Physics

18.1 - Components

6 min

Circuit symbols

Meters and resistors

LDRs and thermistors

note 1 of 3

How resistance depends on length and area

The resistance of a wire depends on the material it's made from, its length, and its cross-sectional area.
The longer the wire, the greater its resistance.
The greater the cross-sectional area, the smaller its resistance.
A long, thin wire therefore has a higher resistance than a short, thick wire of the same material.

note 2 of 3

Light-dependent resistors (LDRs)

A light-dependent resistor (LDR) is a resistor whose resistance depends on the intensity of light falling on it.
In darkness, its resistance is highest.
As the light gets brighter, its resistance falls.
Common applications: automatic night lights, outdoor lighting, and burglar detectors.

A curve falling steeply then levelling off, on axes labelled resistance (Ω) and light intensity, its high end marked dark and low end bright, beside a boxed circuit symbol labelled LDR.

note 3 of 3

Thermistors

A thermistor is a resistor whose resistance depends on temperature.
In cold conditions, its resistance is high.
As the temperature rises, its resistance falls.
Because resistance falls as temperature rises, it's called a negative temperature coefficient (NTC) thermistor.
Common applications: car engine temperature sensors, thermostats, irons, and fire alarms.

A curve falling steeply then levelling off, on axes labelled resistance (Ω) and temperature (°C), its high end marked cold and low end hot, beside a boxed circuit symbol labelled thermistor.

key terms

TermDefinition

Light-dependent resistor (LDR)A resistor whose resistance decreases as the intensity of light falling on it increases.

ThermistorA resistor whose resistance decreases as its temperature increases.

end of notes

Key points to remember

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18.2 - Series CircuitsNext up