Cambridge O Level Physics: Electricity and magnetism (5054)

This guide teaches Section 4, Electricity and magnetism (4.1 to 4.6), of the Cambridge O Level Physics 5054 syllabus for examination in 2026, 2027 and 2028.

Pair it with the revision notes, practice questions, course hub, checklist and diagnostics.

4.1 Magnetism and magnetic fields

Like poles repel and unlike poles attract. A magnet also attracts unmagnetised magnetic material, so only repulsion proves a magnet. Iron, steel, cobalt and nickel are magnetic; copper, aluminium and plastic are not. Magnetic material near a magnet gains induced magnetism: the end nearest an N pole becomes an S pole, so they attract.

Soft iron is magnetised and demagnetised easily, so it makes temporary magnets. Steel keeps its magnetism, so it makes permanent magnets. Permanent magnets suit compasses; electromagnets, which switch off, suit scrapyard cranes.

A magnetic field is a region where a magnetic pole feels a force. Its direction is the way an N pole placed there would be pushed. To plot it, mark the two ends of a plotting compass needle, move the compass so one end sits on the last mark, repeat, then join the marks; the needle’s N end points along the field. Iron filings sprinkled round a magnet show the same pattern.

Around a bar magnet the lines leave the N pole, enter the S pole, never cross, and are closest together where the field is strongest.

4.2 Electrical quantities

Charge and electric fields

Charge is positive or negative, measured in coulombs (C). Unlike charges attract; like charges repel. Rub a polythene rod with a dry cloth and it picks up scraps of paper. Only electrons move: the rod gains them and becomes negative; the cloth loses them and becomes positive.

An electric field is a region where a charge feels a force. Its direction is the direction of the force on a positive charge. Patterns:

• point charge or charged conducting sphere: radial lines, outwards for positive, inwards for negative;
• oppositely charged parallel plates: equally spaced parallel lines from positive to negative (ignore the ends).

Metals and graphite conduct; plastic, glass and rubber insulate. To test a material, put it in a gap in a circuit with a lamp: the lamp lights only for a conductor. In the electron model, a conductor has free electrons that drift through it; an insulator’s electrons are held in their atoms.

Current, e.m.f. and p.d.

Current is charge per unit time: I = Q/t (2.4 C in 8.0 s is 0.30 A). The amp is one coulomb per second (C/s). In a metal the free electrons move from negative to positive, but conventional current runs from positive to negative. Direct current (d.c.) flows one way; alternating current (a.c.) repeatedly reverses.

E.m.f. is the work done by a source moving unit charge round a complete circuit: E = W/Q. P.d. is the work done by unit charge passing through a component: V = W/Q. Both are in volts (1 V = 1 J/C). An ammeter goes in series and a voltmeter in parallel with the component; choose a range just above the expected reading.

Cells in series add their e.m.f.s (three 1.5 V cells give 4.5 V); identical cells in parallel give the e.m.f. of one cell. Doing 54 J of work on 6.0 C means E = 9.0 V.

Resistance

R = V/I. To find a resistance, set several currents with a variable resistor, calculate V ÷ I for each ammeter and voltmeter pair and average.

For a wire of one material, resistance rises in direct proportion to length and falls in inverse proportion to area.

Worked example 2. A 6.0 Ω wire is replaced by the same material, three times as long with twice the area: R = 6.0 × 3 ÷ 2 = 9.0 Ω.

Ohm’s law: so long as temperature does not change, a conductor’s current is directly proportional to its p.d.

• Fixed resistor: a straight line through the origin, because R is constant.
• Filament lamp: a curve that flattens as p.d. rises, because the heating filament’s resistance increases.
• Diode: almost no current one way, and a sharp rise above a small forward p.d. the other way.

Raising a metal’s temperature increases its resistance, because the vibrating atoms collide with the free electrons more often.

4.3 Electric circuits

Components

An NTC thermistor has less resistance when hot; an LDR has less resistance in brighter light; a diode passes current one way only; an LED is a diode that emits light; a fuse melts if the current is too large; a relay is an electromagnet-operated switch. Learn the standard symbol for every component in the syllabus list.

Series and parallel

• Series: the current is the same everywhere, p.d.s add to the supply p.d., and R = R1 + R2 + R3.
• Parallel: currents entering a junction equal currents leaving it, each branch has the same p.d. as the whole arrangement, and for two resistors R = (R1 × R2) ÷ (R1 + R2).

Worked example 3. A 5.0 Ω resistor, in series with a 12 Ω and 4.0 Ω pair in parallel, is on 9.6 V. The pair is 3.0 Ω, the total 8.0 Ω and the current 1.2 A. The 5.0 Ω resistor has 6.0 V, leaving 3.6 V across the pair, so the branch currents are 0.30 A and 0.90 A.

Sensors and potential dividers

Two series resistors share the supply p.d. in proportion to their resistances: R1 ÷ R2 = V1 ÷ V2. A variable potential divider has a slider that picks off any fraction of the supply.

Worked example 4. Across 10 V, R1 = 3.0 kΩ and R2 = 2.0 kΩ in series: V1 = 10 × 3 ÷ 5 = 6.0 V and V2 = 4.0 V.

As an input sensor, an NTC thermistor or LDR is in series with a fixed resistor. When the thermistor warms, its resistance falls, so its share of the p.d. falls and the fixed resistor’s share rises. An LDR does the same with light.

4.4 Practical electricity

Uses include heating, lighting, battery charging, motors and electronics. In a lighting circuit, parallel lamps each get the full supply p.d. and work independently, so one failing leaves the rest on.

Power is P = IV and energy is E = IVt. A kilowatt-hour (kW h) equals the energy a 1 kW appliance transfers while running for 1 hour. Cost = kW h × price per kW h.

