Motor effect: F = BIL and the left-hand rule
Motor effect: a current-carrying conductor in a magnetic field feels a force — greatest when current is perpendicular to the field, zero when parallel. Direction from Fleming's left-hand rule: thumb = force, first finger = field (N→S), second finger = conventional current. Magnitude ( in T, in A, in m); doubling any one doubles . A d.c. motor uses a split-ring commutator to reverse the coil current each half-turn, so it turns one way.
(E) Induction needs changing flux
An e.m.f. is induced whenever the magnetic flux (linkage) through a coil changes — not only when a conductor moves through a field. Two routes: (1) relative motion of magnet and coil; (2) a changing current in a nearby coil, with no motion. Faraday (qualitative): induced e.m.f. ∝ the rate of change of flux linkage — move faster, use a stronger magnet, or add turns. Lenz's law: the induced current opposes the change causing it (energy conservation).
(E) Transformers: V1/V2 = N1/N2
Two coils on a soft-iron core. a.c. in the primary makes a changing flux in the core that induces an alternating e.m.f. in the secondary; d.c. gives steady flux and no e.m.f., so a transformer needs a.c.. Voltage: . Ideal (power conserved): , so a step-up transformer raises the p.d. and lowers the current. The grid steps p.d. up for transmission (low current → less heating), then steps it down for consumers.
Drawn from real examiner reports.
Use the LEFT hand for the motor effect
Use the LEFT hand for the motor effect (force on a current) and the right hand for the generator effect (induced current). Swapping hands, or swapping the field and current fingers, reverses the predicted direction. Fleming's left hand: thumb = force/motion, first finger = field (N→S), second finger = conventional current, held mutually perpendicular.
June 2023 Paper 22 (Extended): "the direction of the force on an electron in a magnetic field was not well understood."
(E) Faster motion → larger e.m.f.
By Faraday's law the induced e.m.f. ∝ the rate of change of flux linkage, so moving the magnet faster changes the flux more rapidly → a larger induced e.m.f. and larger current. Believing speed has no effect is a common error. "Produces more electricity" is too vague; write "the induced e.m.f. increases because the rate of change of flux increases".
June 2024 Paper 11 Q30 (0625_s24_er.pdf): "a common misconception of both stronger and weaker candidates was that the speed of the magnet did not affect the magnitude of the induced e.m.f." November 2023 Paper 22 Q31: "some candidates chose option C, showing that they had not recognised that moving a wire more quickly in a magnetic field induces a greater e.m.f."
(E) Define e.m.f. by work per charge
e.m.f. is NOT just "the voltage of the source". Cambridge needs two elements: the work done (energy transferred) by the source in moving unit charge around a complete circuit; unit volt (V). The wrong form "the p.d. it provides" scores zero. Full mark-scheme wording: "the electrical work done by a source in moving unit charge around a complete circuit".
November 2024 Paper 42 Q7(a) — two marks for the e.m.f. definition, requiring both "work done" and "per unit charge".
(E) No current flows in the iron core
No current passes through the iron core — the primary and secondary are electrically isolated. a.c. in the primary → changing flux in the core → this induces an e.m.f. in the secondary. Energy is carried by the magnetic field, not by charge crossing the core. A common error is to imagine an alternating current flowing in the core itself.
June 2023 Paper 22 Q32 (0625_s23_er.pdf) — examiner explicitly named the alternating-current-in-core misconception.
(E) Step-up: secondary has more turns
"Step-up" does not mean more turns on the primary. In a step-up transformer the secondary has more turns: , so ; the current then falls (). June 2023 Paper 13 Q32: many wrongly thought a step-up transformer has more turns on the primary. Match "more turns" to the coil with the higher potential difference.
June 2023 Paper 13 Q32 (0625_s23_er.pdf): "few candidates answered correctly — misconception that step-up has more turns on primary than secondary."
(E) Transformers need a.c., not d.c.
A transformer does not change d.c. into a.c., and it does not work on d.c. at all. Only a changing current makes a changing flux, and only a changing flux induces an e.m.f. in the secondary; steady d.c. gives steady flux and zero output. A transformer only changes the size of an a.c. potential difference.
November 2023 Paper 22 Q33 (0625_w23_er.pdf): "some candidates thought that a transformer converts d.c. to a.c."
(E) Induction chain: flux → rate → e.m.f.
"Explain" induction questions give one mark per link — write numbered steps: (1) resistance/current changes → (2) magnetic field changes → (3) flux through the coil changes → (4) e.m.f. induced. Miss a link and you lose that mark.
State the direction of every change
State the DIRECTION of every change, not just that something changed. "The field changes" scores zero; "the field increases" scores. Examiners flag answers that say a quantity "changed" without saying increased or decreased — attach it to current, field, flux and e.m.f.
Fleming left-hand rule finger map
Hold your LEFT hand with thumb, first and second fingers mutually perpendicular: thumb = force (motion), first finger = field (N→S), second finger = conventional current (+ to −). Check you used the LEFT hand for a motor and the RIGHT hand for a generator.
(E) Transformer calc: keep primary/secondary
For transformer sums pick the right equation: for turns and voltage, or for the ideal power link. Keep the primary (subscript 1) and secondary (subscript 2) quantities apart. Rearrange before substituting; carry units (V, A).
| Relationship | Formula | Quantities |
|---|---|---|
| Force on conductor (motor effect) | = force (N), = magnetic flux density (T), = current (A), = length in field (m) | |
| Transformer turns ratio | = primary/secondary p.d. (V); = primary/secondary turns | |
| Ideal transformer (power) | = primary/secondary current (A) | |
| Current ratio (derived) | Current ratio is the inverse of the turns ratio | |
| Charge | = charge (C), = current (A), = time (s) |
Key definitions:
Define electromotive force (e.m.f.).
A step-down transformer is connected to a 230 V a.c. supply. The primary coil has 1150 turns and the secondary coil has 200 turns.
Calculate the potential difference across the secondary coil. Show your working.
