Understanding Electricity and magnetism: IGCSE Physics Explained

Electricity and magnetism in global perspective

From the high-voltage transmission lines crossing the Swiss Alps to the undersea cables linking continents beneath the Atlantic, electricity and magnetism shape the infrastructure of modern civilisation. The principles you'll study in IGCSE Physics (0625) are the same ones that engineers in Tokyo, Nairobi, and São Paulo apply daily to design power grids, medical scanners, and electric vehicles. This section of the syllabus is one of the largest and most frequently examined, and for good reason: it connects fundamental physics to the technologies that define how billions of people live.

The topic divides into five interconnected strands: magnetism, electrical quantities, electric circuits, electrical safety, and electromagnetic effects. Each strand builds on the previous, and the Cambridge exam rewards candidates who can trace those connections fluently.

Key equations at a glance

Simple phenomena of magnetism

Magnetism begins with a deceptively simple observation: certain materials attract or repel one another without physical contact. A bar magnet has two poles, designated north (N) and south (S). Like poles repel; unlike poles attract. This rule, familiar to anyone who has played with fridge magnets, underpins every electromagnetic device from a compass needle aligning with the Earth's field to the steering magnets inside a particle accelerator at CERN.

• Magnetic materials: iron, steel, nickel, and cobalt respond to magnetic fields. Most other metals (copper, aluminium, gold) do not.
• Induced magnetism: placing an unmagnetised magnetic material near a magnet causes it to become temporarily magnetised. The near end acquires the opposite polarity to the pole it faces, which is why attraction always occurs first.
• Permanent vs temporary magnets: steel retains magnetism after the external field is removed (permanent magnet). Soft iron loses its magnetism quickly (temporary magnet). This distinction matters in applications: a compass needle must be permanent, while the core of an electromagnet must be temporary so it can be switched on and off.

A magnetic field is the region around a magnet where a magnetic pole experiences a force. Field lines run from north to south outside the magnet, never cross, and are closest together where the field is strongest. The Earth itself acts as a giant magnet, its geographic north pole sitting near its magnetic south pole, which is why the north-seeking end of a compass points roughly northward.

Electrical quantities

Electricity, at its core, is the movement of charged particles. In metallic conductors, those particles are free electrons drifting through a lattice of positive ions. Two quantities govern every circuit: current and potential difference.

Charge and current

Electric charge is measured in coulombs (C). Current, the rate of flow of charge, is defined by I = Q / t, where I is in amperes, Q in coulombs, and t in seconds. One ampere means one coulomb of charge passes a point each second. Conventional current flows from positive to negative in the external circuit, opposite to the actual electron flow. Both conventions appear in IGCSE questions, so knowing which is which prevents confusion.

Electromotive force and potential difference

A cell or battery provides the energy that drives charge around a circuit. The energy per unit charge supplied by the source is called the electromotive force (e.m.f.), while the energy per unit charge converted in a component is the potential difference (p.d.). Both are measured in volts, but they describe different things: e.m.f. is the total energy input, p.d. is the energy output at a specific component. The relationship V = W / Q connects them, where W is the energy transferred in joules.

Resistance and Ohm's law

Resistance measures how much a component opposes current. Ohm's law states that, for an ohmic conductor at constant temperature, V = IR, where R is resistance in ohms. Not all components obey this linear relationship. A filament lamp, for instance, increases in resistance as it heats up: its I-V graph curves. A diode conducts in one direction only, showing negligible current for reverse bias and a steep rise in current beyond the threshold voltage in forward bias. Exam questions frequently ask candidates to sketch or interpret these characteristic graphs.

Electric circuits

Circuit analysis is the practical heart of this topic, and the Cambridge exam tests it relentlessly. The rules for series and parallel circuits differ in ways that reward careful, systematic thinking.

The key insight: in series, current is the conserved quantity and voltage is shared. In parallel, voltage is the conserved quantity and current is shared. Mixing these up is the single most common error candidates make in circuit questions.

