GCSE · Physics · AQA · Spec 8463

Thermistors and LDRs

Two tiny components that let circuits react to the world around them.

What you need to know

  • A thermistor's resistance DECREASES as temperature INCREASES.
  • An LDR's resistance DECREASES as light intensity INCREASES.
  • Both are non-ohmic components — their resistance changes with conditions.
  • The equation V = I R links potential difference, current and resistance for both components.

The big picture

A thermistor is a resistor whose resistance falls as temperature rises — useful in thermostats and temperature-sensing circuits. An LDR (light-dependent resistor) works the same way with light: its resistance falls as light intensity increases, making it ideal for automatic lighting. Both are non-ohmic components, meaning they don't obey Ohm's law, but the relationship between potential difference, current and resistance still follows V = I R for any fixed condition. Understanding how their resistance changes — and in which direction — is the key idea examiners test.

TONIGHT'S REVISION

Thermistors and LDRs

How resistance responds to temperature and light — and why circuits use that.

Physics · Electric circuits

Thermistor and LDR in a Circuit

Explore how changing conditions alter resistance and current

Two lamps · 6 V supply

Dashed line = current flow

6 VL1L2

Voltage

6 V

Total R

Resistance falls from ~2000 Ω (cold) to ~200 Ω (hot)

Current

Current increases as resistance drops

Brightness

Component 'opens up' to more current — analogy: lamp gets brighter

Thermistor — temperature rises. As temperature increases, the thermistor's resistance decreases. With the same supply voltage, a lower resistance means a higher current flows through the circuit — V = I R rearranged as I = V ÷ R.

Exam line: Both components DECREASE in resistance as their stimulus increases — remember the direction.
Watch out: Students often confuse the direction: more temperature or more light both DECREASE resistance, not increase it.

Stop and Think

Read carefully — both sensors are involved. Commit to your answer before revealing.

A street light uses an LDR to switch on automatically. Which statement correctly describes what happens at dusk as light intensity falls?

How a Thermistor Controls a Thermostat

Follow one molecule of 'decision' through the circuit — each stage explains WHY the next happens.

Thermistors and LDRs

?

Reason it through

Why does more light falling on an LDR allow more current to flow in the circuit?

Link 1 of 3

First link · your turn

What does increasing light intensity do to the LDR's internal structure?

2
Locked — reveal the link above first
3
Locked — reveal the link above first

Relationship matrix

Tap any cell to reveal it. Tap a column header to read one property down every item.

Stimulus increasesStimulus decreasesTypical applicationOhmic or non-ohmic?
Thermistorresponds to temperature
LDRresponds to light intensity

Each cell hides a short answer and the reason behind it. Predict before you tap.

AQA 8463 — Paper 1

AQA Exam Answer: Thermistors and LDRs

Does your answer hit the mark-scheme phrases? Tap to check.

Question

Explain how an LDR could be used to switch a light on automatically when it gets dark. [3 marks]

Student answer

As light intensity decreases, the resistance of the LDR increases. This causes the current in the circuit to decrease. The decrease in current is detected and used to switch the light on.

Method marks0/3

Equations you need

Taken directly from the exam-board specification.

V = I R

V = potential difference (V) · I = current (A) · R = resistance (Ω)

Learn it — you must recall this in the exam

Key points

1Thermistor: hotter → lower resistance → more current flows.
2LDR: brighter → lower resistance → more current flows.
3Both components DECREASE in resistance as their stimulus increases — this direction is the most-tested fact.
4Thermistors are used in thermostats; LDRs are used to switch lights on automatically in the dark.
5Even though they are non-ohmic, V = I R still applies at any given instant.

Worked example

Problem

A thermistor in a warm room has a resistance of 500 Ω. The potential difference across it is 3 V. Calculate the current through the thermistor.

🧠

Memory hook

Think 'MORE energy in, LESS resistance out' — more heat into a thermistor, more light onto an LDR: resistance falls both times. Same rule, two different stimuli.

