Resistance in Sensory Resistors (CIE A Level Physics)

• 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

• • As the potential difference across the LDR increases, the current 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

• 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

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