Worked example 1: Series circuit

A 12 V battery is connected in series with a 4 Ω resistor and an 8 Ω resistor. Find the current and the p.d. across each resistor.

1. Total resistance: R = 4 + 8 = 12 Ω
2. Current: I = V / R = 12 / 12 = 1.0 A (same through both resistors)
3. P.d. across 4 Ω resistor: V = IR = 1.0 x 4 = 4.0 V
4. P.d. across 8 Ω resistor: V = IR = 1.0 x 8 = 8.0 V
5. Check: 4.0 + 8.0 = 12 V, which matches the battery e.m.f.

Worked example 2: Parallel circuit

A 6 V battery is connected across two resistors in parallel: 3 Ω and 6 Ω. Find the current through each resistor and the total current.

1. P.d. across each resistor = 6 V (same in parallel)
2. Current through 3 Ω: I = V / R = 6 / 3 = 2.0 A
3. Current through 6 Ω: I = V / R = 6 / 6 = 1.0 A
4. Total current: 2.0 + 1.0 = 3.0 A
5. Combined resistance: 1/R = 1/3 + 1/6 = 3/6 = 1/2, so R = 2 Ω. Check: I = 6/2 = 3.0 A. Consistent.

Electrical energy and power

Every electrical component converts energy. A lamp converts electrical energy to light and thermal energy. A motor converts it to kinetic energy. The rate of energy conversion is power, measured in watts.

• Energy transferred: E = IVt (current x voltage x time)
• Power: P = IV. Substituting Ohm's law gives two alternative forms: P = I²R and P = V²/R

The three forms of the power equation each suit different problem types. Use P = IV when both current and voltage are given. Use P = I²R when you know the current and resistance but not the voltage. Use P = V²/R when you know the voltage and resistance but not the current. Choosing the right form saves time and avoids unnecessary intermediate calculations.

Worked example 3: Electrical energy

A 230 V kettle draws a current of 10 A and takes 3 minutes to boil water. Calculate the energy transferred.

1. Convert time: 3 minutes = 180 s
2. E = IVt = 10 x 230 x 180 = 414 000 J = 414 kJ

Electrical safety

Household wiring carries lethal voltages. The safety mechanisms tested in IGCSE Physics reflect engineering standards used from European ring mains to North American split-phase systems, though the specific voltages differ by country.

• Fuses: thin wire that melts when current exceeds its rating, breaking the circuit. Always connected in the live wire. The fuse rating should be slightly above the normal operating current of the appliance.
• Circuit breakers: electromagnetic switches that trip when current is too high. Faster than fuses, resettable, and increasingly standard worldwide.
• Earthing: the metal case of an appliance is connected to earth via a third wire. If a live wire touches the case, a large current flows to earth, blowing the fuse or tripping the breaker before anyone receives a shock.
• Double insulation: appliances with plastic casings need no earth wire because the user can never touch a conducting surface. Marked with a double-square symbol.

The three-pin plug wiring is a favourite exam question. The live wire (brown) carries the alternating current. The neutral wire (blue) completes the circuit. The earth wire (green and yellow striped) is a safety path. The fuse sits in the live wire, never the neutral, because cutting the live wire removes the dangerous voltage from the appliance.

Electromagnetic effects

The deep connection between electricity and magnetism, first demonstrated by Hans Christian Oersted in Copenhagen in 1820 and formalised by Michael Faraday in London shortly after, remains one of the most elegant unifications in physics. A current-carrying conductor produces a magnetic field; a changing magnetic field induces a current. These two principles generate the entire technology of electric motors, generators, and transformers.

Electromagnets

A solenoid, a coil of wire carrying current, produces a magnetic field similar to that of a bar magnet. Inserting a soft iron core dramatically strengthens the field. The strength of an electromagnet increases with more turns, greater current, or a core with higher magnetic permeability. Electromagnets power devices from hospital MRI machines in Seoul to scrapyard cranes in Rotterdam, and the relay, an electromagnet-operated switch, allows a small current in one circuit to control a much larger current in another.

