Electricity and Magnetism
What this domain tests
Charge, current, potential difference, resistance, Ohm's law, series and parallel circuits, electrical power, and the magnetic effects of currents including the motor effect and electromagnetic induction.
Core rules
- Q = It; V = IR for a conductor at constant temperature; R = ρL/A.
- Series: same current, voltages add, R = R₁ + R₂. Parallel: same voltage, currents add, 1/R = 1/R₁ + 1/R₂.
- Power P = VI = I²R = V²/R; energy in kWh = power in kW × time in hours.
- EMF is energy per unit charge from the source; terminal p.d. V = E − Ir with internal resistance r.
- Force on a current-carrying wire F = BIL sinθ, direction by Fleming's left-hand rule.
- Changing magnetic flux induces an EMF (Faraday); transformers obey V₁/V₂ = N₁/N₂.
Worked example
A 12 V supply of negligible internal resistance feeds a 6 Ω and a 3 Ω resistor in parallel, in series with a 4 Ω resistor. Parallel part: 1/Rp = 1/6 + 1/3 = 3/6, so Rp = 2 Ω. Total R = 2 + 4 = 6 Ω, hence I = 12/6 = 2 A. Voltage across the parallel pair = 2 × 2 = 4 V, and the current splits 0.67 A through 6 Ω and 1.33 A through 3 Ω - inversely proportional to resistance.
Traps that cost marks
- Adding parallel resistances directly instead of using reciprocals.
- Placing an ammeter in parallel or a voltmeter in series.
- Confusing kWh (energy) with kW (power).
- Ignoring internal resistance and mis-stating terminal p.d.
- Reversing the transformer turns ratio.