Electromagnetic effects
Electromagnetic induction, the a.c. generator, transformers, the magnetic effect of a current, and the motor effect.
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Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 4.5.1Describe an experiment to demonstrate electromagnetic induction and state the factors affecting the size of the induced e.m.f.
- 4.5.2Describe the construction and action of a simple a.c. generator.
- 4.5.3Describe the structure and use of a transformer and recall and use Vp/Vs = Np/Ns.
- 4.5.4Describe the force on a current-carrying conductor in a magnetic field and use Fleming's left-hand rule.
9 minute read
Induction, generators and transformers
Electromagnetic induction
When a conductor cuts through magnetic field lines — or the field through a coil changes — an e.m.f. is induced across the conductor. If the circuit is complete, a current flows.
The classic demonstration: move a bar magnet into a coil connected to a sensitive galvanometer. The needle deflects while the magnet moves and returns to zero when it stops. Reverse the motion and the deflection reverses.
The induced e.m.f. is larger if you increase:
- the speed of movement
- the number of turns on the coil
- the strength of the magnetic field
Lenz's law in one sentence: the induced effect always opposes the change causing it. That is why a generator gets harder to turn when you draw more current from it — and it is a direct consequence of conservation of energy.
The a.c. generator
A coil rotates in a magnetic field. As it turns, the rate at which it cuts field lines changes, so the induced e.m.f. varies sinusoidally. Slip rings and brushes connect the rotating coil to the external circuit, and because they never swap contacts, the output alternates.
Peak e.m.f. occurs when the coil is parallel to the field (cutting field lines fastest), and the output is zero when the coil is perpendicular to the field.
The transformer
A transformer has a primary coil and a secondary coil wound on a soft-iron core.
- Alternating current in the primary produces a changing magnetic field.
- The soft-iron core carries this changing field to the secondary coil.
- The changing field induces an alternating e.m.f. in the secondary.
The turns-ratio relationship is Vp / Vs = Np / Ns. More turns on the secondary means a step-up transformer.
For an ideal (100% efficient) transformer, power in equals power out, so VpIp = VsIs. Stepping voltage up therefore steps current down.
Why the grid uses high voltage
Power lost as heat in transmission cables is P = I²R. Because the loss depends on the square of the current, transmitting at high voltage and low current dramatically reduces the energy wasted.
The motor effect
A current-carrying wire in a magnetic field experiences a force. Use Fleming's left-hand rule: first finger = field (N to S), second finger = current (conventional, + to −), thumb = motion (force). This is the basis of the d.c. motor and the loudspeaker.
A transformer only works with a.c. Connect it to a d.c. supply and the field is steady, nothing changes, and no e.m.f. is induced in the secondary.
Think of it like this
Induction is like a bicycle dynamo: the harder you pedal (the faster the field changes), the brighter the lamp — and the more the dynamo resists your pedalling.
Worked examples
Method, step by step
A transformer has 200 turns on the primary and 1200 turns on the secondary. The primary voltage is 12 V. Calculate the secondary voltage.
- 1Vp / Vs = Np / Ns, so Vs = Vp × (Ns / Np)
- 2Vs = 12 × (1200 / 200)
- 3Vs = 12 × 6
Vs = 72 V (a step-up transformer)
An ideal transformer steps 240 V down to 12 V. The output current is 2.0 A. Calculate the input current.
- 1For an ideal transformer, VpIp = VsIs
- 2240 × Ip = 12 × 2.0 = 24
- 3Ip = 24 / 240
Ip = 0.10 A
Common misconceptions
- Thinking a transformer works on d.c. It needs a *changing* magnetic field, so it requires a.c.
- Saying current is induced. Strictly, an e.m.f. is induced; current only flows if the circuit is complete.
- Using the left-hand rule for generators. Left hand is for the motor effect; the generator effect uses the right hand.
In the exam
- Transformer questions almost always want the three-step chain: alternating current → changing magnetic field → induced e.m.f. in the secondary. Write all three.
- For "why is electricity transmitted at high voltage", the answer must mention that lower current reduces heating losses because P = I²R.
- When asked how to increase an induced e.m.f., give the standard three: move faster, more turns, stronger magnet.