GCSE · Physics · AQA · Spec 8463
Thermistors and LDRs
Two components can make a circuit notice heat or light without anyone touching a switch.
Physics · Sensing circuits
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Switch sensor ↑ · tap for the rule
As temperature increases, a thermistor's resistance decreases.
Increase the stimulus. In both cases, resistance falls.
Predict, then check
Think about the direction of each change.
The temperature around a thermistor increases and the light intensity on an LDR increases. What happens to their resistances?
V = I R
Tap the quantity you want to find. The triangle shows you the formula.
Cover potential difference, current or resistance to reveal its rearranged formula, then plug in numbers to solve.
For AQA Physics in the 2025–2027 series, the full physics equation sheet is provided with the papers.
Find the resistance
Problem
At one instant in a model sensor circuit, the potential difference across the sensor is 6.0 V and the current through it is 0.020 A. Calculate its resistance.
Equations you need
Taken directly from the exam-board specification.
V = potential difference (V) · I = current (A) · R = resistance (Ω)
Learn it — you must recall this in the exam
WHAT YOU'VE LEARNED
A quick recap of today's lesson.
What you need to know
- Thermistor: temperature increases → resistance decreases.
- LDR: light intensity increases → resistance decreases.
- A thermistor can be used in a thermostat; an LDR can be used to switch a light on when it becomes dark.
- Thermistors and LDRs are non-ohmic components: their resistance changes with the relevant condition.
- At a stated condition, V = I R links potential difference, current and resistance.
- For AQA Physics, V = I R is classified as recall_and_apply. For the 2025–2027 exam series, the full equation sheet also prints recall-and-apply equations including this one.
The big picture
A thermistor and an LDR are both non-ohmic components whose resistance changes with conditions. A thermistor's resistance decreases as temperature increases; an LDR's resistance decreases as light intensity increases. That makes thermistors useful in thermostats and LDRs useful in circuits that switch a light on when it becomes dark. At a stated condition, V = I R links potential difference, current and resistance.
Key points
Worked example
Problem
An LDR has a potential difference of 6.0 V across it and a resistance of 600 Ω at a particular light intensity. Calculate the current. Then state what happens to the current if the light intensity increases while the potential difference stays the same.
⚠ Watch out
Reversing the trend. A hotter thermistor has lower resistance, and a more brightly lit LDR has lower resistance. When the stimulus falls, the resistance rises.
★ Exam tip
For AQA Physics, V = I R keeps its enduring recall_and_apply classification. For the 2025–2027 exam series the full physics equation sheet also prints it. Keep the sensor trend separate from the equation: first identify how resistance changes, then use V = I R if numerical values are given.
Memory hook
More heat or more light → less R. Thermistor: hotter, lower resistance. LDR: brighter, lower resistance.
Check yourself
An LDR is connected at a fixed potential difference. A cloud blocks some of the light. What happens to the LDR's resistance and to the current?
Flashcards
(12)What happens to a thermistor's resistance as its temperature increases?
What happens to an LDR's resistance as light intensity increases?
What trend do thermistors and LDRs have in common?
Give one application of a thermistor.
Give one application of an LDR.
Are thermistors and LDRs ohmic or non-ohmic components?
What equation links potential difference, current and resistance?
What do V, I and R represent in V = I R?
A component has a potential difference of 6 V and a resistance of 300 Ω. What current flows?
At fixed potential difference, a thermistor gets hotter and its resistance falls. What happens to the current?
At fixed potential difference, light intensity on an LDR increases. What happens to its resistance and current?
For AQA Physics in the 2025–2027 exam series, is V = I R printed on the provided equation sheet?
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.
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