# 9.3.2 Resistance in Sensory Resistors

### Resistance in a Light-Dependent Resistor

• A light-dependent resistor (LDR) is a non-ohmic conductor and sensory resistor
• Its resistance automatically changes depending on the light energy falling onto it (illumination)
• As the light intensity increases, the resistance of an LDR decreases
• This is shown by the following graph:

Graph of light intensity and resistance for an LDR

• LDRs can be used as light sensors, so, they are useful in circuits which automatically switch on lights when it gets dark, for example, street lighting and garden lights
• In the dark, its resistance is very large (millions of ohms)
• In bright light, its resistance is small (tens of ohms)

Resistance of an LDR depends on the light intensity falling on it

#### Worked example

• As the potential difference across the LDR increases, the light intensity increases causing its resistance to decrease
• Ohm’s law states that V = IR
• The resistance is equal to V/I or 1/R = I/V = gradient of the graph
• Since R decreases, the value of 1/R increases, so the gradient must increase
• Therefore, I increases with the p.d with an increasing gradient

### Resistance in a Thermistor

• A thermistor is a non-ohmic conductor and sensory resistor
• Its resistance changes depending on its temperature
• As the temperature increases the resistance of a thermistor decreases
• This is shown by the following graph:

Graph of temperature and resistance for a thermistor

• Thermistors are temperature sensors and are used in circuits in ovens, fire alarms and digital thermometers
• As the thermistor gets hotter, its resistance decreases
• As the thermistor gets cooler, its resistance increases

The resistance through a thermistor is dependent on the temperature of it

#### Worked Example

A thermistor is connected in series with a resistor R and a battery.

The resistance of the thermistor is equal to the resistance of R at room temperature.

When the temperature of the thermistor decreases, which statement is correct?

A.     The p.d across the thermistor increases
B.     The current in R increases
C.     The current through the thermistor decreases
D.     The p.d across R increases

• The resistance of the thermistor increases as the temperature decreases
• Since the thermistor and resistor R are connected in series, the current I in both of them is the same
• Ohm’s law states that V = IR
• Since the resistance of the thermistor increases, and I is the same, the potential difference V across it increases
• Therefore, statement A is correct

### Author: Ashika

Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.
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