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AQA GCSE Physics · Topic 4.2 Electricity
Edexcel International GCSE Physics · Section 2 Electricity · 2 (c) Energy and voltage in circuits
I–V Graphs, Resistance and Sensors
In this lesson you'll learn to: plot and explain the I–V graphs of a fixed resistor, a
filament lamp and a diode, use V = I × R, describe how a thermistor and an LDR respond
to temperature and light, in the order the mark scheme wants, and carry out the resistance of a wire
practical (an AQA required practical, and a common context question for everyone).
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Prefer to watch? Scroll to the bottom for the video on this topic
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A video walkthrough is coming soon — it will appear at the bottom of this page
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Start here — have a play
Don't read anything yet. On tab 1, plot the I–V graph of each component in both directions, then try the Mystery component and work out what it is. On tab 2, heat the thermistor and move the lamp towards the LDR. On tab 3, close the switch, record a reading, open the switch, move the crocodile clip and repeat until you have a graph. Then leave the switch closed for a while and see what happens.
Resistance against length
Crocodile clip
50 cm
Your reading
| length / cm | p.d. / V | current / A | resistance / Ω |
|---|
The method: set the clip, close the switch, record, then open the switch before you move the clip. Try leaving it closed for a while and recording again: watch where that point lands.
Component
Current against potential difference (I–V graph)
Variable resistor
high
low
Slide the variable resistor to plot the graph
What is it?
Sensor
Resistance against temperature
Current against temperature
Bunsen burner
Off
Lamp distance
close
far
60 cm
Read the meters, then link the three
Meters are ideal: the ammeter has no resistance and no current flows through the voltmeter.
▲ Every reading is calculated from the circuit, so the graphs have the same shapes you will get in the lab.
Got the graphs? Now learn to explain them.
Exam questions on these practicals ask for the method, the control variables, an evaluation, and an explanation of each graph's shape. The notes below cover all four.
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Revision notes
Resistance: V = I × R
⭐ The equation
potential difference = current × resistance V = I × R
V in volts (V), I in amps (A), R in ohms (Ω). Rearranged: R = V ÷ I, which is how every practical on this page works out the resistance.
For a fixed p.d., a bigger resistance means a smaller current.
Resistors and I–V graphs
An I–V graph plots current (y) against potential difference (x). Its shape tells you how the resistance behaves.
| Component | The graph | What it means |
|---|---|---|
| Fixed resistor (ohmic conductor) | Straight line through the origin | Current is directly proportional to p.d. The resistance stays constant, as long as the temperature stays constant. A metal wire at constant temperature behaves the same way. |
| Filament lamp | S-shaped curve, the same both ways | As the p.d. increases the current increases by less and less. The resistance increases. |
| Diode | Flat, then a sharp rise just after about 0.7 V | Current flows in one direction only. The resistance is very high in the reverse direction. |
⭐ Why the lamp's resistance increases
As the current increases, the filament gets hotter. The ions in the metal vibrate more, so the free electrons collide with them more often. More collisions means a higher resistance.
Practical: I–V characteristics
AQA calls this a required practical; for Edexcel it is the 2.9 practical (your workbook does it with a filament lamp). Same method for both.
- Put the component in series with an ammeter and a variable resistor, with a voltmeter in parallel across the component.
- Change the variable resistor to change the p.d. Record the current and p.d. each time.
- Swap the connections on the supply (reverse it) and repeat, to get the negative half of the graph.
- Plot current (y) against p.d. (x). A diode needs a protective resistor in series, or the current shoots up past 0.7 V and damages it.
Sensors: thermistors and LDRs
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Thermistor
As the temperature increases, the resistance decreases, so the current increases.
Used in thermostats, fire alarms and car engine temperature sensors.💡
LDR (light-dependent resistor)
As the light intensity increases, the resistance decreases, so the current increases.
Used in automatic night lights and street lights.The thermistor practical
- Put the thermistor in a beaker of water with a thermometer, connected to a cell and an ammeter, with a voltmeter across it.
- Heat the water and take readings every 10 °C. Work out R = V ÷ I each time.
- Stir the water before each reading so the temperature is even.
- Use a small current, so the thermistor doesn't heat itself up.
- Safety: take care with the hot water and the heater, and don't touch the hot beaker.
AQA onlyRequired practical: resistance of a wire
Edexcel students: this is not one of your practicals, but it uses everything above
(R = V ÷ I, a hot wire's resistance going up), so it is a favourite context for a 4- or 6-mark question. Worth a read.
