A thermistor is a temperature-dependent resistor whose resistance decreases as temperature increases. Free GCSE AQA Physics notes and 15 questions answered.
Updated 20 Mar 2026 · Revise Right Team
Thermistors, LDRs, and Non-Linearity is part of Current, potential difference and resistance in GCSE AQA Higher Physics.
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GCSE AQA Higher
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What this lesson covers
A thermistor is a temperature-dependent resistor whose resistance decreases as temperature increases. Free GCSE AQA Physics notes and 15 questions answered.
Explain how the resistance of thermistors and LDRs changes with temperature and light intensity
Describe applications of thermistors and LDRs in practical circuits
Analyse I-V graphs to determine whether circuit components are linear or non-linear
Calculate resistance values from I-V graphs and relate them to component properties
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Use this lesson to stay inside Current, potential difference and resistance while connecting the detail back to the wider Electricity topic.
Electricity
Current, potential difference and resistance
GCSE AQA Higher Physics
Revision notes
Read the core explanations from Thermistors, LDRs, and Non-Linearity before testing yourself from memory.
1. Why Some Resistors Change
Most resistors have fixed values - a 10Ω resistor stays at 10Ω. But what if you need a resistor that responds to its environment? That's where thermistors and LDRs come in - they're the smart sensors of the electronics world.
These components break the rules. Their resistance isn't constant - it changes based on what's happening around them. This makes them perfect for automatic systems that need to respond to temperature or light changes.
2. Thermistors - Temperature Sensors
Think of a thermistor like a temperature-controlled tap. As temperature rises, the 'tap' opens wider, allowing more current to flow. The resistance drops dramatically - from thousands of ohms when cold to just a few ohms when hot.
This happens because thermistors are made from semiconductor materials. When you heat them up, more electrons break free and become available to carry current. More charge carriers means lower resistance.
Thermistors are everywhere in modern technology. Your car's engine management system uses them to monitor coolant temperature. Central heating systems use them in thermostats to maintain room temperature automatically.
3. Light Dependent Resistors (LDRs)
LDRs work like electronic eyes. In darkness, they have very high resistance - sometimes millions of ohms. Shine light on them and their resistance plummets to just a few hundred ohms.
The science is similar to thermistors but triggered by light instead of heat. Photons hit the semiconductor material and give electrons enough energy to break free. More light means more free electrons, which means lower resistance.
Worked example: Calculating LDR Resistance Change
An LDR has resistance 1MΩ in darkness and 500Ω in bright light. If connected to a 9V battery, calculate the current in both conditions.
$I = \frac{V}{R}$ — Current equals voltage divided by resistance
$I_{dark} = \frac{9V}{1,000,000Ω} = 9 \times 10^{-6}A$ — Very small current in darkness - 9 microamps
$I_{bright} = \frac{9V}{500Ω} = 0.018A$ — Much larger current in light - 18 milliamps
$\frac{I_{bright}}{I_{dark}} = \frac{0.018}{9 \times 10^{-6}} = 2000$ — Current increases by factor of 2000 when light shines on LDR
Answer: Dark: 9μA, Bright: 18mA - current increases 2000 times
This massive change in current makes LDRs perfect for switching circuits. Street lights use them to turn on automatically when it gets dark. Security lights use them to stay off during the day and activate at night.
4. Practical Applications
The real power of thermistors and LDRs comes when you use them in circuits that make decisions. Here's how they work in practice:
LDR applications work similarly. Garden lights contain LDRs that detect when daylight fades. The changing resistance triggers a switch that turns the LED on. No human intervention needed - the circuit makes the decision automatically.
5. Non-Linear Behaviour and I-V Graphs
Normal resistors give straight-line I-V graphs, so doubling the voltage doubles the current. Thermistors and LDRs are non-linear components, so their I-V graphs are curved. For a thermistor, the curve changes because the component heats up and its resistance changes. For an LDR, the resistance depends on the light intensity falling on it, not simply on current flowing through it.
Here's why: when current flows through a thermistor, it heats up. Higher temperature means lower resistance. Lower resistance means more current can flow. This creates a curved relationship - not the straight line of Ohm's law.
Worked example: Analysing a Thermistor I-V Graph
A thermistor I-V graph shows: at 2V, I = 0.1A; at 4V, I = 0.3A. Calculate the resistance at each voltage and explain the pattern.
