GCSE Physics Electricity Notes
Electricity is about charge moving around a circuit. Get current, potential difference and resistance straight and the whole topic clicks.
Key concepts
- Current (I)
- The rate of flow of charge, measured in amps (A). Measured with an ammeter placed in series.
- Potential difference (V)
- The energy transferred per unit charge, measured in volts (V). Measured with a voltmeter placed in parallel.
- Resistance (R)
- How much a component opposes current, measured in ohms (Ω). Higher resistance means less current for the same p.d.
- Current is the same at every point in a series circuit.
- Potential difference is shared between components in series and is the same across each branch in parallel.
- An ammeter goes in series; a voltmeter goes in parallel.
Key formulas
Ohm's law
V = I × R
Potential difference = current × resistance.
Charge
Q = I × t
Charge (C) = current (A) × time (s).
Electrical power
P = V × I
Also P = I²R.
Required practicals
- Investigate how the resistance of a wire changes with its length.
- Investigate the I–V characteristics of a resistor, a filament lamp and a diode.
Worked example
A 12 V supply drives a current of 0.5 A through a resistor. Find the resistance.
- 1Write down the equation: V = I × R.
- 2Rearrange for R: R = V ÷ I.
- 3Substitute: R = 12 ÷ 0.5.
- 4Calculate: R = 24 Ω.
Answer: R = 24 Ω
Common mistakes
✗ Connecting an ammeter in parallel or a voltmeter in series.
✓ Ammeter in series (measures flow through), voltmeter in parallel (measures across).
✗ Forgetting units, e.g. writing 24 instead of 24 Ω.
✓ Always quote the unit — answers without units lose the final mark.
✗ Mixing up current and potential difference.
✓ Current = flow of charge (amps); potential difference = energy per charge (volts).
Quick check
Try these, then reveal the answers.
What is the unit of resistance?Reveal answer
The ohm (Ω).
A current of 2 A flows for 5 s. How much charge passes? (Q = I × t)Reveal answer
Q = 10 C
Where should a voltmeter be connected?Reveal answer
In parallel with the component.
Exam tips
- Command words: 'state' needs a short fact, 'explain' needs a reason, 'calculate' needs working and units.
- Always rearrange the equation before substituting numbers — it earns a method mark.
- For I–V graphs, a straight line through the origin means resistance is constant (an ohmic conductor).
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Frequently asked questions
What is the difference between current and potential difference?+
Current is the rate of flow of charge (amps); potential difference is the energy transferred per unit charge (volts).
Why does a filament lamp's resistance increase?+
As current increases the filament heats up, ions vibrate more and oppose the flow of charge, so resistance rises.
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