Electromagnetic Effects: Induction, the Motor Effect and Transformers

Electromagnetic effects covers two interactions between a wire and a magnetic field: induction (movement makes electricity) and the motor effect (electricity makes movement). Motors, generators and transformers regularly fill 8-10 marks on Paper 4, making this the heaviest-weighted Extended section in the electricity topic. The ideas are physical, not mathematical.

What are the two effects you must keep separate?

Electromagnetic induction (movement makes electricity). An e.m.f. is induced across a conductor when it moves through a magnetic field, or when the field through a circuit changes. Faster movement, a stronger magnet or more turns on a coil each increase the induced e.m.f. Reversing the motion or the field reverses it. This is how generators work.

The motor effect (electricity makes movement). A current-carrying wire in a magnetic field experiences a force. Fleming’s left-hand rule gives the direction: thumb = force (motion), first finger = field (N to S), second finger = conventional current. Reversing either the current or the field reverses the force. A d.c. motor uses this on a coil; the split-ring commutator reverses the current every half turn so the coil keeps spinning one way. Increase the turning effect with more current, a stronger field or more turns.

A plain current-carrying wire also creates its own magnetic field: circular field lines around a straight wire, a bar-magnet-like pattern around a solenoid. That is the basis of electromagnets and relays.

How does a transformer work and what is the equation?

A transformer changes the size of an alternating voltage. Alternating current in the primary coil creates a changing magnetic field in the soft-iron core. The changing field passes through the secondary coil and induces an alternating e.m.f. in it. Transformers need a.c.: a steady d.c. input induces nothing.

In words: primary voltage over secondary voltage equals primary turns over secondary turns.

Equation	Symbols	Applies
Transformer equation	$\dfrac{V_p}{V_s} = \dfrac{N_p}{N_s}$	Extended
Ideal transformer power	$V_p \times I_p = V_s \times I_s$ (100% efficient)	Extended

Step-up transformers ($N_s > N_p$) raise voltage for transmission. Step-down transformers lower it for homes. Transmitting at high voltage means low current for the same power, and lower current cuts the $I^2 R$ heating loss in the cables.

Worked Exam Question

A transformer steps 240 V mains down for a 12 V garden lamp. The primary coil has 800 turns. (a) Calculate the number of turns on the secondary coil. (3 marks) (b) Explain why this transformer cannot work from a battery. (2 marks)

Solution. (a) Equation: $\dfrac{V_p}{V_s} = \dfrac{N_p}{N_s}$. Rearrange: $N_s = \dfrac{N_p \times V_s}{V_p}$. Substitute: $N_s = \dfrac{800 \times 12}{240}$. Answer: $N_s = 40$ turns. Sense check: voltage drops 20×, so turns drop 20×. ✓ (b) A battery supplies d.c., which gives a constant magnetic field in the core. With no changing field through the secondary coil, no e.m.f. is induced.

Mark scheme:

• M1: transformer equation stated.
• M1: correct rearrangement and substitution.
• A1: 40 turns.
• B1: d.c. gives a constant / unchanging field.
• B1: induction needs a changing field, so no secondary e.m.f.

Common Mistakes

• Inverting the transformer ratio. Fix: keep primary on top throughout, $\dfrac{V_p}{V_s} = \dfrac{N_p}{N_s}$, and sense-check that step-down means fewer secondary turns.
• Using the right hand for the motor effect. Fix: left hand for the motor effect. Check current direction is conventional (+ to −).
• Saying transformers “create” energy. Fix: an ideal transformer conserves power; stepping voltage up steps current down.
• Forgetting why the commutator exists. Fix: it reverses the coil current every half turn to keep rotation continuous.
• “High voltage transmission because it travels faster.” Fix: high voltage → low current → less $I^2 R$ heating loss in cables.

Exam Technique Tip

Six-mark transformer or transmission questions reward a numbered causal chain. For transmission: state the power, link high voltage to low current via $P = IV$, link low current to less heating in cables, conclude less energy wasted. Four ordered steps with the two equations named typically secure 5-6 of the 6 marks. Vague “more efficient” statements without the current link score 2 at best.

How This Is Examined

A CS subtopic with the sharpest Core/Extended split in the topic. Core candidates (Papers 1 and 3) describe induction qualitatively, recognise the motor effect and know what transformers do. Extended candidates (Papers 2 and 4) apply Fleming’s left-hand rule, use the transformer equation and the ideal-power relation, and explain high-voltage transmission. Malaysia’s own grid transmits at up to 500 kV, a usable example. Paper 4 often ends with a 6-mark extended response here. Practical papers rarely touch this subtopic, so prioritise theory-paper drills.