Electricity
What is tested
This domain tests current, potential difference, Ohm's law, resistance, series and parallel circuits, heating effect, electric power and domestic wiring. Numerical questions often combine resistance and power.
Core rules
- Current is rate of flow of charge; potential difference is work done per unit charge. Ohm's law: V = IR at constant temperature.
- Resistance depends on length, area of cross-section, material and temperature.
- Series: R = R1 + R2 + ...; current is the same. Parallel: 1/R = 1/R1 + 1/R2 + ...; voltage is the same.
- Heating effect: H = I^2Rt. Electric power: P = VI = I^2R = V^2/R. Energy billed in kWh.
- In domestic circuits, live, neutral and earth wires are used; fuses and earthing are safety measures. Ammeter is connected in series and voltmeter in parallel.
Worked example
Resistors 6 Ω and 3 Ω are connected in parallel across 6 V. 1/Rp = 1/6 + 1/3 = 1/2, so Rp = 2 Ω. Total current I = V/Rp = 6/2 = 3 A. Power P = VI = 6 × 3 = 18 W. The 6 Ω branch carries 1 A and the 3 Ω branch carries 2 A.
Traps
Parallel resistors have the same voltage, not the same current. Ammeter has very low resistance; voltmeter has very high resistance. Do not mix up kWh and joule. In series, current is common.