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
Which graph best represents the way in which the current I through an LDR depends upon the potential difference V across it?
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- 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
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
