Electricity
What the exam tests
Electricity tests current, potential difference, Ohm's law, resistance and resistivity, series and parallel circuits, the heating effect, electric power and domestic wiring. Numerical problems on equivalent resistance and energy consumption are standard.
Core rules
- Current I = Q/t, measured in amperes by an ammeter connected in series.
- Potential difference V = W/Q, measured in volts by a voltmeter connected in parallel.
- Ohm's law: V = IR, provided temperature is constant.
- Resistance R = ρl/A: it increases with length and resistivity and decreases with cross-sectional area.
- Series: same current; R = R1 + R2 + R3. Parallel: same voltage; 1/R = 1/R1 + 1/R2 + 1/R3.
- Heating effect: H = I2Rt. Power P = VI = I2R = V2/R.
- Electrical energy is sold in kWh: 1 kWh = 3.6 × 106 J.
- A fuse is a low-melting-point wire in series with the live wire; it melts on overload.
Worked example
A 6 Ω and a 3 Ω resistor are connected in parallel across a 6 V battery. Find the total current. Parallel resistance: 1/R = 1/6 + 1/3 = 1/2, so R = 2 Ω. Total current I = V/R = 6/2 = 3 A. The branch currents are 6/6 = 1 A and 6/3 = 2 A, which add to 3 A. This checks the junction rule.
Traps that cost marks
- Connecting an ammeter in parallel or a voltmeter in series.
- Adding resistances directly in a parallel circuit.
- Saying kWh is a unit of power; it is a unit of energy.
- Forgetting that the equivalent resistance of a parallel combination is less than the smallest resistor.