What is a light-dependent resistor and how does its resistance vary?

A light-dependent resistor is a semiconductor component whose resistance falls sharply as the light falling on it becomes brighter, from perhaps megohms in darkness to a few hundred ohms in bright light.

Why is a negative temperature coefficient thermistor suitable as the sensing element in a heating control circuit?

Its resistance falls markedly as it gets hotter, so when it is used as one arm of a potential divider the output potential difference changes substantially with temperature and can be used to operate a switch.

Describe how the output of a potential divider changes with the light level when a light-dependent resistor is the upper arm and a fixed resistor is the lower arm.

In darkness the light-dependent resistor has a very high resistance so most of the supply is dropped across it and the output across the fixed resistor is small, while in bright light its resistance collapses and the output rises towards the supply potential difference.

How is the light level at which a street lamp switches on decided in a light-sensing potential divider?

It is set by the value of the fixed resistor in the other arm, which is usually made variable so that the output reaches the switching threshold at whatever light level the light-dependent resistor's resistance corresponds to.

CCEA A-Level Physics 1210

Physical Quantities, Scalars and Vectors

Topic overview Physical quantities and SI base units Prefixes and standard form Scalars and vectors Resolving vectors and finding resultants Coplanar forces in equilibrium

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Linear Motion and Dynamics

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Newton's Laws of Motion

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Current, Charge and Potential Difference

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Internal Resistance and Potential Dividers

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Internal Resistance and Potential Dividers

Potential dividers in lighting and heating control circuits

3 things to know
Unit AS 1

Overview

Recall questions on the light-dependent resistor and the thermistor as sensing elements, how the output changes with light or temperature, the effect of swapping the sensor and the fixed resistor, using the output to switch a lamp or a heater, a frost-warning circuit, and worked calculations for both sensors.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Describe how a potential divider containing a thermistor can be used to switch a heater on when a room becomes cold.

Key term

What is the effect of exchanging the positions of the sensor and the fixed resistor in a sensing potential divider?

Key term

How is the varying output of a sensing potential divider used to switch a lamp or a heater on and off?

Questions from this subtopic

Each one with the model answer underneath.

1
What is a light-dependent resistor and how does its resistance vary?
Model answer
A light-dependent resistor is a semiconductor component whose resistance falls sharply as the light falling on it becomes brighter, from perhaps megohms in darkness to a few hundred ohms in bright light.
2
Why is a negative temperature coefficient thermistor suitable as the sensing element in a heating control circuit?
Model answer
Its resistance falls markedly as it gets hotter, so when it is used as one arm of a potential divider the output potential difference changes substantially with temperature and can be used to operate a switch.
3
Describe how the output of a potential divider changes with the light level when a light-dependent resistor is the upper arm and a fixed resistor is the lower arm.
Model answer
In darkness the light-dependent resistor has a very high resistance so most of the supply is dropped across it and the output across the fixed resistor is small, while in bright light its resistance collapses and the output rises towards the supply potential difference.
4
What is the effect of exchanging the positions of the sensor and the fixed resistor in a sensing potential divider?
Model answer
The sense of the output is reversed, so a circuit whose output rose with increasing light or temperature now gives an output that falls, which is how the same components can switch a device either on or off at a threshold.
5
How is the varying output of a sensing potential divider used to switch a lamp or a heater on and off?
Model answer
The output is fed to the input of a transistor or the input of a comparator, which switches its output state once the potential difference passes a set threshold, and that output operates a relay or drives the load directly.
6
How is the light level at which a street lamp switches on decided in a light-sensing potential divider?
Model answer
It is set by the value of the fixed resistor in the other arm, which is usually made variable so that the output reaches the switching threshold at whatever light level the light-dependent resistor's resistance corresponds to.

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