Cambridge IGCSE Physics 0625: Electricity and magnetism – Revision Notes

These notes condense Topic 4, Electricity and magnetism (sections 4.1–4.5), of the Cambridge IGCSE Physics 0625 syllabus for examination in 2026, 2027 and 2028. Core content is for every candidate; Supplement content, marked Extended only, is examined only on Papers 2 and 4. For full explanations and worked examples, use the study guide for this topic.

Other pages: 0625 course hub, printable 0625 checklist, Core practice questions, Extended practice questions.

Equations to recall

The syllabus asks you to “recall and use” all of these.

Equation	Quantities and units	Tier
R = V/I	resistance Ω, p.d. V, current A	Core
P = IV	power W	Core
E = IVt	energy J, time s	Core
energy in kW h = power in kW × time in h	kilowatt-hour	Core
Vp/Vs = Np/Ns	transformer voltages and turns	Core
I = Q/t	charge C	Extended only
E = W/Q (e.m.f.) and V = W/Q (p.d.)	work J, charge C	Extended only
R = R₁ + R₂ + … (series)	Ω	Core
1/R = 1/R₁ + 1/R₂ (parallel)	Ω	Extended only
R₁/R₂ = V₁/V₂ (potential divider)	Ω, V	Extended only
IpVp = IsVs (100% efficient transformer)	A, V	Extended only
P = I²R (power lost in cables)	W	Extended only

4.1 Magnetism: key facts

• Like poles repel, unlike poles attract. A magnet attracts unmagnetised iron or steel by induced magnetism. Only repulsion proves an object is a magnet.
• Soft iron: easy to magnetise, loses it easily: temporary magnets, electromagnet cores. Steel: keeps its magnetism: permanent magnets.
• Magnetic materials: iron, steel, nickel, cobalt. Non-magnetic: copper, aluminium, plastic, wood.
• Magnetic field: region where a magnetic pole experiences a force. Direction = force on an N pole. Lines go N → S outside the magnet.
• Bar magnet field: lines from N round to S, arrows N → S, never crossing, closest at the poles.
• Plotting: compass for direction (mark the needle tip, move the compass on, join the dots), iron filings for pattern.
• Extended only: forces come from interacting fields; closer lines = stronger field.

4.2 Charge, current, p.d. and resistance

Must-know definitions

• e.m.f.: electrical work done by a source in moving a unit charge around a complete circuit.
• p.d.: work done by a unit charge passing through a component.
• Current (Extended only): charge passing a point per unit time.
• Electric field (Extended only): region where a charge experiences a force; direction = force on a positive charge.
• Kilowatt-hour: energy transferred by a 1 kW appliance in 1 hour.

Charging by friction. Only electrons move. The object that gains electrons becomes negative; the one that loses them becomes positive. Rubbed polythene becomes negative, rubbed acetate positive; a charged rod picks up small pieces of paper.

Conductors vs insulators. Conductors (metals) have free electrons; insulators do not.

Meters. Ammeter in series; voltmeter in parallel. Pick the range just above the expected reading. Analogue: check the zero, avoid parallax. Digital: move to a higher range if it shows overload.

Resistance experiment. Ammeter in series with the component, voltmeter across it, variable resistor to change the current; several V and I readings; R = V/I for each, then the mean.

Field patterns (Extended only). Point charge and charged sphere: radial lines (outwards from +). Parallel plates: parallel, evenly spaced lines from + to −.

Current direction (Extended only). Conventional current + → −; electrons − → +.

Resistance of a wire. Longer → more resistance; thicker → less. Extended only: R ∝ length, R ∝ 1/area.

I–V graphs (Extended only)

Component	Shape	Why
Fixed resistor	straight line through origin	R constant
Filament lamp	curve, flattening at higher V	filament heats, R increases
Diode	zero in reverse; rises steeply after a small forward p.d.	conducts one way only

Method in steps: cost of electricity

1. Convert power to kW.
2. Convert time to hours.
3. Energy (kW h) = kW × h.
4. Cost = energy × price per kW h.

4.3 Circuits

Must-know distinctions: series vs parallel

	Series	Parallel
Current	same everywhere	source current larger than each branch; Extended: branch currents add to the total
p.d.	Extended: shares add to the total	Extended: same across each branch
Combined R	sum of resistors	less than the smallest resistor
One lamp fails	all go out	others stay lit

• Circuit symbols: learn them from the “Electrical symbols” page of the syllabus; the study guide describes each one and what it does.
• Cells in series, facing the same way: e.m.f.s add.
• Lighting circuits are parallel: each lamp gets full p.d., has its own switch, and stays on if another fails.
• NTC thermistor: resistance falls as temperature rises. LDR: resistance falls as light increases.
• Potential divider: the bigger resistance gets the bigger share of the p.d.
• Extended only: diodes and LEDs conduct one way; an LED emits light.
• Extended only: the junction rule holds because charge is not used up or stored at a junction.

Method in steps: mixed circuit (Extended only)

1. Combine each parallel group with 1/R = 1/R₁ + 1/R₂.
2. Add series resistances to get the total.
3. I = e.m.f. / total R.
4. p.d. across each part = I × R of that part.
5. Split the current between branches using I = V/R for each branch; check they add up.

