🌙 In the dark
The resistance of an LDR is very high. It can be around a million ohms. Very little current can pass through it.
Energy and Voltage in Circuits » LDRs, Thermistors, Lamps and LEDs
What you'll learn this session
Study time: 30 minutes
Edexcel spec: 2.11, 2.12
- How the resistance of an LDR changes with illumination
- How the resistance of a thermistor changes with temperature
- Where LDRs and thermistors are used
- How lamps and LEDs show that there is a current in a circuit
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Unlock This CourseResistors that react to the world around them
A fixed resistor has the same resistance wherever you put it. Two components are different. Their resistance depends on what is happening around them. A light-dependent resistor (LDR) responds to light. A thermistor responds to temperature. You met their circuit symbols in the lesson Electrical Units and Circuit Symbols. Now you need to know how they behave.
The spec asks you to describe the qualitative variation of their resistance. Qualitative means in words, without numbers. So you must be able to say which way the resistance goes, but you will not be asked to calculate it from the brightness or the temperature.
Key terms:
- Light-dependent resistor (LDR): a resistor whose resistance decreases as the light falling on it gets brighter.
- Illumination: how brightly something is lit. Also called light intensity.
- Thermistor: a resistor whose resistance decreases as its temperature increases.
- Indicator: a lamp or LED that lights to show there is a current in a circuit.
The LDR and illumination
The rule for an LDR is short: the brighter the light, the lower the resistance.
🌞 In bright light
The resistance is low. It can drop to a few hundred ohms. A much larger current can pass through it.
You can see this with a simple circuit: a battery, an ammeter and an LDR in one loop. Cover the LDR with your hand and the ammeter reading falls. Shine a torch on it and the reading rises. The battery has not changed, so the resistance of the LDR must have changed.
The graph in words. Put illumination on the horizontal axis and resistance on the vertical axis. The line starts very high on the left, where it is dark. It falls steeply at first as the light gets brighter. Then it becomes less and less steep and levels off at a low value. It is a smooth downward curve, not a straight line, and it never touches the horizontal axis because the resistance never reaches zero.
The thermistor and temperature
The rule for a thermistor has the same pattern: the hotter the thermistor, the lower its resistance. When it cools down, its resistance goes back up.
Here are some example readings for one thermistor.
| Temperature in °C | Resistance in Ω |
|---|---|
| 20 | 2000 |
| 40 | 1000 |
| 60 | 500 |
| 80 | 300 |
| 100 | 200 |
Look at how the resistance falls. Between 20 °C and 40 °C it drops by 1000 Ω. Between 80 °C and 100 °C it drops by only 100 Ω. Different thermistors have different values, but the pattern is the same.
The graph in words. Put temperature on the horizontal axis and resistance on the vertical axis. The line starts high on the left, where it is cold. It falls steeply at first and then levels off as the thermistor gets hotter. This is the same shape as the LDR graph. Only the label on the horizontal axis is different.
Do not mix it up with a filament lamp
In the lesson Current-Voltage Graphs you saw that the resistance of a metal filament increases when it gets hot. A thermistor does the opposite. Its resistance decreases when it gets hot. Examiners like to test whether you know the difference.
Using LDRs and thermistors
Because their resistance changes, LDRs and thermistors work as sensors. A change in light or temperature becomes a change in current, and a circuit can use that change to switch something on or off.
🌃 Automatic night lights
An LDR senses the daylight. When it gets dark, the resistance of the LDR rises. The circuit detects this and switches the light on. Street lamps and garden lights work this way.
🏠 Thermostats
A thermistor senses the temperature of a room, an oven or a fridge. As the temperature changes, its resistance changes. The circuit uses this to switch the heating or cooling on and off.
🚨 Fire alarms
A thermistor sits in the alarm. In a fire it heats up and its resistance falls. The current in the circuit rises, and this sets off the alarm.
In an exam answer about a use, always give the full chain: what changes around the component, what happens to its resistance, and what happens to the current.
