O-Level and SEC G3 Physics K323
P19: Electromagnetic Induction
Identify changing flux, induced current direction, generator action, and transformer relationships.
Reviewed for the 2027 cohort on 19 July 2026. The official syllabus remains authoritative for assessable wording and paper details.
Core notes
Electromagnetic induction requires a changing magnetic field through a circuit. K323 applies this to direction, induced e.m.f. magnitude, a.c. generators, ideal transformers and high-voltage transmission.
Induction and Lenz response
An e.m.f. is induced when magnetic flux through a circuit changes. The change can come from relative motion, changing field strength, changing coil area or orientation, or changing current in a nearby circuit.
The induced direction opposes the change producing it. This Lenz response is consistent with energy conservation. A faster change, stronger field or more coil turns generally produces a larger induced e.m.f.
A.C. generators
A simple generator rotates a coil in a magnetic field or rotates a magnet relative to a coil. The changing flux induces an alternating e.m.f. Slip rings and brushes connect a rotating coil to the external circuit without reversing the output connection every half-turn.
For steady rotation in a uniform field, the voltage-time graph alternates positive and negative. Faster rotation increases frequency and usually peak e.m.f.; a stronger field or more turns increases peak e.m.f.
Transformers and transmission
A transformer uses alternating current in a primary coil to create changing magnetic flux in an iron core, inducing e.m.f. in a secondary coil. It does not operate continuously from steady d.c. because the required flux change is absent after switching.
For an ideal transformer, input power equals output power. Transmitting the same power at higher voltage uses lower current, reducing cable heating because power loss depends strongly on current. A step-up transformer raises transmission voltage and a step-down transformer reduces it near users.
Formulae and relationships
| Relationship | Use |
|---|---|
| Relate ideal transformer voltages and turns. | |
| Apply ideal transformer power conservation. | |
| Explain cable heating loss. |
Worked examples
Example 1: An ideal transformer has primary turns, secondary turns and primary voltage. Find secondary voltage.
- Use .
- So .
- Therefore .
Answer: , so it is a step-down transformer.
Chapter checkpoint
Use these three moves to organise the topic before attempting a mixed or practical question.
- Identify a changing magnetic flux as the condition for induction.
- Predict how speed, field strength, turns, and direction affect induced e.m.f. and current.
- Connect generators and transformers to induction with input, output, and energy losses distinguished.
Official outcome coverage
K323 P19: 6 mapped outcomes, references P19(a), P19(b), P19(c), P19(d), P19(e), P19(f). Check the official K323 syllabus.
The outcome wording is not reproduced here. The relevant official syllabus remains authoritative for exact assessable scope.
Practical and data connection
Move a magnet or coil systematically and use meter deflection to compare induced effects while controlling direction and speed.
Exam traps and retrieval check
Avoid these traps
- Saying any magnetic field induces e.m.f. without requiring a change.
- Using split rings in the simple a.c. generator description instead of slip rings.
- Claiming high voltage itself reduces power transmitted rather than reducing current for the same power.
Check from memory
What is the essential condition for induction?
Changing magnetic flux through a circuit.
Why does a transformer require a.c.?
A.c. produces changing magnetic flux in the core.
Why transmit at high voltage?
For the same power, current is lower and cable heating loss is reduced.
Pure versus Combined scope
This is a standalone Pure Physics K323 topic. It has no matching top-level Combined Physics owner in K326 or K327, so Combined students should not add it unless their current syllabus or school scope explicitly requires it.
Shared explanation source
Eclat has a related explanation in its existing IP library. It can help with the shared concept, but its IP extensions and school-sensitive scope are not automatically part of K323. Open the related IP explanation.