4.4 Electrical safety

• Hazards: damaged insulation, overheating cables, damp conditions, overloaded plugs, extension leads and sockets.
• Three wires: live (line), neutral, earth. Switch and fuse go in the live.
• Fuse: melts if current exceeds rating. Trip switch: opens automatically, can be reset. Choose the rating just above normal current (I = P/V).
• Metal casing: earthed, so a fault sends a large current to earth and the fuse melts.
• Double-insulated (plastic casing): no earth wire needed; the fuse still protects the circuit and cabling.

4.5 Electromagnetic effects

Must-know distinctions: motor vs generator

	Motor effect (4.5.4–4.5.5)	Induction (4.5.1–4.5.2)
In	current + field	motion + field
Out	force / turning	induced e.m.f.
Rule (Extended only)	Fleming’s left hand: field, current, force	Fleming’s right hand: field, induced current, motion
Bigger effect	more turns, more current, stronger field	faster motion, stronger field, more turns
Connection (Extended only)	split-ring commutator + brushes (d.c. motor)	slip rings + brushes (a.c. generator)

• Induced e.m.f. when a conductor cuts field lines or the field linking it changes. Extended only: it opposes the change causing it.
• Generator graph (Extended only): e.m.f. is maximum when the coil plane is parallel to the field; zero when perpendicular.
• Field around a straight wire: concentric circles (right-hand grip rule). Solenoid: like a bar magnet; nearly uniform inside.
• Extended only: field weaker further from the wire; more current, stronger field; reverse the current, reverse the field.
• Relay: small current in a coil closes a switch in another circuit, e.g. a car ignition switching the large starter-motor current. Loudspeaker: varying current in a coil in a magnet’s field vibrates the cone.
• Force experiment: a wire between the poles of a U-shaped magnet jumps when current flows; reverse the current or swap the poles and it moves the other way.
• Electron beams (Extended only): treat as a current in the opposite direction, then use the left-hand rule.
• Transformer: soft-iron core; step-up has Ns > Np. Extended only: changing current in the primary → changing field in the core → induced e.m.f. in the secondary. Needs a.c.
• High-voltage transmission: a step-up transformer at the power station raises the voltage; step-down transformers near users lower it again. Smaller current, less heating in cables. Extended only: loss = I²R, so ten times the current means one hundred times the loss.

Quick self-test

1. (Extended) A current of 0.50 A flows for 2.0 minutes. Find the charge.
2. A lamp has 6.0 V across it and 0.25 A through it. Find its resistance.
3. (Extended) Find the combined resistance of 20 Ω and 30 Ω in parallel.
4. Three 1.5 V cells are joined in series, all the same way round. Find the total e.m.f.
5. An 800 W microwave is used for 15 minutes a day for 30 days. Electricity costs 25 cents per kW h. Find the cost.
6. (Extended) A 3.0 kΩ and a 1.0 kΩ resistor are in series across 12 V. Find the p.d. across the 1.0 kΩ resistor.
7. A transformer has 400 primary turns and a 230 V input. Its output is 11.5 V. Find the number of secondary turns.
8. A 1.2 kW hairdryer runs on 230 V. Choose a fuse from 3 A, 5 A and 13 A.
9. (Extended) A wire’s length is doubled and its cross-sectional area halved. By what factor does its resistance change?
10. A 60 W lamp runs on 230 V. Find the current, to 2 significant figures.
11. Explain why the switch must be in the live wire.
12. (Extended) A beam of electrons moves left to right through a magnetic field. Which direction do you use for “current” in Fleming’s left-hand rule?

Answers

1. Q = It = 0.50 × 120 = 60 C.
2. R = 6.0/0.25 = 24 Ω.
3. 1/R = 1/20 + 1/30 = 5/60, so R = 12 Ω (less than 20 Ω, as expected).
4. 3 × 1.5 = 4.5 V.
5. 0.80 kW × 0.25 h × 30 = 6.0 kW h; 6.0 × 25 = 150 cents.
6. The total is 4.0 kΩ and the 1.0 kΩ takes 1/4 of it: 3.0 V (and 9.0 V across the 3.0 kΩ; 3.0/1.0 = 9.0/3.0).
7. Ns = 400 × 11.5/230 = 20 turns.
8. I = 1200/230 = 5.2 A, so the 13 A fuse (a 5 A fuse would melt in normal use).
9. ×2 for length, ×2 for halving the area: 4 times larger.
10. I = 60/230 = 0.26 A.
11. With the switch off, the appliance is disconnected from the live (high-voltage) wire, so it is safe to touch inside. In the neutral, the appliance would stay live.
12. Right to left: conventional current is opposite to the electron flow.

Where marks are usually lost

• Writing “the current is used up” in a series circuit; current is the same everywhere.
• Giving 1/R as the answer for a parallel pair; a combined value bigger than the smallest resistor is always wrong.
• Leaving minutes in I = Q/t or E = IVt, or mixing W with hours when finding kW h.
• Picking a fuse equal to or below the working current; the rating must be just above it.
• Saying the earth wire “stops the current”; it carries a fault current so the fuse melts.
• Describing induction without saying the conductor must move relative to the field, or the field must change.
• Placing the generator’s peak e.m.f. when the coil is perpendicular to the field; that is where it is zero.
• Explaining the commutator as “keeping the current flowing”; it reverses the coil current every half turn.
• For transmission, saying higher voltage “reduces resistance”; it reduces the current, and loss = I²R.

Official syllabus

Cambridge International, Cambridge IGCSE Physics 0625 syllabus for examination in 2026, 2027 and 2028, Topic 4 Electricity and magnetism (sections 4.1–4.5). Take the 0625 Core diagnostic or 0625 Extended diagnostic to check your readiness.