Electrical Components (Edexcel IGCSE Physics (Modular))
Revision Note
Written by: Ashika
Reviewed by: Caroline Carroll
Thermistors & LDRs
Environmental conditions, such as temperature and light intensity, can influence the resistance of resistors, such as
Thermistors
Light-dependent resistors (LDRs)
Thermistors
The resistance of a thermistor depends on its temperature
The resistance of a thermistor is high in cold conditions and low in hot conditions
As the temperature increases the resistance of a thermistor decreases
As the temperature decreases the resistance of a thermistor increases
The resistance of a thermistor depends on its temperature
The relationship between resistance and temperature for a thermistor can be shown on a graph
The graph of resistance against temperature for a thermistor shows a curve indicating these quantities are inversely proportional to each other
Light-dependent resistors (LDRs)
The resistance of a light-dependent resistor (LDR) depends on the light intensity on it
The resistance of an LDR is high in dark conditions and low in bright conditions
As the light intensity increases the resistance of an LDR decreases
As the light intensity decreases the resistance of an LDR increases
The resistance of an LDR depends on the intensity of light on it
The relationship between resistance and light intensity for an LDR can be shown on a graph
The graph of light intensity against temperature for an LDR shows a curve indicating these quantities are inversely proportional to each other
Lamps & LEDs
Lamps and light-emitting diodes (LEDs) illuminate (light up) when a current flows through them
This makes them useful for indicating the presence of a current in a circuit
Light-emitting diodes (LEDs)
LEDs are a type of diode
This means they only allow current to flow through them in one direction
Therefore, in a circuit, an LED will only light up if it is placed in the correct direction
The circuit symbol for an LED is as follows:
LEDs can be used to indicate the presence of a current as they illuminate when current flows through them. The same is true for lamps
Examiner Tips and Tricks
Make sure you learn the various symbols mentioned on this page. Many of them are very similar with small differences denoting what they do:
Two arrows pointing towards a symbol mean that it is light-dependent
Two arrows pointing away mean that it is light-emitting
Symbols are sometimes drawn with circles around them (e.g. the LDR). These circles are often optional (although not in the case of meters and bulbs).
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