Movement and Interactions: Electricity — AQA Combined Science: Synergy
This topic covers current, resistance and potential difference, circuits, mains electricity, power and the National Grid.
Electric current
Current (I) is the rate of flow of charge, measured in amperes (A): $$Q = I \times t$$ (charge in coulombs = current × time). A circuit needs a complete loop and a source of potential difference.
Current, resistance and potential difference
- Potential difference (V) is the energy transferred per unit charge (volts).
- Resistance (R) opposes current (ohms).
Ohm's law: $V = I \times R$.
For an ohmic conductor at constant temperature, current is directly proportional to potential difference (straight-line I–V graph). A filament lamp's resistance increases as it heats (curved graph); a diode allows current one way only.
Series and parallel circuits
- Series: current the same everywhere; potential difference shared; total resistance = sum of resistances.
- Parallel: potential difference the same across each branch; current shared; total resistance is less than the smallest resistor.
Circuit elements
Thermistor — resistance falls as temperature rises (temperature sensors). LDR — resistance falls as light increases (light sensors).
Required practical: investigating how the resistance of a wire varies with length, and the I–V characteristics of components.
Direct and alternating currents
- Direct current (d.c.) flows in one direction (from cells/batteries).
- Alternating current (a.c.) repeatedly reverses direction. UK mains is a.c., 230 V, 50 Hz.
Mains cables
A three-core cable: live (brown) carries ~230 V (dangerous), neutral (blue) completes the circuit, earth (green/yellow) is a safety wire that stops the casing becoming live. A fuse melts if too much current flows. Touching the live wire is dangerous because of the large potential difference between it and earth.
Power
Power is the rate of energy transfer: $$P = V \times I \qquad P = I^2 \times R$$ Energy transferred: $E = P \times t$ and $E = Q \times V$.
Power and domestic appliances
Appliances transfer energy from the mains to useful forms; the energy transferred depends on power and time switched on. The power rating tells you how much energy is transferred per second.
The National Grid
Step-up transformers raise the voltage for transmission; step-down transformers lower it for use. High voltage means lower current, reducing energy lost as heat in the cables, so transmission is efficient.
Exam tips
- Know what stays the same and what is shared in series vs parallel.
- Learn V = IR, Q = It, P = VI and E = Pt, and practise rearranging.
- Learn the three wires by colour and the role of the fuse and earth wire.
- Explain why the National Grid uses high voltage (lower current → less energy wasted).