Cambridge IGCSE Physics 4 Electricity and Magnetism Exam Questions

About Cambridge IGCSE Physics 4 Electricity and Magnetism

Electricity and magnetism in Cambridge IGCSE Physics 0625 covers simple magnetism, electrical quantities, electric circuits, electrical safety and electromagnetic effects. You need to work with charge, current, electromotive force, potential difference, resistance, energy and power, analyse series and parallel circuits including sensors and potential dividers, and explain fuses, circuit breakers and earthing. Electromagnetic effects include induction, the a.c. generator, the magnetic effect of a current, the d.c. motor and the transformer. Extended candidates add further equations and Fleming's rules. Examiners test circuit calculations heavily, along with practical work on resistance and explanations of why electricity is transmitted at high voltage.

What you need to know

• 4.1 Magnetism Like poles repel and unlike poles attract. Magnetic materials can be magnetised by induction. Soft iron is used for temporary magnets (electromagnet cores) and steel for permanent magnets. Field lines go from north to south and are closer where the field is stronger.
• 4.2 Charge and current Objects become charged by friction as electrons are transferred. Current is the rate of flow of charge, I = Q / t, measured with an ammeter in series. Conventional current flows from positive to negative, opposite to electron flow.
• 4.2 E.m.f., p.d. and resistance Potential difference is measured with a voltmeter in parallel. Resistance R = V / I. A fixed resistor at constant temperature has a straight-line I-V graph through the origin; a filament lamp's resistance increases as it gets hotter. Resistance increases with length and decreases with cross-sectional area.
• 4.2 Energy and power P = IV and E = IVt. The kilowatt-hour is a unit of energy used to calculate the cost of electricity.
• 4.3 Circuits In series, the current is the same everywhere and the p.d. is shared; total resistance R = R1 + R2. In parallel, the current divides and the combined resistance is less than any branch (Extended: 1/R = 1/R1 + 1/R2). Learn thermistors, light-dependent resistors, diodes, LEDs and potential dividers.
• 4.4 Electrical safety Hazards include damaged insulation, overheating cables and damp conditions. Fuses and circuit breakers break the circuit if the current is too high; earthing and double insulation protect users from a live metal case.
• 4.5 Induction and generators A changing magnetic field linked with a conductor induces an e.m.f.; it is larger with a stronger magnet, faster motion or more turns. An a.c. generator uses a rotating coil with slip rings. Extended candidates use Fleming's right-hand rule.
• 4.5 Motors and transformers A current-carrying conductor in a magnetic field experiences a force (Fleming's left-hand rule); a d.c. motor uses a split-ring commutator. Transformer: Vp / Vs = Np / Ns. High voltage transmission reduces current and so reduces power loss in cables (Extended: P = I^2 R and IpVp = IsVs).

Common mistakes in 4 Electricity and Magnetism

• Connecting a voltmeter in series or an ammeter in parallel. Ammeters go in series; voltmeters go in parallel across the component.
• Saying current is used up in a circuit. Current is the same at every point in a series circuit; energy is transferred, not current.
• Adding resistances in parallel as if they were in series. The combined resistance of parallel resistors is always less than the smallest one.
• Saying a transformer works on direct current. A changing magnetic field is needed, so it only works with alternating current.
• Saying the fuse protects the user. The fuse protects the wiring and appliance; the earth wire together with the fuse protects the user.

Try three 4 Electricity and Magnetism exam-style questions