Electricity and magnetism: current, voltage, resistance, circuits and magnetic effects
What this domain tests
Electricity questions ask you to use current, voltage and resistance in circuits, apply Ohm's law, combine series and parallel resistors, calculate power and energy consumption, and explain household safety. Magnetism covers poles and fields, electromagnets, the motor effect and electromagnetic induction.
Core rules
- Current I = Q/t, in amperes; it is the same everywhere in a series loop. An ammeter is connected in series and has very low resistance.
- Voltage U = W/Q, in volts: the energy transferred per unit of charge. A voltmeter is connected in parallel and has very high resistance.
- Ohm's law: R = U/I, valid for a metallic conductor at constant temperature. Resistance grows when the wire is longer, thinner or hotter.
- Series: the current is common, voltages add, R = R₁ + R₂ + …
- Parallel: the voltage is common, currents add, 1/R = 1/R₁ + 1/R₂ + …; the total is always smaller than the smallest branch resistance.
- Power and energy: P = UI = I²R = U²/R, and W = Pt. The kilowatt-hour is an energy unit: 1 kWh = 3.6 MJ.
- Mains: 230 V at 50 Hz. The fuse or circuit breaker sits in the live wire; earthing gives a fault current a safe path; wet hands dangerously lower the body's resistance.
- Magnetism: unlike poles attract, external field lines run from N to S; a current creates a circular field around a wire and a strong field inside a coil; a current-carrying coil in a magnetic field turns (motor), and a changing field induces a voltage (generator).
Worked example
Resistors of 4 Ω and 12 Ω are connected in parallel to a 6.0 V supply. Find the total resistance, the total current and the branch currents.
1/R = 1/4 + 1/12 = 3/12 + 1/12 = 4/12, so R = 3 Ω. I = U/R = 6.0/3 = 2.0 A. Branch currents: 6.0/4 = 1.5 A and 6.0/12 = 0.50 A; they add to 2.0 A, which checks the result. Total power P = UI = 6.0 × 2.0 = 12 W.
Traps that cost marks
- Adding parallel resistances directly instead of adding their reciprocals.
- Using the supply voltage across one resistor of a series circuit.
- Confusing kW (power) with kWh (energy) on an electricity bill.
- Forgetting to convert mA to A (divide by 1000) or minutes to hours.
- Believing resistance grows simply because the voltage grows - for an ohmic resistor at constant temperature it does not.
- Thinking current is used up as it travels round a series circuit.