Cambridge IGCSE Physics 0625: Electricity and magnetism – Study Guide

This guide teaches Topic 4, Electricity and magnetism, of the Cambridge IGCSE Physics 0625 syllabus for examination in 2026, 2027 and 2028: sections 4.1 to 4.5. Core content is for everyone. Supplement content is examined only on the Extended papers (Paper 2 and Paper 4) and is labelled Extended only. See the 0625 course hub and the printable 0625 checklist.

What this topic covers

Section	Core	Extended only
4.1 Magnetism	Poles, induced magnetism, soft iron vs steel, field lines	Fields interacting; line spacing
4.2 Electrical quantities	Charge, current, e.m.f., p.d., R = V/I, P = IV, E = IVt, kW h	Electric fields; I = Q/t; E = W/Q; V = W/Q; I–V graphs; R ∝ L, R ∝ 1/A
4.3 Circuits	Symbols, series and parallel rules	Diodes, LEDs; junction rule; parallel R; divider equation
4.4 Safety	All content	–
4.5 Electromagnetic effects	Induction, fields of currents, motor effect, transformer	Generator; direction rules; commutator; IpVp = IsVs; I²R

Every equation here is “recall and use”: learn them.

4.1 Simple phenomena of magnetism

Like poles repel; unlike poles attract. A magnet attracts unmagnetised magnetic material (iron, steel, nickel, cobalt) by induced magnetism: the material becomes a magnet with the opposite pole facing the magnet. Non-magnetic materials (copper, aluminium, plastic) are not attracted. Because an unmagnetised piece of iron is attracted to either pole, attraction does not prove an object is magnetised: repulsion is the test for a magnet.

Soft iron magnetises easily but loses its magnetism when the field is removed (a temporary magnet). Steel is harder to magnetise but keeps it (a permanent magnet).

A magnetic field is a region in which a magnetic pole experiences a force. Its direction at a point is the direction of the force on an N pole there, so lines run N to S outside a bar magnet.

Drawing the field of a bar magnet. Lines leave the N pole and curve round to the S pole, with an arrow on each line pointing from N to S. The lines never cross, are closest together at the poles and spread out away from the magnet. Draw them symmetrically on both sides of the magnet.

Plotting it. Place the magnet on paper. Put a small plotting compass near the N pole and mark the position of its needle’s tip with a dot; move the compass so its tail is on that dot and mark again; repeat, then join the dots and add the arrow the needle shows. Repeat from several starting points. Iron filings sprinkled on card over the magnet (tap the card gently) show the pattern but not the direction.

Uses: permanent magnets in loudspeakers and motors; electromagnets where magnetism must switch off (relays, scrap-yard cranes).

Extended only. Magnetic forces are due to interactions between magnetic fields. Closely spaced lines mean a strong field.

4.2 Electrical quantities

Charge (4.2.1)

There are two kinds of charge, positive and negative. Like charges repel; unlike charges attract. Rub a polythene rod with a cloth: electrons move from cloth to rod, so the rod becomes negative and the cloth positive. Only electrons move when solids are charged by friction; the positive charges stay where they are.

Experiments. Producing charge: rub an insulating rod (polythene or acetate) with a dry cloth. Detecting it: a charged rod picks up small pieces of paper. Showing both kinds: hang a charged polythene rod in a paper stirrup so it can turn; a second charged polythene rod held near one end pushes it away (like charges), while a charged acetate rod, which becomes positive when rubbed, pulls it closer (unlike charges).

Conductor or insulator? Connect a cell, a lamp (or an ammeter) and two crocodile clips in series, then clip the sample between the clips. If the lamp lights (or the ammeter shows a current), the sample is a conductor; if not, it is an insulator.

Electron model. In a metal, some electrons are free to move between the atoms, so charge can flow through it: metals such as copper and aluminium are good conductors. In an insulator (plastic, rubber, glass, dry wood) the electrons are held in their atoms and cannot move through the material.

Extended only. Charge is measured in coulombs (C). An electric field is a region in which a charge experiences a force; its direction is the force on a positive charge. Field lines point radially outwards from a positive point charge or charged sphere, and run parallel and evenly spaced from + to − between charged plates.

Current, e.m.f. and p.d. (4.2.2–4.2.3)

Current is related to the flow of charge. In a metal, a current is a flow of free electrons through the metal. Measure it with an ammeter in series. Direct current (d.c.) flows in one direction only (from a cell or battery); alternating current (a.c.) keeps reversing direction (the mains supply).

e.m.f. is the electrical work done by a source in moving a unit charge around a complete circuit. p.d. is the work done by a unit charge passing through a component. Both are measured in volts (V), with a voltmeter connected in parallel (across the source or component).

