Magnetism: Synthesis and Exam Skills

You have now met every idea in Topic P4 — from the field of a simple bar magnet to the transformers that power the National Grid. This final lesson draws those threads together. The big themes of P4 are really two mirror-image effects: the motor effect (current + field → force/movement) and the generator effect (movement + field → induced voltage). Almost every device in the topic is one or other of these. This lesson, part of Topic P4 (Magnetism and magnetic fields) of OCR Gateway Science A, brings the motor and generator effects side by side, pulls together the Higher-tier equations $F = BIL$ and the transformer relations, picks apart the most common exam mistakes, and walks through a worked multi-step problem of the kind that ties several ideas together.

By the end of this lesson you should be able to compare the motor and generator effects and the devices based on each, select and use the correct P4 equation, avoid the classic P4 exam mistakes, and work through a multi-step magnetism problem with confidence.

The Two Big Effects, Side by Side

The whole of P4 hangs on two opposite effects. Getting them clearly separated in your mind is the single most valuable exam skill in this topic.

	Motor effect	Generator effect
What goes in	Current in a wire + a magnetic field	Movement of a wire (or magnet) + a magnetic field
What comes out	A force / movement	An induced potential difference (and current)
Fleming's rule	Left hand	Right hand
Devices	Electric motor, loudspeaker	Generator/dynamo, microphone, transformer
Higher equation	$F = BIL$	(transformer) $\dfrac{V_p}{V_s} = \dfrac{N_p}{N_s}$

Notice the pleasing symmetry: a motor and a generator are built from almost identical parts (a coil, magnets, a commutator or slip rings), but run in opposite directions — put electricity in and a motor gives you movement; put movement in and a generator gives you electricity. The same pairing links the loudspeaker (motor effect: electricity → sound) and the microphone (generator effect: sound → electricity).

graph LR
  E[Electrical energy] -->|motor effect| M[Movement]
  M -->|generator effect| E

Exam Tip: Decide first which effect a question is about. Current in, force out → motor effect → left hand. Movement in, voltage out → generator effect → right hand. Choosing the right effect (and the right hand) before anything else stops most P4 mistakes.

Choosing the Right Equation

P4 has a small set of equations, and Higher-tier questions reward picking the correct one quickly. Identify what you are given and what you are asked for.

If the question is about…	…use	Tier
Force on a current-carrying wire	$F = BIL$	Higher
Transformer voltages and turns	$\dfrac{V_p}{V_s} = \dfrac{N_p}{N_s}$	Higher
An ideal transformer's currents	$V_p I_p = V_s I_s$	Higher

Two reminders that protect easy marks:

Units first. In $F = BIL$ , the length must be in metres and the current in amperes to give a force in newtons. Convert centimetres to metres before substituting.
Keep primary and secondary together. In the transformer equation, all the primary quantities belong on one side of the ratio and the secondary on the other — don't mix a primary voltage with a secondary number of turns.

Exam Tip: Write down the equation first, then substitute, then give the answer with a unit. Examiners award method marks for the equation and substitution even if the final arithmetic slips — so never jump straight to a number.

A Worked Multi-Step Problem (Higher)

Many of the best exam questions combine two ideas. Here is a worked example that uses both transformer equations together.

A power station produces $25 \text{ kW}$ of electrical power. A step-up transformer raises the voltage from $5\,000 \text{ V}$ to $100\,000 \text{ V}$ for transmission. The transformer can be treated as ideal.

(a) Calculate the current in the primary coil. (b) Calculate the current in the secondary (transmission) cable. (c) The transformer's primary coil has $400$ turns. Calculate the number of turns on the secondary coil. (Higher)

Part (a) — primary current. The power delivered to the primary is $P = V_p I_p$ , so:

$I_p = \dfrac{P}{V_p} = \dfrac{25\,000}{5\,000} = 5.0 \text{ A}$

(Note $25 \text{ kW} = 25\,000 \text{ W}$ .)

Part (b) — secondary current. For an ideal transformer, $V_p I_p = V_s I_s$ , so:

$I_s = \dfrac{V_p I_p}{V_s} = \dfrac{5\,000 \times 5.0}{100\,000} = \dfrac{25\,000}{100\,000} = 0.25 \text{ A}$

The voltage was stepped up $\times 20$ (from $5\,000$ to $100\,000 \text{ V}$ ), so the current was stepped down $\times 20$ (from $5.0$ to $0.25 \text{ A}$ ) — exactly as expected, keeping the power at $25 \text{ kW}$ .

Part (c) — secondary turns. Using $\dfrac{V_p}{V_s} = \dfrac{N_p}{N_s}$ , rearranged for $N_s$ :

$N_s = N_p \times \dfrac{V_s}{V_p} = 400 \times \dfrac{100\,000}{5\,000} = 400 \times 20 = 8\,000 \text{ turns}$

Answers: (a) $5.0 \text{ A}$ ; (b) $0.25 \text{ A}$ ; (c) $8\,000$ turns. The low transmission current of $0.25 \text{ A}$ is exactly why the grid steps the voltage up — it keeps the $I^2 R$ heating loss in the cables tiny.

Exam Tip: In a multi-step problem, carry your answers forward: the current you find in part (a) feeds straight into part (b). Always sanity-check with the step-up/step-down logic — if the voltage went up $\times 20$ , the current must come down $\times 20$ .

Common P4 Exam Mistakes

The topic has a handful of classic traps. Knowing them in advance is worth several marks.

Wrong hand. Using the left hand for a generator question (or the right hand for a motor question). Decide the effect first.
Field-line errors. Drawing a bar magnet's field with no arrows, with arrows pointing S→N outside, or with lines that cross. Lines run N→S outside, never cross, and bunch at the poles.
Soft vs hard iron. Saying an electromagnet uses a steel core. It uses soft iron so it loses magnetism when switched off; steel would stay magnetic.
Slip rings vs commutator. Swapping them over — slip rings → a.c. generator, split-ring commutator → d.c. dynamo (and motor).
d.c. on a transformer. Forgetting that a transformer needs a.c.; on d.c. the field doesn't change and the output is zero.
Forgetting "no change, no induction". Claiming a current is induced in a stationary wire or for a magnet held still in a coil. Induction needs a changing field.
Unit slips in $F = BIL$ . Leaving the length in centimetres instead of metres, giving an answer 100× too big.
Motor/generator muddle for sound devices. Saying a loudspeaker uses the generator effect or a microphone the motor effect — it is the other way round.

Exam Tip: Before writing a field diagram, mentally tick the three rules — arrows N→S outside, never cross, closest at the poles. Before a calculation, tick the units. These two habits remove the most common P4 errors.

A Revision Checklist for P4

When you revise this topic, a short checklist catches the points examiners return to year after year. Try to answer each from memory:

Magnetism: Synthesis and Exam Skills

Magnetism: Synthesis and Exam Skills

The Two Big Effects, Side by Side

Choosing the Right Equation

A Worked Multi-Step Problem (Higher)

Common P4 Exam Mistakes

A Revision Checklist for P4

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