Physics / Unit 10: Magnetism and Modern Physics
10.4 Induction, Generators and Transformers
Notes
A changing magnetic field through a coil induces a voltage and current in it. This is electromagnetic induction (Faraday's law).
- Faster change, a stronger magnet or more turns of wire give a larger induced voltage. A magnet that is not moving induces nothing.
- A generator spins a coil in a magnetic field, changing mechanical energy into electrical energy. Power plants and wind turbines use generators.
- A transformer changes AC voltage: $\displaystyle \frac{V_s}{V_p} = \frac{N_s}{N_p}$. More secondary turns: step-up. Fewer: step-down. Transformers need AC.
- Example: 120 V on a 100-turn primary, 500-turn secondary: $\displaystyle V_s = 120 \times \frac{500}{100} = 600 \text{ V}$.
Practice
A magnet is held still inside a coil connected to a meter. What does the meter show?
Induction needs a changing magnetic field; a still magnet does not change.
A transformer has 2,000 turns on its primary and 100 turns on its secondary. The primary voltage is 240 V. What is the secondary voltage?
$\displaystyle V_s = 240 \times \frac{100}{2000} = 12 \text{ V}$ (step-down).
Which device changes mechanical energy into electrical energy?
A generator uses induction from a spinning coil and magnetic field.
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