Worked example: explaining a change in current
A thermistor is connected in series with a battery and an ammeter. The thermistor is moved from a warm room into a freezer. Explain what happens to the ammeter reading.
Step 1: say what changes. The temperature of the thermistor decreases.
Step 2: say what happens to the resistance. The resistance of the thermistor increases.
Step 3: say what happens to the current. The voltage of the battery is the same, so a higher resistance means a smaller current. The ammeter reading falls.
An optional experiment
The spec suggests an experiment to show the relationship between voltage, current and resistance for a component whose resistance varies with temperature, light intensity or pressure. It is an optional extra, but it is worth understanding. For a thermistor:
- Connect the thermistor in series with a battery and an ammeter. Connect a voltmeter in parallel with the thermistor.
- Put the thermistor in a beaker of cold water with a thermometer.
- Record the temperature, the voltage and the current.
- Warm the water gently. Take the three readings again every 10 °C.
- Calculate the resistance at each temperature using resistance = voltage ÷ current.
For example, at 20 °C the readings are 4.0 V and 0.002 A, so the resistance is 4.0 ÷ 0.002 = 2000 Ω. At 60 °C they are 4.0 V and 0.008 A, so the resistance is 4.0 ÷ 0.008 = 500 Ω. For an LDR the idea is the same. You change the illumination by moving a lamp closer to the LDR in a darkened room.
Lamps and LEDs as indicators
You cannot see a current. So how do you know there is one? One way is to use an ammeter. A simpler way is to put a lamp or an LED in the circuit. If it lights, there is a current in it. This is all an indicator is.
💡 Lamps
A lamp lights when there is a current in it, whichever way round it is connected. A lit lamp tells you the circuit is complete. A lamp that goes out tells you the current has stopped.
🔴 LEDs
LED stands for light-emitting diode. It is a diode that gives out light when there is a current in it. Like any diode, it lets current pass in one direction only. So an LED lights only when it is connected the right way round.
Indicators are all around you. The small light on a phone charger, the standby light on a television and the light on a kettle's switch all tell you the same thing: there is a current in this circuit. Most indicator lights today are LEDs because they need only a very small current to light.
That small current matters. Too large a current would damage an LED, so it is normally connected in series with a protective resistor.
Common mistakes
- Writing 'the resistance changes' with no direction. Say that it decreases as the light gets brighter or the temperature gets higher.
- Getting the LDR backwards. Bright light gives a low resistance. Darkness gives a high resistance.
- Saying the resistance of a thermistor increases when it is heated. That is a filament lamp.
- Describing either graph as a straight line. Both are curves that fall steeply and then level off.
- Assuming that an LED that is not lit means there is no supply. It may simply be connected the wrong way round.
Exam-style question
A student connects a battery, an ammeter and an LDR in series.
(a) Describe how the resistance of an LDR varies with illumination. [2 marks]
(b) The student switches off the lights in the room. Explain what happens to the reading on the ammeter. [2 marks]
(c) The student replaces the LDR with an LED. The LED does not light. The battery is working and all the connections are good. Suggest why the LED does not light, and state what the student should do. [2 marks]
(d) A thermistor is used in a fire alarm. Explain how a fire changes the current in the thermistor. [3 marks]
Model answer
(a) As the illumination increases, the resistance decreases (1). The resistance is very high in the dark and low in bright light, or it falls steeply at first and then levels off (1).
(b) The illumination decreases, so the resistance of the LDR increases (1). So the current decreases and the ammeter reading falls (1).
(c) The LED is connected the wrong way round, and it only passes current in one direction (1). Reverse the connections to the LED (1).
(d) The fire increases the temperature of the thermistor (1). The resistance of the thermistor decreases (1). So the current in the thermistor increases (1).
Exam tip
Parts (b) and (d) are both three-link chains: the surroundings change, the resistance changes, the current changes. Write one short sentence for each link and use the words increases or decreases every time. Words like 'changes' or 'is affected' do not earn the mark.