★ Exam tip

AQA questions often ask you to 'explain how the circuit responds' to a change in temperature or light. Always state the two-step chain: stimulus increases → resistance decreases → current increases (for a fixed supply voltage). One mark goes on each link in that chain.

⚠ Watch out

Thinking resistance INCREASES when temperature or light increases — it is the opposite: both thermistors and LDRs DECREASE in resistance as their stimulus goes up.

✓

Check yourself

Without looking: in which direction does an LDR's resistance change when a cloud passes over it, blocking the light?

Flashcards

(23)
What happens to a thermistor's resistance as temperature increases?
It decreases — hotter means lower resistance.
What does LDR stand for?
Light-Dependent Resistor.
What happens to an LDR's resistance as light intensity increases?
It decreases — brighter means lower resistance.
Name one application of a thermistor.
A thermostat (e.g. in a central heating system or electric oven).
Name one application of an LDR.
Automatic street lighting — the light switches on when it gets dark.
Are thermistors ohmic or non-ohmic components?
Non-ohmic — their resistance changes with conditions.
Write the equation linking potential difference, current and resistance.
V = I R
What are the units of resistance?
Ohms (Ω).
A thermistor is in a circuit with a fixed supply. The temperature rises. What happens to the current?
Current increases — lower resistance allows more current to flow.
A thermistor has resistance 2000 Ω and 6 V across it. What is the current?
I = V ÷ R = 6 ÷ 2000 = 0.003 A (3 mA).
An LDR is in a circuit. A cloud blocks the light. What happens to the LDR's resistance?
It increases — less light means higher resistance.
What does 'non-ohmic' mean?
The component does not have a constant resistance — it changes with conditions such as temperature or light.
In which direction does a thermistor's resistance change when it cools down?
Resistance increases — cooler means higher resistance.
An LDR has 12 V across it and carries a current of 0.04 A. What is its resistance?
R = V ÷ I = 12 ÷ 0.04 = 300 Ω.
Give the stimulus and the effect for a thermistor in one sentence.
As temperature increases, the thermistor's resistance decreases.
Give the stimulus and the effect for an LDR in one sentence.
As light intensity increases, the LDR's resistance decreases.
Why would an LDR be used in an automatic security light?
When it gets dark, the LDR's resistance increases, which can trigger a switch to turn the light on.
Does V = I R apply to non-ohmic components?
Yes — it still applies at any given instant, but the resistance value itself changes with conditions.
A thermistor at low temperature has high resistance. What does that mean for current in the circuit (fixed voltage)?
Low current — high resistance restricts the flow of charge.
What is the key difference between an ohmic resistor and a thermistor?
An ohmic resistor has constant resistance; a thermistor's resistance changes with temperature.
On which AQA paper are thermistors and LDRs assessed?
Paper 1.
A thermistor reads 400 Ω at 20 °C and 100 Ω at 60 °C. Which temperature allows more current to flow for the same voltage?
60 °C — lower resistance means more current flows.
Name the two variables that affect the resistance of an LDR and a thermistor respectively.
LDR: light intensity. Thermistor: temperature.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

Learn Thermistors and LDRs properly — interactive practice, marked questions and flashcards.

Start this lesson free

More AQA GCSE Physics topics

See the full AQA Physics curriculum →

How this lesson was checked. This AQA GCSE Physics (specification 8463)lesson was published through Lightbulb Learning's human-designed editorial process — the educational standards, accuracy rules and publication checks it must pass were authored and approved by Philip Halpin. It passed subject-specific assessment, automated educational checks and technical publication verification before going live (publication checks completed 2 August 2026). Published pages are monitored, human spot-checking is ongoing across the lesson library, and anything found wrong is corrected or withdrawn. How our lessons are made and checked. Spotted a mistake? Email hello@lightbulblearning.co and we'll review it.

Helpful guides for parents and students