Thermistors, LDRs & Diodes (Oxford AQA IGCSE Physics)
Revision Note
Thermistors
The resistance of a thermistor decreases as the temperature increases
Resistance and temperature are inversely proportional to each other
Thermistor temperature and resistance relationship
Thermistor resistance-temperature graph
Applications of thermistors
A thermistor is a temperature sensor and is regularly used as a thermostat
This means it automatically regulates temperature or activates a device when the temperature reaches a certain point
Thermistors are found in:
Ovens
Refrigerators
Fire alarms
Digital thermometers
Boilers
They are commonly used to regulate and monitor the temperature in environments where it must be carefully controlled e.g. food and beverage factories
Digital thermometer
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
Examiner Tip
In your exam, you must be able to describe the applications of thermistors in circuits including thermostats.
Remember the shape of the temperature-resistance graph, as it is a common exam question to draw and interpret. The graph should not touch the x-axis, as it implies 0 resistance which is only possible in something called a superconductor.
LDRs
The resistance of an LDR decreases as light intensity increases
Resistance and light intensity are inversely proportional to each other
LDR light intensity and resistance relationship
LDR resistance-light intensity graph
Applications of LDRs
LDRs are used as light sensors
They are useful in circuits that automatically switch lights on when it gets dark,
For example, street lighting and garden lights
In the dark, an LDR's resistance is large (millions of ohms)
In bright light, its resistance is small (tens of ohms)
Street lights
Diodes
The current through a diode flows in one direction only
So the ‘forward’ resistance is low and the ‘reverse’ resistance is very high
The direction is shown by the triangular arrow of the diode symbol
When current flows in the correct direction it is called forward bias
When current flows in the reverse direction it is called reverse bias
The I–V graph for a diode has a unique shape
When the diode is in forward bias, the graph shows a sharp increase in voltage and current (on the right side of the graph)
When the diode is switched around, in reverse bias, the graph shows a flat line where the current is zero at all voltages (on the left side of the graph)
Current-potential difference graph for a diode
Worked Example
The I–V characteristics of two electrical components X and Y are shown.
Which statement is correct?
A. The resistance of X increases as the current increases
B. At 2 V, the resistance of X is half the resistance of Y
C. Y is a diode and X is a resistor
D. X is a resistor and Y is a filament lamp
Answer: C
Line X on the I-V graph is linear
This means the graph has a constant gradient, I/V and the resistance is therefore also constant (since gradient = 1/R)
This is the I-V graph for a conductor at constant temperature e.g. a resistor
Line Y on the I-V graph starts with zero gradient and then the gradient increases rapidly
This means it has infinite resistance at the start which then decreases rapidly
This is characteristic of a device that only has current in one direction like a diode
Therefore the answer is C
Worked Example
Which circuit diagram correctly represents a circuit with current flowing through?
Answer: B
For a circuit to be connected, the switch must be closed
This is either circuit B or D
The other circuit symbol is a diode
Diodes only allow current to flow in one direction
Since the current flow is from positive to negative, a forward-biased diode must point in this direction for the current to flow
This is seen in circuit B
Examiner Tip
In your examination, you may be asked to plot, analyse or compare current-potential difference graphs for different components. Learn the shape of the graphs for the components mentioned above and what this means in terms of the relationship between current and potential difference.
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