp = 240 V, Np = 800 turns and Ns = 40 turns. Using Vp/Vs = Np/Ns: Vs = 240 × 40 / 800 = 12 V. Fewer secondary turns means a step-down transformer.
Remember
- Step-up: more turns on the secondary; step-down: fewer.
- For an ideal transformer, input power = output power.
- Increase an electromagnet's strength with more current, more turns, or a soft-iron core.