Cognito · AQA Physics 2026/27Motor effect and generators
Key facts
Magnetic fields
Magnetic field lines go from north to south pole outside the magnet.
Closer field lines = stronger field.
Earth has a magnetic field — used for navigation.
Electromagnet — current through a coil of wire creates a magnetic field. Strength increased by: more turns, larger current, iron core.
Uses: electric motors, loudspeakers, circuit breakers.
Motor effect (F = BIL)
A current-carrying conductor in a magnetic field experiences a force — the motor effect.
F = BIL (force = magnetic flux density × current × length). Units: N.
Fleming's Left Hand Rule: thumb = Force (motion), index = Field, middle = Current.
Force is maximum when current is perpendicular to field. Zero when parallel.
Reversing current OR field reverses direction of force.
Electric generators (induction)
Electromagnetic induction — moving a conductor through a magnetic field induces a voltage.
Generator: rotating coil in magnetic field → changing flux → induced EMF → current.
Increase induced EMF: faster rotation, stronger magnet, more turns on coil.
Simple generator with slip rings → AC output. With split-ring commutator → DC output.
Transformers
Only work with AC (changing field needed for induction).
Vp/Vs = np/ns.
Vp × Ip = Vs × Is (ideal transformer — 100% efficient).
Step-up: ns > np → Vs > Vp. Step-down: ns < np → Vs < Vp.
Real transformers lose energy as heat in the core (eddy currents) and resistance of wire.
Exam questions — 4 questions · 10 marks
⚡ Extended questions included. Show all working for calculation questions. The AI marks against the real AQA mark scheme.
1 markFleming's left hand ruleAQA 8463 P2 style
A horizontal wire carrying current to the right is placed in a magnetic field pointing upward. Using Fleming's Left Hand Rule, in which direction does the force on the wire act?
A Upward
B Downward
C Out of the page (towards you)
D Into the page
4 marksMotor effect calculationAQA 8463 P2 style
A wire of length 0.3 m carries a current of 4 A in a magnetic field of flux density 0.5 T. Calculate the force on the wire. Explain what happens to the force if the current is doubled. (4 marks)
Hint: F = BIL. For the second part, F ∝ I so doubling I doubles F.
+40 XP
4 marksGenerator vs motorAQA 8463 P2 style
Explain the difference between how an electric motor and an electric generator work. What is the energy transformation in each? (4 marks)
Hint: Motor: electricity IN → movement OUT (motor effect). Generator: movement IN → electricity OUT (induction). Both have coil + magnetic field.
+40 XP
1 markWhy transformers only work with ACAQA 8463 P2 style
Why do transformers only work with alternating current and not direct current?
A DC current is too weak to produce a magnetic field in the core
B AC current constantly changes direction, producing a changing magnetic field in the primary coil that induces a voltage in the secondary coil — DC produces a constant field that induces nothing
C DC causes the transformer to overheat and melt the coil
D AC has a higher voltage than DC so it can overcome the resistance of the coil