IGCSE Physics: magnetism and electromagnetism

Key ideas, board by board

Magnets: like poles repel and unlike poles attract; magnets attract magnetic materials such as iron, steel, nickel and cobalt. Magnetism can be induced in a magnetic material placed in a field. Soft iron magnetises and demagnetises easily, so it is used for temporary magnets and electromagnet cores; steel keeps its magnetism, so it is used for permanent magnets. Field lines run from north to south outside a magnet, and the closer the lines, the stronger the field. Both boards expect you to plot a field with a compass or iron filings. Cambridge Supplement adds that magnetic forces come from interacting fields and that line spacing shows field strength. Edexcel 6.7 asks how two magnets produce a uniform field.

Electromagnetism: Cambridge Core covers field patterns around a straight wire and a solenoid, relays and loudspeakers, the force on a current-carrying conductor (reversed by reversing either current or field), and the factors that increase a motor's turning effect (more turns, more current, stronger field). Supplement adds the left-hand rule for relative directions, deflection of charged particle beams, and the split-ring commutator. Edexcel examines the motor effect, loudspeakers and the left-hand rule on both papers (6.12 to 6.14), but electromagnet construction, field patterns for wire, coil and solenoid, and forces on moving charged particles are 6.9P to 6.11P (Paper 2 only).

Induction: a voltage (e.m.f.) is induced when a conductor cuts magnetic field lines or the field through a coil changes. It is bigger with a stronger magnet, faster movement and more turns. Cambridge Supplement adds that the induced e.m.f. opposes the change causing it, and the a.c. generator with slip rings and e.m.f.-time graphs. Edexcel covers induction and generators (6.15, 6.16) on both papers. Transformers: in Cambridge the construction, step-up and step-down, the turns equation and high-voltage transmission are Core, while the operating principle, IpVp = IsVs and P = I^2 R are Supplement. In Edexcel all transformer content is 6.17P to 6.20P.

The three left- and right-hand ideas

Neither syllabus requires the name "Fleming's right-hand rule"; Cambridge asks you to state and use the relative directions of force, field and induced current.

Worked example 1: transformer

Question: a transformer steps 230 V down to 11.5 V. The primary coil has 2000 turns. (a) How many turns on the secondary? (b) The output current is 2.0 A. Assuming 100% efficiency, find the primary current.

(a) Vp / Vs = Np / Ns, so Ns = Np x Vs / Vp = 2000 x 11.5 ÷ 230 = 100 turns. It is a step-down transformer because Ns is less than Np.

(b) Vp x Ip = Vs x Is, so Ip = (11.5 x 2.0) ÷ 230 = 23 ÷ 230 = 0.10 A. The power on each side is 23 W. Stepping the voltage down steps the current up by the same factor.

Worked example 2: why we transmit at high voltage (Cambridge Supplement)

Question: a power station sends 1.0 MW through cables of total resistance 5.0 ohms. Compare the power wasted at 10 kV and at 400 kV.

At 10 kV: I = P / V = 1 000 000 ÷ 10 000 = 100 A. Power lost = I^2 R = 100^2 x 5.0 = 50 000 W, which is 5% of the power sent.

At 400 kV: I = 1 000 000 ÷ 400 000 = 2.5 A. Power lost = 2.5^2 x 5.0 = 31 W (to 2 significant figures). Raising the voltage 40 times cuts the current 40 times and the losses 1600 times. Step-down transformers then reduce the voltage to a safe level for homes.

Common mistakes

• Saying all metals are magnetic. Aluminium and copper are not.
• Drawing field lines that cross, or arrows going from south to north outside a magnet.
• Mixing up the left-hand rule (motor) with the right-hand grip rule (field around a wire).
• Saying a transformer works with direct current. It needs a changing (alternating) current to produce a changing field in the core.
• Writing that a step-up transformer increases power. It increases voltage and decreases current; power out cannot exceed power in.
• Saying an induced voltage appears when a magnet is held still inside a coil. There must be relative movement or a changing field.
• Confusing slip rings (a.c. generator) with a split-ring commutator (d.c. motor).

Exam technique and mark-scheme language

For "explain how a transformer works" the chain is: alternating current in the primary coil produces a changing magnetic field in the soft-iron core; the changing field passes through the secondary coil; this induces an alternating voltage in the secondary. Missing "changing" or "alternating" usually costs the first mark.

For motors and loudspeakers, link the force to the field: the current in the coil creates a magnetic field that interacts with the permanent magnet's field, producing a force. In a loudspeaker, the alternating current reverses the force, so the cone vibrates at the frequency of the current. For induction factors, give the three standard ones: strength of the magnet, speed of movement and number of turns.

How one-to-one lessons help with this topic

Magnetism is visual and three-dimensional, which is hard to follow from a flat textbook diagram. A tutor uses the shared whiteboard to sketch field patterns and practise the left-hand rule step by step, then drills the transformer calculations and explanation chains until the student writes them in the order examiners expect. For Edexcel students, the tutor makes sure the Paper 2 transformer content is not left until the last weeks.

Self-check

1. Describe the difference between soft iron and steel as magnetic materials.
2. Draw field lines around a bar magnet, a straight wire and a solenoid.
3. Describe how to plot a field with a plotting compass.
4. Use Fleming's left-hand rule to find the direction of a force.
5. List three ways to increase the turning effect of a motor and the induced e.m.f. of a generator.
6. Explain how a transformer works.
7. Use Vp / Vs = Np / Ns and Vp Ip = Vs Is.
8. Explain why electricity is transmitted at high voltage.