Topic 7 of 8 · Form 4
Magnetism and electromagnetism
Magnetic poles, induced magnetism, magnetic field lines (N to S), temporary (soft iron) and permanent (steel) magnets.
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Magnetism
Magnetic poles, induced magnetism, magnetic field lines (N to S), temporary (soft iron) and permanent (steel) magnets.
Electromagnetism
- Magnetic field around a wire and solenoid; right-hand grip rule.
- Electromagnets and relays.
- The motor effect: force on a current-carrying conductor in a magnetic field (Fleming's left-hand rule); d.c. motors with split-ring commutators.
Electromagnetic induction
An e.m.f. is induced when a conductor cuts magnetic field lines or the field through a coil changes. The size increases with speed, field strength and number of turns. Lenz's law: the induced current opposes the change causing it. Uses: a.c. generators.
Transformers
Vp/Vs = Np/Ns; for an ideal transformer, VpIp = VsIs. High-voltage transmission reduces current and therefore I²R power losses in cables.
Worked example: A transformer steps 240 V down to 12 V. The primary has 1 200 turns. Find the secondary turns and the primary current if the secondary current is 2 A.
Ns = 1 200 × 12 ÷ 240 = 60 turns; Ip = 12 × 2 ÷ 240 = 0.1 A
Check yourself
- State Fleming's left-hand rule.
- Name two ways of increasing the induced e.m.f. in a generator.
- Why is electricity transmitted at high voltage?
Show answers
- First finger – field, second finger – current, thumb – motion (force).
- Any two: rotate faster, stronger magnet, more turns, larger coil area.
- Lower current reduces heating losses (I²R) in the cables.
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