GCSE Physics

Electricity Revision Guide

GCSE Physics | Foundation & Higher

Key Facts

Current (I) is the rate of flow of charge, measured in amps (A).
Voltage (V) is the energy transferred per unit charge, measured in volts (V).
Resistance (R) opposes current flow, measured in ohms (Ω).
Ohm's law: V = I × R
Power: P = I × V (also P = I²R)
Charge: Q = I × t
Energy transferred: E = Q × V (also E = I × V × t)

Current, Voltage and Resistance

Electric current is the flow of charge (electrons) around a circuit. For current to flow, there must be a complete circuit and a source of potential difference (voltage). Voltage is like the "push" that drives the current. Resistance is anything that opposes the flow of current, such as a resistor, bulb or wire.

Ohm's law (V = IR) tells us that for a given resistance, increasing the voltage increases the current proportionally. This only holds true for ohmic conductors (like a resistor at constant temperature). Non-ohmic conductors include filament lamps (resistance increases as they heat up), diodes (only allow current in one direction), and thermistors (resistance decreases as temperature increases).

Series Circuits

In a series circuit, components are connected one after another in a single loop. The current is the same through every component. The voltage is sharedbetween components (the voltages add up to the supply voltage). Total resistance is the sum of all individual resistances: R_total = R₁ + R₂ + R₃.

If one component breaks, the whole circuit stops working because there is only one path for current.

Parallel Circuits

In a parallel circuit, components are connected on separate branches. The voltage across each branch is the same (equal to the supply voltage). The current is sharedbetween branches (the branch currents add up to the total current). Adding more resistors in parallel decreases the total resistance because there are more paths for current to flow.

If one branch breaks, the others continue working. This is why homes use parallel circuits.

Electrical Power and Energy

Power is the rate of energy transfer, measured in watts (W). The key equations are P = IV and P = I²R. A 60W bulb transfers 60 joules of energy every second. Energy transferred can be calculated using E = Pt or E = IVt. For domestic appliances, energy is measured in kilowatt-hours (kWh): energy (kWh) = power (kW) × time (hours).

Mains Electricity

UK mains supply is 230V AC (alternating current) at 50 Hz. The three-pin plug contains a live wire (brown, 230V), a neutral wire (blue, 0V), and an earth wire (green and yellow, safety). The earth wire and fuse protect against electric shock. If a fault causes the live wire to touch the metal case, current flows through the earth wire, blowing the fuse and disconnecting the circuit.

Worked Examples

Example 1: A 12V battery drives a current of 3A through a resistor. Find the resistance.

V = IR, so R = V / I = 12 / 3 = 4Ω

Answer: 4 ohms

Example 2: Two resistors of 6Ω and 4Ω are connected in series. Find the total resistance and the current from a 20V supply.

Total R = 6 + 4 = 10Ω

I = V / R = 20 / 10 = 2A

Answer: 10Ω total resistance, 2A current

Example 3: A kettle uses 10A from a 230V supply. Find the power.

P = IV = 10 × 230 = 2300W (2.3kW)

Answer: 2300W or 2.3kW

Example 4: A 2kW heater runs for 3 hours. Calculate the energy used in kWh and the cost at 30p per kWh.

Energy = 2 × 3 = 6 kWh

Cost = 6 × 30p = 180p = £1.80

Answer: 6 kWh, costing £1.80

Common Mistakes

Confusing series and parallel rules (current same in series, voltage same in parallel).
Forgetting to convert units (e.g. milliamps to amps: 500mA = 0.5A, kW to W: 2kW = 2000W).
Using V = IR for non-ohmic conductors (like filament lamps where resistance changes with temperature).
Mixing up current and voltage when describing circuits.
Confusing the earth wire with the neutral wire. The earth is a safety feature, not part of the normal circuit.

Exam Tips

Learn all the equations: V=IR, P=IV, P=I²R, Q=It, E=QV, E=IVt. You must be able to rearrange them.
Always show your working in calculations. Even if you get the wrong answer, you can earn method marks.
Draw clear circuit diagrams using the correct symbols. Practise drawing series and parallel circuits.
For 6-mark questions about circuits, describe what happens to current, voltage, and resistance systematically.
Remember: adding resistors in series increases total resistance; adding them in parallel decreases it.

Practice Questions

A resistor has a resistance of 15Ω. The current through it is 2A. What is the voltage?
Three resistors of 2Ω, 3Ω and 5Ω are in series with a 30V battery. Find the current.
A hairdryer is rated at 1800W on a 230V supply. Find the current.
Explain why the total resistance decreases when resistors are added in parallel.
A current of 0.5A flows for 2 minutes. Calculate the charge transferred.
A 3kW oven runs for 2.5 hours. Calculate the cost at 34p per kWh.

Recommended Revision Guides

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CGP GCSE Physics Complete Revision and Practice — The go-to guide covering all topics.
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Oxford Revise AQA GCSE Physics — Visual revision with exam practice.