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CIE A Level Physics復習筆記9.3.2 Resistance in Sensory Resistors

Category: A-level課程, 教材筆記, 福利干貨 Date: 2022年8月16日下午4:05

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

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

- 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