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 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
- 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
