Electromagnetic induction

Moving a wire through a magnetic field, or changing the field through a coil, induces a voltage. Generators and transformers depend on this.

Key facts

  • A voltage is induced when the magnetic field through a coil changes.
  • Faster movement, stronger magnet or more turns give a bigger voltage.
  • Transformers: Vs/Vp = Ns/Np; step-up raises the voltage, step-down lowers it.
  • For an ideal transformer VpIp = VsIs.
  • The National Grid uses high voltage to reduce energy lost as heat.

Worked examples

Example 1

A transformer steps 230 V down to 12 V. The primary coil has 2300 turns. How many turns on the secondary?

  1. Ns = 2300 × 12 ÷ 230 = 120

Answer: 120