$R_1 = \frac{V}{I} = \frac{2V}{0.1A} = 20Ω$ — Using Ohm's law to find resistance at first point
$R_2 = \frac{V}{I} = \frac{4V}{0.3A} = 13.3Ω$ — Resistance at the second point is lower
$R_1 > R_2$ (20Ω > 13.3Ω) — Resistance decreased as voltage increased
Higher voltage → More current → More heating → Lower resistance — This creates the curved I-V graph characteristic of thermistors
Answer: R₁ = 20Ω, R₂ = 13.3Ω. Resistance decreases due to self-heating effect.
You can use I-V graphs to decide whether a component is linear or non-linear. A straight line through the origin shows constant resistance. A curve shows that the resistance is changing. Where the graph becomes steeper, the current increases more for each extra volt, so the resistance is decreasing. This is how the graph links to the function and properties of thermistors and LDRs.
6. Exam Technique and Common Mistakes
Exam questions on thermistors and LDRs often test whether you understand the direction of change. Many students get confused about which way the resistance changes.
When describing applications, examiners want you to explain the complete chain of events. Don't just say 'the thermistor detects temperature' - explain how the resistance change leads to a voltage change that triggers the switching action.
Understanding these components opens up the world of automatic control systems. From car engines to smart homes, thermistors and LDRs are the sensors that make technology respond intelligently to the world around us.
Sources and content standards
Published by Revise Right. Use the official specification reference to check the requirements for your course. Content updated: 2026-03-20.
A thermistor is a temperature-dependent resistor whose resistance decreases as temperature increases. It is made from semiconductor materials that become more conductive when heated, and it is used in thermostats and engine coolant monitoring.
What is an LDR?
An LDR (light dependent resistor) is a light-sensitive resistor whose resistance decreases as light intensity increases. In darkness it can have millions of ohms of resistance, which plummets to a few hundred ohms in bright light; it is also called a photoresistor.
What does a thermistor I-V graph show?
A thermistor I-V graph is curved because current heats the thermistor, lowering its resistance so more current flows. For example, at 2V and 0.1A the resistance is 20Ω, but at 4V and 0.3A it is 13.3Ω, so resistance decreases as voltage increases.
How does an LDR work?
An LDR works because photons hit its semiconductor material and give electrons enough energy to break free. More light means more free electrons, which means lower resistance, so its resistance decreases as light intensity increases.
What does a thermistor graph look like?
A thermistor graph shows resistance decreasing as temperature increases, from thousands of ohms when cold to just a few ohms when hot. Its I-V graph is curved rather than a straight line, because current heats the thermistor and lowers its resistance.
What are thermistors and LDRs used for?
Thermistors are used as temperature sensors, for example to monitor engine coolant temperature and in central heating thermostats. LDRs are used in light-sensing circuits such as street lights, security lights and garden lights that switch on automatically when it gets dark.
How does light intensity affect the resistance of an LDR?
The resistance of an LDR decreases as light intensity increases. In darkness an LDR can have millions of ohms of resistance, but in bright light its resistance falls to just a few hundred ohms.
How does the resistance of a thermistor change with temperature?
The resistance of a thermistor decreases as temperature increases. Thermistors are made from semiconductor materials, and heating them frees more electrons to carry current, so resistance drops from thousands of ohms when cold to just a few ohms when hot.
What is a non-linear component?
A non-linear component is a circuit component that does not obey Ohm's law, so its current-voltage graph is not a straight line because its resistance changes. Thermistors and LDRs are non-linear, while normal resistors give straight-line I-V graphs.
Why does a thermistor's resistance decrease when heated?
A thermistor's resistance decreases when heated because it is made from semiconductor materials. Heating them up makes more electrons break free and become available to carry current, and more charge carriers means lower resistance.
How do street lights turn on automatically?
Street lights turn on automatically using LDRs. When it gets dark, the resistance of the LDR rises, and this change in resistance triggers a switch that turns the light on with no human intervention.
What is a photoresistor?
A photoresistor is another name for a light dependent resistor (LDR), a light-sensitive resistor whose resistance decreases as light intensity increases. LDRs are used in street lights, security lights and garden lights that switch on automatically when it gets dark.
How do you tell if a component is linear from its I-V graph?
A component is linear if its I-V graph is a straight line through the origin, showing constant resistance. A curve shows that the resistance is changing; where the graph becomes steeper, the resistance is decreasing.
How much does the current through an LDR change from dark to light?
An LDR with resistance 1 MΩ in darkness and 500 Ω in bright light, connected to a 9 V battery, carries 9 μA in the dark and 18 mA in the light. Using I = V/R, the current increases by a factor of 2000.
How do you explain a thermistor in a control circuit?
A thermistor in a control circuit is explained as a complete chain of events: the temperature changes, the thermistor's resistance changes, and this leads to a voltage change that triggers the switching action. Simply saying the thermistor detects temperature is not enough.