Using meters (analogue and digital).

• Choose a range just above the value you expect. On a 0–10 A range a current of about 0.3 A barely moves an analogue pointer (or shows few digits on a digital meter); on a 0–1 A range it is read far more precisely. A range that is too low overloads the meter.
• Analogue meter: check the pointer reads zero before connecting; read the scale with your eye directly in front of the pointer to avoid parallax; find what one small division is worth on the range in use.
• Digital meter: read the display directly; if it shows an overload sign, switch to a higher range; if it shows too few significant figures, switch to a lower one.
• Connect the + terminal of the meter towards the + terminal of the supply.

Extended only. Charge is measured in coulombs (C). Current is the charge passing a point per unit time: I = Q/t. e.m.f. E = W/Q; p.d. V = W/Q. Conventional current flows + to −; electrons flow − to +.

Worked example (Extended). A battery does 54 J of work moving 6.0 C of charge around a circuit. Find its e.m.f.

E = W / Q = 54 / 6.0 = 9.0 V

Worked example (Extended). 0.40 A flows through a 9.0 V motor for 5.0 minutes. Find the charge and the energy.

Q = I t = 0.40 × (5.0 × 60) = 120 C
W = Q V = 120 × 9.0 = 1080 J

Resistance (4.2.4)

R = V/I, in ohms (Ω). 3.0 V across a resistor with 0.15 A through it gives R = 3.0/0.15 = 20 Ω.

Experiment: finding a resistance. Connect the component in series with a cell (or low-voltage supply), a switch, an ammeter and a variable resistor; connect a voltmeter across the component only. Adjust the variable resistor to get several different currents and record V and I each time. Calculate R = V/I for each pair and take the mean. Switch off between readings so the component does not heat up. (Extended only: alternatively plot V against I; for a resistor of constant resistance the points lie on a straight line through the origin, and its gradient is R.)

V / V	I / A	R = V/I / Ω
1.0	0.050	20
2.0	0.10	20
3.0	0.15	20

A longer wire has more resistance; a thicker wire (larger cross-sectional area) has less.

Extended only. R is directly proportional to length and inversely proportional to cross-sectional area. Example: a 3.0 m wire has 6.0 Ω; find R for 1.5 m of the same metal with twice the area.

half the length:      6.0 × 1/2 = 3.0 Ω
twice the area:       3.0 × 1/2 = 1.5 Ω

I–V graphs (Extended only).

• Fixed resistor: straight line through the origin; R constant.
• Filament lamp: curve getting less steep; the hot filament’s resistance increases.
• Diode: no current in reverse; in the forward direction current flows once the p.d. passes a small value.

Energy and power (4.2.5)

Electric circuits transfer energy from a source (a cell or the mains supply) to the circuit components and then into the surroundings: a lamp transfers it by light and by heating, a heater by heating. P = IV and E = IVt. One kilowatt-hour (kW h) is the energy a 1 kW appliance transfers in 1 hour (1 kW h = 1000 W × 3600 s = 3.6 × 10⁶ J).

Worked example. A 12 V lamp takes a current of 2.0 A. Find its power and the energy it transfers in 10 minutes.

P = I V = 2.0 × 12 = 24 W
E = I V t = 2.0 × 12 × (10 × 60) = 14 400 J

Worked example. A 1.5 kW heater runs 3.0 hours a day for 30 days at 20 cents per kW h.

energy = 1.5 kW × (3.0 h × 30) = 135 kW h
cost   = 135 × 20 = 2700 cents

4.3 Electric circuits

Circuit symbols (4.3.1)

You must draw and interpret circuit diagrams using the symbols printed on the “Electrical symbols” page of the syllabus (check your drawings against it). In words:

Component	Symbol	What it does in a circuit
Cell	long thin line (+) beside a short thick line (−)	source of e.m.f.
Battery	two or more cells joined in a row	e.m.f.s add when the cells face the same way
Power supply (d.c. or a.c.)	two small open circles, marked + and − (d.c.) or with ~ between them (a.c.)	laboratory supply
Generator	a square containing G	source of e.m.f. from motion
Switch	a break in the line with a sloping bar	open: no current; closed: current flows
Fixed resistor	a rectangle	limits the current
Variable resistor	a rectangle with a diagonal arrow through it	resistance can be changed to vary the current
Thermistor (NTC)	a rectangle crossed by a diagonal line with a short flat end	resistance falls as temperature rises
LDR	a rectangle with two arrows pointing in towards it	resistance falls as light gets brighter
Heater	a rectangle divided into sections	transfers energy by heating
Potential divider	a rectangle with an arrow touching its side (sliding contact)	gives a variable fraction of the supply p.d.
Lamp	a circle with a cross inside	lights when current flows
Motor	a circle with M	turns when current flows
Bell	a dome shape on two legs	rings when current flows
Ammeter / voltmeter	a circle with A / a circle with V	measures current (in series) / p.d. (in parallel)
Magnetising coil	a line of loops	becomes an electromagnet when current flows
Transformer	two coils with vertical lines (the core) between them	changes an alternating voltage
Fuse	a small rectangle with the wire running through it	melts if the current is too large
Relay coil	a small rectangle on a line	its magnetism operates a switch in another circuit