The DC motor

When a current-carrying coil sits in a magnetic field, it experiences a turning force. The motor effect, described by Fleming's left-hand rule (thumb = motion, first finger = field, second finger = current), converts electrical energy into rotational kinetic energy. A split-ring commutator reverses the current direction every half turn, keeping the coil spinning continuously in the same direction. The motor spins faster with stronger current, more turns on the coil, or a stronger magnetic field.

Electromagnetic induction

Faraday's discovery runs in reverse: moving a conductor through a magnetic field, or changing the magnetic field through a coil, induces an e.m.f. (and a current if the circuit is complete). The induced e.m.f. increases with faster movement, a stronger field, or more turns on the coil. Lenz's law states that the direction of the induced current opposes the change that produced it, a consequence of energy conservation.

The AC generator

A coil rotating in a magnetic field produces an alternating e.m.f. that varies sinusoidally. Slip rings and brushes maintain continuous contact with the external circuit. The output reverses polarity each half rotation, producing alternating current. The generator is the motor in reverse: mechanical energy in, electrical energy out. Power stations from hydroelectric dams in Brazil to wind farms in Denmark all rely on this principle.

Transformers

A transformer consists of two coils (primary and secondary) wound on a shared soft iron core. An alternating current in the primary coil produces a changing magnetic field in the core, which induces an alternating e.m.f. in the secondary coil. The voltage ratio follows the turns ratio:

V_p / V_s = N_p / N_s

A step-up transformer (N_s > N_p) increases voltage; a step-down transformer (N_s < N_p) decreases it. For an ideal transformer with no energy losses, power in equals power out: V_pI_p = V_sI_s. This means stepping up the voltage steps down the current proportionally, which is precisely why national grids transmit electricity at very high voltages: lower current means less energy wasted as heat in the cables (P = I²R).

Worked example 4: Transformer

A step-down transformer has 4600 turns on the primary coil and 200 turns on the secondary. The input voltage is 23 000 V. Calculate the output voltage and, assuming the transformer is ideal with an input current of 2 A, find the output current.

1. V_s = V_p x N_s / N_p = 23 000 x 200 / 4600 = 1000 V
2. For an ideal transformer: V_pI_p = V_sI_s
3. I_s = V_pI_p / V_s = 23 000 x 2 / 1000 = 46 A

The voltage dropped by a factor of 23, so the current increased by a factor of 23. Power is conserved: 23 000 x 2 = 46 000 W = 1000 x 46.

Common mistakes and how to avoid them

Self-check questions

Work through each problem fully before checking the answers.

1. A charge of 450 C flows through a lamp in 3 minutes. Calculate the current.
2. Three resistors of 2 Ω, 3 Ω, and 6 Ω are connected in parallel across a 12 V supply. Find the total resistance and the total current drawn from the supply.
3. A 2300 W electric heater is connected to a 230 V supply. What is the current, and which standard fuse (3 A, 5 A, or 13 A) should be used?
4. A transformer steps 11 500 V down to 230 V. The primary coil has 5000 turns. How many turns does the secondary coil have?
5. Explain why the core of an electromagnet is made from soft iron rather than steel.

Exam strategy for electricity and magnetism

This section of the IGCSE Physics syllabus carries substantial weight across Papers 2, 4, and 6. Circuit calculation questions appear on nearly every sitting, making fluency with V = IR, P = IV, and the series/parallel rules non-negotiable. Draw circuit diagrams clearly, label all values, and work methodically through each branch or loop.

For qualitative questions on electromagnetic effects, examiners reward precise language. Don't write "the wire moves" when you mean "the wire experiences a force in the direction given by Fleming's left-hand rule." Don't write "the magnet makes electricity" when you mean "the changing magnetic flux through the coil induces an e.m.f." Each technical term carries marks.

Practical questions (Papers 5 and 6) frequently involve circuit assembly, ammeter and voltmeter placement, and I-V characteristic experiments. Remember: ammeters go in series with the component being measured; voltmeters go in parallel across it. Swapping them is a common practical error that yields nonsensical readings and costs marks in the planning and evaluation sections alike.