P7 Magnetism and electromagnetism — Section Overview

Magnetism and electromagnetism link two forces that are fundamentally the same — the electromagnetic force. This section covers permanent magnets and fields, the motor effect, electromagnetic induction, and how generators and transformers work.

What this section covers

Sub-topic	Key ideas
P7.1 Permanent magnets	Poles; attraction/repulsion; field lines (N→S externally)
P7.2 Earth's magnetic field	Compass alignment; plotting fields
P7.3 The motor effect	F = BIl; Fleming's left-hand rule; DC motor; loudspeaker
P7.4 Induced potential	Generator effect; Faraday's law; alternator and dynamo; microphone
P7.5 Transformers	Step-up/step-down; turns ratio; power equation; National Grid

Key principles

Permanent magnets attract magnetic materials (iron, steel, nickel, cobalt) and exert forces on other magnets. Field lines run from N to S externally; they show direction the N-pole of a compass points. Strength shown by density of field lines.

Motor effect (F = BIl): A current-carrying conductor in a magnetic field experiences a force. Direction given by Fleming's left-hand rule (thumb = thrust, index = field, middle = current). DC motor uses a split-ring commutator to reverse current direction every half-turn.

Electromagnetic induction: Moving a conductor through a field (or changing flux through a coil) induces a potential difference. Lenz's Law: induced current direction opposes the change causing it.

Transformers (ideal): V_p/V_s = n_p/n_s = I_s/I_p and V_p × I_p = V_s × I_s. Step-up: more turns on secondary → higher voltage, lower current. The National Grid uses step-up to 400 kV to reduce current and energy loss in cables (P_loss = I²R).

Exam focus

• Learn Fleming's left-hand rule — practise physical gesture.
• For transformers: "step-up" = higher voltage on secondary; more secondary turns.
• Show P = IV clearly when doing Grid calculations.