Extended only. A diode (a triangle pointing at a bar) lets current through in one direction only, the way the triangle points; in the reverse direction its resistance is very high. An LED is a diode symbol with two arrows pointing outwards: it conducts one way only and emits light when it conducts.

Series and parallel (4.3.2)

Series: the current is the same everywhere; resistances add; e.m.f.s of cells in series add.

Parallel: the source current is larger than each branch current. Two resistors in parallel have less resistance than either alone. Lighting circuits use parallel lamps: each gets the full p.d., can be switched separately, and stays lit if another fails.

Extended only. Current into a junction = current out, because charge is not used up or stored. Series p.d.s add to the total. Parallel branches share the same p.d. For two resistors in parallel, 1/R = 1/R₁ + 1/R₂.

Worked example (Extended). Four 1.5 V cells in series; an 8.0 Ω resistor in series with 6.0 Ω and 3.0 Ω in parallel.

e.m.f.          = 4 × 1.5 = 6.0 V
pair:     1/R = 1/6.0 + 1/3.0, so R = 2.0 Ω
total:    8.0 + 2.0 = 10 Ω;   I = 6.0 / 10 = 0.60 A
p.d.s:    8.0 Ω: 4.8 V;   pair: 1.2 V   (sum 6.0 V)
branches: 1.2/6.0 = 0.20 A;   1.2/3.0 = 0.40 A   (sum 0.60 A)

Potential dividers (4.3.3)

For a constant current, p.d. increases with resistance, so the larger of two series resistors takes the larger share of the supply.

Extended only. R₁/R₂ = V₁/V₂. With 9.0 V across 2.0 kΩ and 4.0 kΩ, V₁ = 3.0 V and V₂ = 6.0 V.

Variable potential divider (Extended only). Two resistors in series across a supply, with the output taken across one of them, R₂. If one resistor is a variable resistor, thermistor or LDR, the output changes with its resistance: when R₂ increases, V₂ increases (and V₁ falls, as they always add to the supply p.d.). If R₂ is an NTC thermistor and it warms up, its resistance and its share of the p.d. both fall; if R₂ is an LDR and it gets darker, its resistance and V₂ both rise, which can switch on a lamp at dusk. A single resistor with a sliding contact (the potential divider symbol) does the same job: moving the contact changes the fraction of the supply p.d. taken off.

4.4 Electrical safety

Hazards: damaged insulation (exposed live wire, shock), overheating cables (fire), damp conditions (water conducts, shock) and overloading plugs, extension leads and sockets (excess current overheats cables).

Mains wiring has live (line), neutral and earth wires. The switch goes in the live wire so that “off” disconnects the appliance from the high voltage.

A fuse melts when the current exceeds its rating; a trip switch opens automatically and can be reset. Choose a rating just above the normal current.

Worked example. A 920 W, 230 V appliance; fuses of 3 A, 5 A and 13 A.

I = P / V = 920 / 230 = 4.0 A    → choose the 5 A fuse

A metal casing must be earthed: if the live wire touches it, a large current flows to earth and the fuse melts. A double-insulated appliance has a non-conducting casing and no earth wire; its fuse still protects the circuit and cabling.

4.5 Electromagnetic effects

Electromagnetic induction (4.5.1)

An e.m.f. is induced when a conductor moves across a magnetic field, or the field linking it changes. Push a magnet into a coil joined to a sensitive meter: the needle deflects, returns to zero when the magnet stops, and deflects the other way as it is pulled out. The e.m.f. is larger with faster movement, a stronger magnet and more turns.

Extended only. The induced e.m.f. opposes the change causing it. Fleming’s right-hand rule: thumb = motion, first finger = field, second finger = induced current.

The a.c. generator (4.5.2) – Extended only

A coil rotates in a field (or a magnet rotates in a coil). Slip rings and brushes connect a rotating coil to the circuit. The e.m.f. is largest when the plane of the coil is parallel to the field (sides cut lines fastest) and zero when it is perpendicular (sides move along the lines). Peak and trough are half a turn apart; faster spinning gives higher peaks and more cycles per second.

