AQA GCSE Physics · Electricity
I–V characteristics required practical
Investigate the I–V characteristics of a filament lamp, a diode and a resistor at constant temperature.
You vary the potential difference across a component and measure the current through it, in both directions. The shape of the graph shows how its resistance behaves.
Equipment
- Filament lamp, diode (with a protective resistor) and fixed resistor
- Variable resistor (or variable power supply)
- Ammeter and voltmeter
- Power supply and switch
- Connecting leads
Variables
- Independent you change
- Potential difference across the component
- Dependent you measure
- Current through the component
- Control you keep the same
- Same component for each set of readings
- Same circuit and meters
Method
- 1
Set up the circuit with the component, an ammeter in series, a voltmeter in parallel across the component, and a variable resistor.
- 2
Adjust the variable resistor to change the p.d. Record the current and p.d. each time.
- 3
Take readings in steps (e.g. every 1 V) up to about 6 V.
- 4
Swap the connections to the power supply so the current flows the other way, and repeat to get negative values.
- 5
Repeat for each component.
- 6
Plot current (y-axis) against potential difference (x-axis).
Worked example
Readings for a filament lamp.
| p.d. (V) | Current (A) | Resistance (Ω) |
|---|---|---|
| −6.0 | −0.30 | 20.0 |
| −4.0 | −0.25 | 16.0 |
| −2.0 | −0.16 | 12.5 |
| 0.0 | 0.00 | – |
| 2.0 | 0.16 | 12.5 |
| 4.0 | 0.25 | 16.0 |
| 6.0 | 0.30 | 20.0 |
Working
R = V ÷ I
At 2.0 V: 2.0 ÷ 0.16 = 12.5 Ω
At 6.0 V: 6.0 ÷ 0.30 = 20.0 Ω
Conclusion
The lamp’s graph curves: as the current increases, the filament gets hotter and its resistance increases, so the current rises less for each extra volt. A fixed resistor gives a straight line through the origin; a diode only lets current flow one way.
Risks
- Filament lamp gets hotDon’t touch the bulb while it’s on.
- Components can overheat at high currentUse a protective resistor with the diode and switch off between readings.
Common mistakes
- Putting the ammeter in parallel. It must be in series; the voltmeter is in parallel.
- Only taking positive readings. Reverse the supply to see how components behave both ways (essential for the diode).
- Saying the lamp has constant resistance. Its resistance increases as it heats up.
Better results
- Take more readings where the graph curves most.
- Switch off between readings so the resistor and diode don’t heat up.
- Repeat readings and take a mean current at each p.d.
Launching November 2026
7 exam questions on this practical.
Practise the method, the variables, the maths and what the results mean, then revise the Electricity flashcards. It opens to students soon: join the waitlist and we’ll email you once, when it does.