Aim: investigate how the length of a wire affects its resistance, with the wire kept at a constant temperature.
| Variable | In this practical |
|---|---|
| Independent (you change) | length of wire |
| Dependent (you measure) | resistance, worked out from R = V ÷ I |
| Control (keep the same) | temperature, the same wire (material and thickness), the same current |
1
Set up
Cells, a switch, an ammeter and a variable resistor in series with the wire, taped to a metre ruler. A voltmeter in parallel across the length being tested.
2
Measure
Clip across 10 cm. Close the switch, read the ammeter and voltmeter, then open the switch. Work out R = V ÷ I.
3
Repeat
Move the clip 10 cm further each time, up to 100 cm. Plot resistance (y) against length (x).
⭐ Why open the switch between readings?
Safety: a current heats the wire, so it can get hot enough to burn you.
Accuracy: a hotter wire has a higher resistance. Opening the switch keeps the wire at a constant temperature, so the only thing changing the resistance is the length. On tab 3 of the animation, the red points are what happens if you don't.
Adjust the variable resistor so the current is the same for every reading.
The conclusion and the evaluation
The graph is a straight line through the origin, so the resistance is directly proportional to the length. Double the length, double the resistance.
- Directly proportional needs both: a straight line and through the origin. A straight line that misses the origin is only a linear relationship.
- If the line does not pass through the origin, there is a systematic error, often from the resistance of the connecting leads or the clip not sitting exactly on 0 cm. Keep connecting wires short to reduce it.
- Random errors (a clip that doesn't grip well, a meter reading that flickers) are reduced by repeating each length and taking a mean.
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How to write it in the exam
Every "describe how the resistance changes" answer on this page uses the same three links, in the same order. The mark scheme gives one mark per link.
The three-link sentence
As the voltage / temperature / light intensity increases,
the current increases, so the
resistance increases / decreases / stays the same.
what you changewhat the ammeter showswhat that means for R
Exam-style question · filament lamp · 3 marks
Explain why the I–V graph for a filament lamp is a curve.
- As the p.d. increases, the current increases, but by a smaller amount each time (1)
- The filament gets hotter, so the ions vibrate more and the electrons collide with them more (1)
- So the resistance increases (1)
Where marks go missing: "the lamp gets hot" on its own. Say what the heat does: more vibration, more collisions, more resistance.
Exam-style question · street light · 2 marks
A street light uses an LDR. Describe what happens to the resistance of the LDR and the current through it as it gets dark in the evening.
- As the light intensity decreases, the resistance of the LDR increases (1)
- So the current through it decreases (1)
Watch the direction: getting dark is a decrease in light intensity, so every arrow flips.
AQA onlyExam-style question · evaluation · 2 marks
A student left the switch closed for the whole resistance of a wire experiment. Explain how this affected the results.
- The current heated the wire, so its temperature increased (1)
- A hotter wire has a higher resistance, so the later readings were too high (1)
Check your answer has all 3 links: what changed, what the current did, what the resistance did. Name each one, don't just imply it.
📘 Now do it in your workbook
Draw the resistance of a wire circuit and fill in the method, sketch all five resistor graphs with their symbols and the three-link answers, then write up the thermistor practical.
Write V = I × R, sketch all five resistor graphs with their symbols and the three-link answers, write up the filament lamp I–V practical, then the thermistor practical.
AQA GCSE Physics Workbook · ELECTRICITY · pages 5, 7 & 8 Edexcel iGCSE Physics Workbook · ELECTRICITY · pages 8, 9 & 10
⬇ Free: pages 5, 7 & 8 (PDF)
📄 Full workbook PDF · £2.99
Paperback on Amazon
⬇ Free: pages 8, 9 & 10 (PDF)
📄 Full workbook PDF · £2.99
Paperback on Amazon
The full workbook covers the whole electricity topic, with exam-style questions and worked answers.
✅ Can you do it? Tick as you go
0% complete🎉 Nice work! You've ticked off every objective for this spec point.
Don't forget to hit “Mark complete” at the bottom of the lesson.
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Prefer to watch? Here's the whole thing
Video walkthrough — coming soon
Everything above, explained out loud — useful for a last-minute recap, or if you'd rather hear it than read it.
Everything above, explained out loud — useful for a last-minute recap, or if you'd rather hear it than read it. The video is on its way.
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Video coming soon
The walkthrough for this lesson is being filmed and will appear right here. Until then, the animation and the notes above cover every mark.
Now put it into practice
Pages 5, 7 and 8 of the workbook have both required practicals to write up and all five resistor graphs to sketch and explain, each with full answers at the back.
Pages 8, 9 and 10 of the workbook have all five resistor graphs to sketch and explain, the filament lamp I–V practical and the thermistor practical, each with full answers at the back.
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