ZIMSEC O Level · Physics · Form 4
Magnets
Like poles repel; unlike poles attract. Field lines go from north to south. Iron is easily magnetised and demagnetised (soft); steel keeps its magnetism (hard), so it is used for permanent magnets.
Electromagnetism
A current produces a magnetic field (right-hand grip rule for a coil). An electromagnet is a coil with a soft-iron core; its strength increases with more turns and more current. Uses: relays, electric bells, cranes.
The motor effect
A current-carrying wire in a magnetic field feels a force (Fleming's left-hand rule: First finger – Field, seCond finger – Current, thuMb – Motion). This is how electric motors work.
Electromagnetic induction
Moving a wire or magnet so that a conductor cuts magnetic field lines induces a voltage. Used in generators (AC dynamos) at power stations.
Transformers
Change AC voltage. Vp/Vs = Np/Ns. Step-up transformers raise the voltage for transmission on the national grid (less current, less energy lost as heat); step-down transformers lower it for homes.
Worked example: A transformer has 2 000 primary turns and 100 secondary turns. The input is 240 V. Find the output.
Vs = 240 × 100 ÷ 2 000 = 12 V
Check yourself
- Name two ways of making an electromagnet stronger.
- Why is electricity transmitted at high voltage?
- A transformer steps 11 000 V down to 220 V. Find the turns ratio Np : Ns.
Show answers
- More turns on the coil; larger current (or a soft-iron core).
- The current is smaller, so less energy is wasted as heat in the cables.
- 50 : 1
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