Starting with the coil’s plane perpendicular to the field, one turn gives:

Rotation	0°	90°	180°	270°	360°
Coil plane	perpendicular	parallel	perpendicular	parallel	perpendicular
e.m.f.	zero	peak	zero	trough (peak, reversed)	zero

The graph of e.m.f. against time is therefore a wave shape (sinusoidal) crossing zero twice per turn. Its sign reverses after each half turn because each side of the coil moves up through the field and then down.

Magnetic effect of a current (4.5.3)

A straight wire’s field is concentric circles (right-hand grip rule: thumb along the current, fingers show the field). A solenoid’s field is like a bar magnet’s outside and nearly uniform inside; the grip rule with fingers along the current in the turns makes the thumb point to the solenoid’s N end.

Experiment: field patterns. Pass a vertical wire through a hole in a horizontal card (or push a solenoid through a card), and switch on a large current for a short time. Sprinkle iron filings on the card and tap it: they form the pattern. Place small plotting compasses on the card to show the direction.

A relay: a small current magnetises a coil, which attracts a soft-iron armature and pulls a switch closed in a second circuit, so a small, safe current can switch a large one (for example, a car’s ignition switch operating the starter motor). A loudspeaker (in radios, phones and headphones): a changing current in a coil in a permanent magnet’s field makes the coil and the attached cone vibrate, producing sound.

Extended only. The field weakens with distance from a wire and is strong and uniform inside a solenoid. More current gives a stronger field; reversing the current reverses the field (in the field-patterns experiment, every plotting compass needle turns round).

Force on a conductor and the d.c. motor (4.5.4–4.5.5)

Experiment: force on a conductor. Lay a short, bare copper rod across two horizontal bare wires (rails) connected to a low-voltage supply, with the rod between the poles of a U-shaped magnet. When the current is switched on, the rod rolls along the rails. Reverse the current and it rolls the other way; reverse the field (turn the magnet over) and it also rolls the other way; reverse both and it rolls the original way.

A current-carrying coil in a magnetic field experiences a turning effect, increased by more turns on the coil, a larger current or a stronger field.

Extended only. Fleming’s left-hand rule: first finger = field, second = current, thumb = force. For a beam of charged particles, treat the beam as a current: for positive particles in the direction they move, for electrons opposite to their motion, then apply the rule.

The d.c. motor (Extended only). A coil sits between the poles of a magnet. Current flows in opposite directions along the two sides of the coil, so the forces on them are in opposite directions (one up, one down) and the coil turns. The coil is connected to the supply through a split-ring commutator and carbon brushes: the brushes press on the rotating split ring and make sliding contact. Every half turn, as the plane of the coil passes the position perpendicular to the field, the gaps in the split ring pass the brushes and the current in the coil reverses, so the forces keep turning the coil the same way.

The transformer (4.5.6)

A transformer has two coils, the primary (input) and the secondary (output), wound on the same soft-iron core. Vp/Vs = Np/Ns. Step-up: more secondary turns, so Vs > Vp; step-down: fewer secondary turns, so Vs < Vp.

High-voltage transmission. At the power station a step-up transformer raises the voltage to a very high value for the transmission cables; step-down transformers near homes and factories lower it to the mains voltage. Advantages of transmitting at high voltage: for the same power the current is smaller, so less energy is wasted heating the cables, and thinner, lighter, cheaper cables (and fewer pylons) can be used.

Extended only. a.c. in the primary makes a changing field in the core, which induces an alternating e.m.f. in the secondary. At 100% efficiency, IpVp = IsVs.

Worked example. 230 V across 920 primary turns; 48 secondary turns. Find the output voltage.

Vs = 230 × 48 / 920 = 12 V   (step-down)
Extended: Is = 1.5 A  →  Ip = (12 × 1.5) / 230 = 0.078 A

Extended only: P = I²R. Sending 50 MW through 4.0 Ω cables: at 25 kV, I = 2000 A and the loss is 16 MW; at 250 kV, I = 200 A and the loss is 0.16 MW. Ten times the voltage, one hundredth of the loss.

Common errors

• Voltmeter in series or ammeter in parallel.
• Forgetting to invert 1/R for parallel resistors.
• Saying current is “used up” by components.
• Switch or fuse in the neutral wire.
• Generator e.m.f. peak placed at the wrong coil position.
• Mixing up the left-hand (motor) and right-hand (generator) rules.
• Saying a transformer works on d.c.

Next steps

Test recall with the revision notes, then try the Core practice questions and Extended practice questions. Find weak spots with the 0625 Core diagnostic or Extended diagnostic. Next topic: Nuclear physics.

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).