GCSE Physics electricity
Electricity is one of the biggest GCSE Physics topics. Here are the ideas and equations you need.
On this page
- Current, potential difference and resistance
- Series and parallel circuits
- I-V graphs
- Mains electricity
- Energy and power
- The National Grid
- A simple picture of electricity
- Worked examples
- Thermistors and LDRs
- Staying safe with mains electricity
- Common mistakes
- Practice questions
- Frequently asked questions
- Key takeaways
- Where to go next
- How to revise electricity
Current, potential difference and resistance
- Current is the rate of flow of charge: Q = It.
- Potential difference is the energy transferred per unit charge.
- Resistance links them: V = IR.
Series and parallel circuits
- Series: the same current flows everywhere; potential differences add up; total resistance is the sum.
- Parallel: each branch has the same potential difference; currents add up; adding branches lowers the total resistance.
I-V graphs
- A fixed resistor: a straight line through the origin.
- A filament lamp: a curve, because resistance rises as it gets hot.
- A diode: current flows in one direction only.
Mains electricity
- UK mains is alternating current at about 230 V and 50 Hz.
- Plug wires: live (brown), neutral (blue) and earth (green and yellow).
- The earth wire and fuse protect against faults.
Energy and power
- P = VI and P = I²R
- E = Pt and E = QV
The National Grid
Step-up transformers increase the voltage for transmission, which lowers the current and reduces energy losses in the cables. Step-down transformers reduce it again for homes.
See all the GCSE Physics equations.
A simple picture of electricity
Current is the rate of flow of charge, measured in amps. Potential difference, measured in volts, is the energy transferred per unit of charge, and it is what pushes the current round the circuit. Resistance, measured in ohms, is how much a component opposes the current. These three are linked by V = IR. If you keep the potential difference the same and increase the resistance, the current falls.
Worked examples
Example 1: charge
A current of 2 A flows for 30 s. How much charge flows?
- Q = It = 2 × 30 = 60 C.
Example 2: Ohm's law
A 12 V battery is connected to a 4 Ω resistor. Find the current.
- I = V/R = 12/4 = 3 A.
Example 3: series circuit
A 6 V battery is connected to 2 Ω and 4 Ω resistors in series. Find the current and the pd across each resistor.
- Total resistance = 6 Ω, so I = 6/6 = 1 A.
- pd across 2 Ω = 2 V; across 4 Ω = 4 V. They add up to the 6 V supply.
Example 4: parallel circuit
A 12 V supply is connected to two branches, one with a 6 Ω resistor and one with a 3 Ω resistor. Find the total current.
- Each branch has the full 12 V. Currents: 12/6 = 2 A and 12/3 = 4 A.
- Total current = 6 A.
Example 5: power
A 230 V hairdryer draws 5 A. Find its power.
- P = IV = 230 × 5 = 1150 W.
Example 6: energy transferred
A 2 kW heater runs for 3 hours. How much energy does it transfer in joules and in kWh?
- E = Pt = 2000 × 3 × 3600 = 21.6 MJ.
- In kWh: 2 kW × 3 h = 6 kWh.
Example 7: energy and charge
How much energy is transferred when 50 C of charge moves through a pd of 9 V?
- E = QV = 50 × 9 = 450 J.
Thermistors and LDRs
A thermistor's resistance falls as its temperature rises, so it is used in thermostats and fire alarms. A light-dependent resistor's resistance falls as light gets brighter, so it is used in automatic night lights. In exam questions, you may need to explain what happens to the current or a voltage when the temperature or light level changes. Work through it in steps: resistance changes first, then current, then the voltages across each component.
Staying safe with mains electricity
UK mains electricity is alternating current at 230 V and 50 Hz. A three-core cable has a live wire (brown) at 230 V, a neutral wire (blue) near 0 V, and an earth wire (green and yellow) for safety. If a fault makes a metal case live, a large current flows through the earth wire and the fuse melts, cutting off the supply. Touching the live wire is dangerous even when a switch is off, because your body is at 0 V and current can flow through you to earth.
Common mistakes
- Thinking current is used up around a circuit; it is the same everywhere in a series circuit.
- Adding resistors in parallel as if they were in series; total resistance in parallel is less than the smallest resistor.
- Forgetting to convert minutes or hours into seconds for E = Pt.
- Mixing up the colours of the wires in a plug.
Practice questions
Try each one before opening the answer.
Q1. Find the resistance of a lamp with 6 V across it and 0.5 A through it.
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Q2. Three 2 Ω resistors are in series. What is the total resistance?
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Q3. A 60 W bulb is on for 2 minutes. How much energy does it transfer?
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Q4. Why does the National Grid use high voltages?
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Frequently asked questions
What is the difference between ac and dc?
Direct current flows in one direction only, as from a battery. Alternating current keeps changing direction, as in mains electricity.
Why does a filament lamp's graph curve?
As current increases, the filament heats up, and its resistance increases. So the current rises less for each extra volt.
Key takeaways
- V = IR links pd, current and resistance.
- In series, current is the same and pd is shared.
- In parallel, pd is the same and current is shared.
- P = IV and E = Pt connect power and energy.
- The earth wire and fuse protect you from faults.
Where to go next
Electricity links to energy and the required practicals.
How to revise electricity
Learn the circuit symbols first, because every question uses them. Then practise the series and parallel rules with simple numbers until they feel natural. Draw the I-V graphs for a resistor, a filament lamp and a diode from memory and explain each shape. Finally, try longer questions that combine circuits with power and energy, which are common in the higher-mark questions.
Electricity questions reward neat working: label your circuit, write the equation, substitute with units, and the answer follows.
Once these basics are secure, electricity often becomes one of the highest-scoring topics on Paper 1.