How to Revise Topic 4: Electricity and Magnetism

Topic 4 is tied with Topic 1 for the highest mark weighting in IGCSE Physics. It is calculation-heavy and requires strong circuit analysis skills.

What Topic 4 covers

1. Magnetism (magnetic materials, fields, electromagnets)
2. Electrical quantities (charge, current, voltage, resistance, e.m.f.)
3. Electric circuits (series and parallel, V-I characteristics, circuit components)
4. Electrical safety (fuses, earthing, circuit breakers, power and energy)
5. Electromagnetic effects (electromagnetic induction, transformers, motors, generators)

Priority areas

Must-know calculations

• V = IR (Ohm’s Law)
• P = IV, P = I^2R, P = V^2/R (electrical power)
• E = Pt or E = IVt (energy transferred)
• Q = It (charge)
• Total resistance in series: R_total = R1 + R2 + R3
• Total resistance in parallel: 1/R_total = 1/R1 + 1/R2
• Transformer equation: V1/V2 = N1/N2
• Cost of electricity = power (kW) x time (hours) x cost per kWh

Must-know circuit rules

• In series: current is the same everywhere; voltage splits across components; total resistance is the sum
• In parallel: voltage is the same across each branch; current splits at junctions; total resistance is less than the smallest individual resistance
• Ammeter in series, voltmeter in parallel

Must-know concepts

• The difference between current and voltage (current is charge flow; voltage is energy per charge)
• V-I characteristics of resistors, filament lamps and diodes
• How thermistors and LDRs work in circuits
• How a fuse and earth wire protect against electrical faults
• How electromagnetic induction works (Faraday’s Law)
• How a transformer works and why it only works with a.c.
• The motor effect and Fleming’s left-hand rule

Revision strategy for Topic 4

Week 1: Circuits and calculations

• Day 1: Current, voltage and resistance definitions. V = IR practice.
• Day 2: Series circuits: calculate current, voltage drops and total resistance.
• Day 3: Parallel circuits: calculate branch currents, total resistance.
• Day 4: Power and energy calculations. Cost of electricity.
• Day 5: Mixed circuit problems from past papers.

Week 2: Components, safety and electromagnetic effects

• Day 1: V-I characteristics (draw and interpret for resistor, lamp, diode). Thermistors and LDRs.
• Day 2: Electrical safety: fuses, earthing, circuit breakers. Calculate fuse ratings.
• Day 3: Magnetism: fields, electromagnets, motor effect, Fleming’s left-hand rule.
• Day 4: Electromagnetic induction, generators, transformers. Transformer calculations.
• Day 5: Mixed past paper questions on all of Topic 4.

Common mistakes in Topic 4

• Confusing series and parallel circuit rules (voltage splits in series, current splits in parallel).
• Connecting voltmeters in series (they go in parallel).
• Using the wrong power formula (use P = IV for simple circuits, P = I^2R when you have current and resistance but not voltage).
• Forgetting to convert kW to W or hours to seconds when mixing units.
• Confusing the motor effect with electromagnetic induction.
• Not knowing that transformers only work with a.c. (the changing current produces a changing magnetic field).

Self-test checklist

After revision, you should be able to:

• Apply V = IR to calculate any of the three variables.
• Calculate total resistance in series and parallel.
• Calculate power using P = IV, P = I^2R or P = V^2/R.
• Calculate the cost of running an appliance.
• Draw and interpret V-I characteristics for three components.
• Explain how a fuse and earth wire provide safety.
• Apply the transformer equation.
• Explain the motor effect and electromagnetic induction.