Thermistors, LDRs & Diodes (Oxford AQA IGCSE Physics)

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

Exam 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.

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

Exam 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.