IGCSE Physics revision · Electricity & magnetism
Electromagnetic effects
The hardest IGCSE topic by student vote, and the one my former students most often mention when they talk about things finally clicking. It splits cleanly in two: magnets making currents (induction), and currents feeling forces (the motor effect). Decide which half the question is in before writing anything.
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What the syllabus demands
- —Describe electromagnetic induction: an e.m.f. is induced when a conductor cuts field lines or the field through a circuit changes
- —State the factors affecting the size of an induced e.m.f.
- —Describe the a.c. generator with slip rings
- —Describe the motor effect and use Fleming's left-hand rule
- —Explain the turning effect on a current-carrying coil — the d.c. motor with split-ring commutator
- —Describe the transformer: Vp/Vs = Np/Ns, and why high voltage is used for transmission
Definitions that earn marks
Clear definitions to practise — check your course mark scheme
- Electromagnetic induction
- An e.m.f. is induced across a conductor when it moves relative to a magnetic field and cuts field lines, or when the magnetic field through it changes.
- Motor effect
- A current-carrying conductor in a magnetic field experiences a force, provided the current is not parallel to the field.
- Transformer
- Two coils wound on a soft-iron core: an alternating current in the primary produces a changing magnetic field in the core, which induces an alternating e.m.f. in the secondary.
The equations
More equations to practise: the IGCSE formula sheet.
Where the marks die
Common mistakes to check
- 01
Saying a transformer works on d.c. It cannot: the field must be changing to induce an e.m.f. in the secondary. Only a.c. produces a continuously changing field — this is the most common transformer mark lost.
- 02
Fleming's left-hand rule with the wrong hand or the wrong finger assignments: First finger = Field, seCond = Current, thuMb = Motion. Left hand for motors; the right hand belongs to generators.
- 03
Induction answers without 'cutting field lines' or 'changing field'. Moving parallel to the field induces nothing — the conductor must cut lines.
- 04
Mixing the motor and generator hardware: a d.c. motor uses a split-ring commutator (reverses current every half turn to keep it spinning one way); an a.c. generator uses slip rings.
- 05
Transmission answers without the chain: high voltage → same power at lower current → less energy lost as heat in the cables (P = I²R).
One worked example, done properly
Question
A transformer has 400 turns on the primary and 20 turns on the secondary. The primary is connected to a 240 V a.c. supply. Calculate the secondary voltage.
Method
- 1.Vp/Vs = Np/Ns, so 240 ÷ Vs = 400 ÷ 20 = 20.
- 2.Vs = 240 ÷ 20.
Vs = 12 V (a step-down transformer)