Physics 0625 · syllabus 4.5
Cambridge IGCSE Physics 0625 — Electromagnetic effects topical past papers
Electromagnetic induction and a.c. generators, the magnetic effect of a current, relays and loudspeakers, d.c. motors, and transformers in power transmission.
- Questions
- 838
- Years
- 2009–2026
- With mark scheme
- 99%
- Questions 2022–2026
- 293
Anyone can browse these pages. Opening the questions and their mark schemes takes a free Momentum account.
What Electromagnetic effects covers
When a wire moves across a magnetic field, or the magnetic field through a coil changes, an e.m.f. is induced in it, and a current flows if the circuit is complete. Push a magnet into a coil connected to a sensitive meter and the needle flicks; hold the magnet still and the reading drops back to zero. The induced e.m.f. is larger if the movement is faster, the field stronger or the coil has more turns. Extended candidates know that the induced e.m.f. always acts to oppose the change that causes it, work out which way an induced current flows from the directions of the motion and the field, and describe an a.c. generator: a coil turning in a magnetic field, connected through slip rings and brushes, whose e.m.f. peaks when the sides of the coil cut the field fastest and is zero when the plane of the coil is at right angles to the field.
A current produces a magnetic field: circles around a straight wire, and a pattern like a bar magnet's around a solenoid. Extended candidates describe how the field is strongest close to the wire or inside the solenoid, grows with the current and reverses when the current reverses. Electromagnets put the effect to work in relays, where a small current switches a larger one, and in loudspeakers. A current-carrying wire in a magnetic field feels a force, which reverses if either the current or the field is reversed; Extended candidates predict its direction, for beams of charged particles too. In a d.c. motor the forces on the two sides of a coil make it turn, more strongly with more turns, a larger current or a stronger magnet, and Extended candidates explain how the split-ring commutator reverses the current every half turn so that the coil keeps turning the same way.
A transformer has a primary and a secondary coil wound on a soft-iron core. Alternating current in the primary makes a changing magnetic field in the core, which induces an alternating e.m.f. in the secondary, and the voltages are in the ratio of the turns, Vp/Vs = Np/Ns. Electricity is transmitted at high voltage because the same power then needs a smaller current, so less energy is wasted heating the cables. Extended candidates use IpVp = IsVs for a transformer that wastes no energy, and P = I²R for the power lost in the cables.
In the syllabus
The headings this topic has in the Cambridge International 2026–2028 syllabus, as the board prints them.
- 4.5 Electromagnetic effects
What you should be able to do
- Describe an experiment that demonstrates electromagnetic induction, and state what affects the size of the induced e.m.f.
- Describe an a.c. generator and relate its e.m.f.–time graph to the position of the coil (Extended)
- Describe the magnetic field of a straight wire and of a solenoid, and how relays and loudspeakers use it
- Use the left-hand rule to find which way a current-carrying wire, or a beam of charged particles, is pushed in a magnetic field (Extended)
- Explain what makes a d.c. motor turn more strongly and, for Extended, how the split-ring commutator keeps it turning
- Use Vp/Vs = Np/Ns and, for Extended, IpVp = IsVs and P = I²R to explain high-voltage transmission
Key ideas
- An e.m.f. is induced when a conductor cuts a magnetic field or the field through a coil changes, and it opposes the change causing it (the last for Extended)
- The induced e.m.f. is larger with faster movement, a stronger field and more turns
- Transformer: Vp/Vs = Np/Ns; with no energy wasted, IpVp = IsVs (Extended)
- Power lost in cables P = I²R, so halving the current cuts the loss to a quarter (Extended)
- Fleming's left-hand rule: first finger field, second finger current, thumb force (Extended)
How Electromagnetic effects has been examined
From 2022 to 2026, Electromagnetic effects was on Paper 4 · Theory (Extended) in 14 of 14 exam series, worth about 10% of that paper’s marks.
| Paper | Questions | Share of marks | Series |
|---|---|---|---|
| Paper 1 · Multiple Choice (Core) | 104 | 8% | 14 of 14 |
| Paper 2 · Multiple Choice (Extended) | 125 | 10% | 14 of 14 |
| Paper 3 · Theory (Core) | 32 | 8% | 12 of 14 |
| Paper 4 · Theory (Extended) | 32 | 10% | 14 of 14 |
Momentum’s own count, over the questions in its bank for these years: a series is one exam session, every zone included, and a question filed under two chapters counts in both. Median marks per question: 1. The bank files this topic together with neighbouring ones, so these figures count every question in its chapters, including some on those topics.
Filed in Momentum under: Magnets and Currents.
A worked example
A transformer steps 230 V mains down to 11.5 V for a lamp, and its primary coil has 2000 turns. Find the number of turns on the secondary. The lamp takes a current of 4.0 A; assuming no energy is wasted, find the current in the primary.
- Vp/Vs = Np/Ns, so Ns = Np × Vs/Vp = 2000 × 11.5 / 230 = 100 turns
- IpVp = IsVs, so Ip = IsVs/Vp = 4.0 × 11.5 / 230 = 0.20 A
- Check: power in = 230 × 0.20 = 46 W, and power out = 11.5 × 4.0 = 46 W
Answer: 100 turns on the secondary; 0.20 A in the primary
Written by Momentum for this page, with its own numbers; it is not taken from a past paper.
Mistakes that cost marks
- Saying a transformer works on direct current — a steady current makes a steady field, which induces nothing; the primary needs alternating current
- Turning the turns ratio upside down — the larger voltage always goes with the larger number of turns, so a step-down transformer has fewer turns on its secondary
- Saying a magnet resting inside a coil induces a current — an e.m.f. is induced only while the field through the coil is changing
- Saying high-voltage transmission lowers the resistance of the cables — the resistance is unchanged; the current is smaller, so the I²R heating is much smaller
- Pointing the current finger along the direction an electron beam travels — conventional current flows the opposite way to electrons, so reverse it before using the left-hand rule
Exam technique
- In induction experiments, describe what you do and what the meter shows, and what changes when you move the magnet faster, reverse it or hold it still
- Write the transformer equation with each voltage beside its own number of turns, then check that a step-up transformer has given a larger output
- For transmission, give the chain: high voltage, so a small current for the same power, so little I²R heating in the cables, so less energy wasted
- For motors, say what the commutator does every half turn and why, rather than only naming it
Practising in Momentum
- Every question in a chapter as one list, from every paper and year in the bank, newest first or shuffled.
- Each question turns over to its own part of the mark scheme, where the board published one.
- Narrow by paper, year, session, zone and marks, and star the questions worth keeping.
- Pick chapters, a number of questions and a time, and the exam builder draws a mock to sit and mark.
Past-paper questions and mark schemes are the copyright of Cambridge International. Momentum shows them to signed-in learners for revision and does not publish them on these pages.