This mini-lesson walks you through the whole of AQA Topic 4.2 — Electricity: current, potential difference & resistance, the circuit symbols and I–V graphs, series vs parallel circuits, mains electricity & safety, the power and energy equations, and static electricity.
Work through each screen, answer the questions as you go (some are wordy, some are calculations) and collect ⭐ stars. Press Start when you're ready.
Electricity is invisible, so physicists picture a circuit as a loop of water pipes. It's not perfect, but it makes every quantity feel concrete:
The pump is the cell — it provides the push (the potential difference / voltage). The flow rate of water is the current. A narrow section is a resistor: it makes it harder for water to flow, so the flow (current) is smaller.
The key insight: the water is never used up — the same water keeps circulating. In the same way, charge (and current) is not used up going round a circuit. Energy is what gets transferred to the components.
Electric current is the rate of flow of electric charge. For a current to flow, the circuit must be complete (a closed loop) and there must be a source of potential difference (such as a cell) to push the charge.
Charge is measured in coulombs (C), current in amperes (A), time in seconds (s). The bigger the current, the more charge passes a point each second.
A current of 2 A flows for 30 s.
Q = I × t = 2 × 30 = 60 C
Potential difference (p.d. or voltage, in volts, V) is the energy transferred per unit charge — it is the "push" driving the current. Resistance (in ohms, Ω) opposes the flow of charge. They are linked by:
Rearrange it to find any quantity: I = V ÷ R and R = V ÷ I. A larger resistance means a smaller current for the same p.d.
A current of 3 A flows through a 4 Ω resistor.
V = I × R = 3 × 4 = 12 V
AQA expects you to recognise and draw these standard circuit symbols:
An I–V graph plots current against potential difference. For an ohmic conductor (a fixed resistor at constant temperature) the line is straight through the origin — resistance stays constant. Other components are non-ohmic:
Thermistor: its resistance falls as temperature rises. LDR: its resistance falls as light intensity increases. Both are used in sensing circuits.
Required practicals: investigate the I–V characteristics of a resistor, lamp and diode; and measure the resistance of a wire and of resistor combinations in series and parallel.
Series (one single loop): the current is the same everywhere; the p.d. is shared between the components; and the total resistance is the sum — R = R₁ + R₂ + … (adding a resistor always increases total resistance).
Parallel (separate branches): the p.d. is the same across each branch; the current is shared between branches; and the total resistance is LESS than the smallest individual resistance.
Why does adding a resistor in parallel lower the resistance? Think of checkout lanes at a supermarket: opening a second lane gives shoppers another route, so the queue clears faster overall — even though each lane is no wider. Extra parallel paths make it easier for charge to flow, so total resistance drops. (A classic exam trap: students wrongly add parallel resistances.)
Resistors of 3 Ω, 4 Ω and 5 Ω in series.
R = 3 + 4 + 5 = 12 Ω
Each statement describes a circuit. Tap whether it is true of a series or a parallel circuit.
Direct current (d.c.) flows in one direction only — this is what a cell or battery supplies. Alternating current (a.c.) constantly changes direction. UK mains is a.c., at about 230 V and a frequency of 50 Hz.
A UK mains cable has three cores, colour-coded so they are never confused:
Power is the rate of energy transfer (in watts, W). In an electrical circuit you can find it two ways:
The power rating on an appliance (e.g. "2000 W") tells you how fast it transfers energy. A higher current or higher resistance both raise the power dissipated as heat.
A kettle on UK mains: 230 V drives a current of 13 A.
P = V × I = 230 × 13 = 2990 W (≈ 3 kW)
Everyday appliances transfer energy electrically: a kettle to a thermal store, a motor to a kinetic store, a lamp by radiation. The energy transferred depends on power and time, or on charge and p.d.:
The more powerful the appliance and the longer it runs, the more energy it uses — this is what you pay for. Using less power or running for less time both cut the cost.
The National Grid is the network of cables and transformers that carries electricity from power stations to homes. Step-up transformers raise the p.d. for transmission (which lowers the current, reducing energy wasted as heat in the cables), and step-down transformers lower it again for safe use at home.
A 2000 W heater runs for 30 s.
E = P × t = 2000 × 30 = 60 000 J (60 kJ)
Physics only: this static-electricity section is examined on the Physics paper only, not Combined Science.
When two insulators are rubbed together, electrons (which are negative) are transferred from one to the other by friction. The object that gains electrons becomes negatively charged; the one that loses electrons becomes positively charged. Only electrons move — never the positive nuclei.
If enough charge builds up, the p.d. between the object and earth can become large enough to make a spark jump — electrons leap across the gap, ionising the air.
Tap a material, then tap the box it belongs in.
Charge: Q = I t
Ohm's law: V = I R
Series resistance: R = R₁ + R₂ + …
Parallel: total R is less than the smallest branch
Power: P = V I and P = I² R
Energy: E = P t and E = Q V
You've covered all of AQA 4.2 — current, p.d. & resistance, circuit symbols and I–V graphs, series & parallel, mains electricity & safety, power and energy, and static electricity. Press Finish to see your score.
You've worked through Electricity for AQA GCSE Physics. 🎉
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