CIE AS Physics (9702) exams from 2022

Revision Notes

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:

LDR graph, downloadable AS & A Level Physics revision notes

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)

 

LDR diagram, downloadable AS & A Level Physics revision notes

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

 

Worked example

Worked example - LDRs, downloadable AS & A Level Physics revision notes

 

  • 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:

Thermistor graph, downloadable AS & A Level Physics revision notes

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

Thermistor diagram, downloadable AS & A Level Physics revision notes

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.

WE - thermistor in circuit question image, downloadable AS & A Level Physics revision notes

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

Answer: A

  • 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

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