GCSE-readyCore practical: I-V graphs, LDRs and thermistors
Core practical 10.17 measures current and potential difference for a resistor and a filament lamp. You also need to know how the resistance of LDRs and thermistors changes.
The method
2. Use the variable resistor to change the current. Record the current and potential difference each time.
3. Reverse the connections to the supply to get negative values.
4. Plot current (y-axis) against potential difference (x-axis). Calculate resistance using R = V ÷ I.
Switch off between readings so the resistor does not heat up and change its resistance.
What the graphs show
Filament lamp: a curve that gets less steep. As the current rises, the filament gets hotter and its resistance increases.
Diode: current flows in one direction only; there is almost no current in reverse.
You can also compare series and parallel circuits: in series the total resistance is the sum; adding a resistor in parallel lowers the total resistance.
LDRs and thermistors
Thermistor: resistance decreases as temperature increases. Used in thermostats and electronic thermometers.
To investigate them, measure the resistance (V ÷ I, or with an ohmmeter) while changing the light level or the temperature (for example in a water bath).
A filament lamp has 2.0 V across it and 0.25 A through it. At 6.0 V the current is 0.50 A. Find the resistance each time and explain the change.
- R = 2.0 ÷ 0.25 = 8 Ω
- R = 6.0 ÷ 0.50 = 12 Ω
- The higher current heats the filament, so its resistance increases.
Answer: 8 Ω, then 12 Ω: the hotter filament has a higher resistance.
Ammeter in series, voltmeter in parallel, variable resistor to change the current. Fixed resistor: straight line through the origin. Lamp: curve, as resistance rises with temperature. Diode: one direction only. LDR resistance falls with more light; thermistor resistance falls as temperature rises.
- Current
- The rate of flow of charge. Unit ampere (A).
- Potential difference
- The energy transferred per unit charge between two points: E = Q × V. Unit volt (V). The International GCSE often calls it voltage.
- Resistance
- Opposition to current: R = V ÷ I. Unit ohm (Ω).
- Temperature
- K = °C + 273. A change of 1 °C is the same as a change of 1 K.
- Diode
- A component that lets current flow in one direction only.
- Thermistor
- A resistor whose resistance falls as its temperature rises.
All GCSE and International GCSE Physics key terms and definitions →
Try two quick questions
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2 more questions
Try each one, then tap to check.
A component has 4.5 V across it and a current of 0.30 A. Calculate its resistance.
Show the answer
Answer: 15 Ω
R = V ÷ I = 4.5 ÷ 0.30 = 15 Ω
A lamp has 6.0 V across it and a current of 0.15 A. Calculate its resistance.
Show the answer
Answer: 40 Ω
R = V ÷ I = 6.0 ÷ 0.15 = 40 Ω
Check you have got it
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- Pearson Edexcel GCSE Physics (1PH0) specification, topic 10.17, 10.18, 10.19, 10.20, 10.21
- Pearson Edexcel International GCSE Physics (4PH1) specification, topic 2.9, 2.11
- Pearson Edexcel GCSE (9-1) Physics (1PH0) specification
Checked by the GCSE-ready editorial team, 17 October 2026. Last updated 2 October 2026. Spotted a mistake? Tell us and we will fix it.
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