Worked example 5. A 230 V motor takes 0.40 A for 5.0 minutes: E = 230 × 0.40 × 300 = 27 600 J. A 0.75 kW appliance used for 4.0 hours transfers 3.0 kW h, costing 36 cents at 12 cents per kW h.

Electrical safety

Mains hazards: damaged insulation exposes live wires; overheating cables start fires; damp conditions let current pass through a person; overloaded plugs, leads and sockets draw excess current.

A fuse holds a thin wire that melts once the current goes above its rating. A trip switch (circuit breaker) opens when the current exceeds its setting and can be reset. Choose a rating just above normal current. Both go in the live wire so that, once either operates, the appliance is disconnected from the live supply.

A mains circuit has live (line), neutral and earth wires. A switch goes in the live wire; in the neutral wire, the appliance would stay live when off.

Earthing. The earth wire connects a metal case to the ground. If the live wire touches an earthed case, a large current flows to earth, the fuse melts or the trip switch operates, and the case is made safe. A double-insulated appliance has a non-conducting case, so it cannot become live.

4.5 Electromagnetic effects

Electromagnetic induction

Move a magnet in and out of a coil joined to a sensitive voltmeter. An e.m.f. is induced only while the magnet moves; reversing the movement or the pole reverses it. It grows with faster cutting of field lines and with more turns.

Lenz’s law: an induced current acts to oppose the change that causes it. Push an N pole in and the nearer end becomes N, repelling it; pull it out and that end becomes S, attracting it back.

The a.c. generator

A coil rotates in a magnetic field. Slip rings on the coil, touching stationary brushes, carry the output to the circuit without twisting the wires. (Or a magnet spins inside a fixed coil.)

The e.m.f.-time graph is a repeating wave. It peaks (or troughs) when the coil is parallel to the field, as its sides cut field lines fastest, and falls to zero when the coil is at right angles to the field. Turning faster raises the peaks and shortens the period.

Magnetic effect of a current

A straight wire has circular field lines centred on it, spaced further apart farther out. With your right hand, thumb along the conventional current, your fingers curl in the field direction. A solenoid has a field like a bar magnet’s; fingers curled the way the current goes round, the thumb points to the N end. A bigger current strengthens the field; reversing it reverses the field.

A relay uses a small current in an electromagnet to pull an iron armature and close contacts in a separate, larger-current circuit. A loudspeaker has a coil fixed to a cone in the field of a permanent magnet; a varying current gives a varying force, so the cone vibrates and makes sound.

Forces on conductors and the motor

A wire carrying current across a magnetic field feels a force. A loose wire between magnet poles moves when the switch closes. Swap the current’s direction or the field’s and the force flips; swap both and it stays the same.

Fleming’s left-hand rule (left hand, thumb and two fingers at right angles): first finger shows field, second shows current, thumb shows force.

Each of two parallel wires sits in the other’s field. Currents in the same direction attract, because between the wires the fields cancel and each wire is pushed into the weaker field; opposite currents repel, because between them the fields point the same way and are strong.

A coil in a field has a turning effect that increases with more turns, a bigger current and a stronger field. In the d.c. motor, brushes press on a split-ring commutator on the coil. Each half turn it swaps contacts, reversing the coil current so the coil keeps turning the same way.

The transformer

An iron-cored transformer has a primary and secondary coil on a soft-iron core. A.c. in the primary sets up a varying field in the core, and this induces an e.m.f. in the secondary. D.c. gives no changing field, so no output. Step-up means more secondary turns than primary; step-down means fewer. Vp ÷ Vs = Np ÷ Ns.

Worked example 6. A 1150-turn primary on 230 V must give 12 V: Ns = 1150 × 12 ÷ 230 = 60 turns (step-down).

High-voltage transmission. For a given power I = P ÷ V, so raising V lowers the current and the cable p.d.

Worked example 7. 20 kW is sent along cables of resistance 0.80 Ω. At 400 V the current is 50 A, the cable p.d. is 40 V, and the loss is 50 × 40 = 2000 W. At 4000 V the current is 5.0 A, the p.d. is 4.0 V and the loss is 20 W: ten times the voltage cut the loss a hundredfold.

4.6 Uses of an oscilloscope

An oscilloscope displays a waveform as a trace. The Y-gain sets the p.d. per vertical division and the timebase sets the time per horizontal division.

Worked example 8. Y-gain 2.0 V/div, timebase 5.0 ms/div. A wave peaks 3.0 divisions above the centre line and one cycle spans 4.0 divisions. Peak p.d. = 3.0 × 2.0 = 6.0 V; period = 4.0 × 5.0 = 20 ms.

Frequently asked questions

What does Section 4 Electricity and magnetism cover in O Level Physics 5054? It covers magnetism and magnetic fields, electrical quantities (charge, current, e.m.f., p.d. and resistance), electric circuits, practical electricity and safety, electromagnetic effects including induction, motors and transformers, and uses of an oscilloscope.

Why must the fuse and switch be in the live wire? With the fuse or switch in the live wire, the appliance is disconnected from the high voltage when the fuse melts or the switch is off. Putting either in the neutral wire would leave the appliance live.

What is the difference between e.m.f. and p.d.? E.m.f. is the work done by a source moving unit charge round a complete circuit, while p.d. is the work done by unit charge passing through a component. Both are measured in volts, where 1 V = 1 J/C.

What is the difference between earthing and double insulation? Earthing gives fault current a path to ground, so the fuse melts or trip switch operates and the metal case is safe. Double insulation uses a non-conducting case, so the case cannot become live in the first place.

Official syllabus

Cambridge O Level Physics 5054 syllabus for 2026, 2027 and 2028, Cambridge International, Section 4: Electricity and magnetism.