Electricity — AQA Combined Science: Trilogy
Electricity covers circuits, current, potential difference, resistance, mains electricity and the National Grid.
Current, potential difference and resistance
- Current (I) is the rate of flow of electric charge, measured in amperes (A). Current is the same everywhere in a series circuit.
- Potential difference (V), or voltage, is the energy transferred per unit charge, measured in volts (V).
- Resistance (R) opposes the flow of current, measured in ohms (Ω).
Charge equation: $$Q = I \times t$$ (charge in coulombs = current × time)
Ohm's law: $$V = I \times R$$
For a component to work there must be a closed circuit and a source of potential difference (e.g. a cell).
Circuit symbols and diagrams
You must recognise standard symbols: cell, battery, switch, lamp, resistor, variable resistor, ammeter (in series), voltmeter (in parallel), LED, diode, LDR, thermistor and fuse.
I–V characteristics
- Fixed resistor (at constant temperature): current is directly proportional to potential difference — the graph is a straight line through the origin (it is an ohmic conductor).
- Filament lamp: as current increases, the filament gets hotter and resistance increases, so the line curves (an S-shape).
- Diode: current flows in one direction only; very high resistance in reverse.
Required practical: investigating how the resistance of a wire depends on its length, and the I–V characteristics of components.
Special resistors
- Thermistor — resistance decreases as temperature increases. Used in thermostats and temperature sensors.
- Light-dependent resistor (LDR) — resistance decreases as light intensity increases. Used in automatic lighting.
Series and parallel circuits
Series circuits:
- Current is the same through every component.
- The total potential difference is shared between components.
- Total resistance is the sum of the individual resistances (R = R₁ + R₂ + …).
Parallel circuits:
- Potential difference is the same across each branch.
- Current is shared between the branches.
- The total resistance is less than the smallest individual resistance (adding a resistor in parallel provides more paths for the current).
Energy and power in circuits
Power is the rate of energy transfer: $$P = V \times I \qquad P = I^2 \times R$$
Energy transferred: $$E = P \times t \qquad E = Q \times V$$
Everyday appliances transfer energy from the mains to useful forms (e.g. kinetic, thermal). The amount transferred depends on the power and the time it is switched on.
Mains electricity
- UK mains is alternating current (a.c.) at 230 V and a frequency of 50 Hz. Direct current (d.c.) from cells flows in one direction only.
- A three-core cable has three wires:
- Live (brown) — carries the alternating potential difference (~230 V); dangerous.
- Neutral (blue) — completes the circuit (near 0 V).
- Earth (green and yellow) — a safety wire that stops the appliance casing becoming live.
- A fuse (or circuit breaker) melts and breaks the circuit if too much current flows, protecting the appliance and preventing fire. The earth wire and fuse work together: a fault sends a large current to earth, blowing the fuse.
Touching the live wire is dangerous because there is a large potential difference between it and your body (which is at earth potential), so a current flows through you.
The National Grid
The National Grid carries electricity from power stations to consumers.
- Step-up transformers increase the potential difference (voltage) for transmission.
- Step-down transformers decrease it again for safe use in homes.
High voltage means a lower current for the same power, which reduces energy lost as heat in the cables, making transmission efficient.
Exam tips
- Know which quantities stay the same and which are shared in series vs parallel circuits.
- Learn V = IR, Q = It, P = VI and E = Pt, and practise rearranging them.
- Be able to describe the shape and reason for the filament lamp I–V curve (resistance increases as it heats).
- Learn the three wires by colour and the role of the fuse and earth wire in safety.
- Explain why the National Grid uses high voltage (lower current → less energy wasted as heat).