1. 1
   A car manufacturer includes a diode in the circuit connecting the car battery to its onboard electronic control unit, to protect it if the battery is ever connected the wrong way round.

   (a)

   A car manufacturer includes a diode in the circuit that connects the car battery to its onboard electronic control unit, to prevent damage if the battery terminals are ever connected the wrong way round during maintenance. Which property of a diode allows it to perform this protective function?

   [1 mark]

   • AIt allows current to flow equally well in both directions
   • BIt allows current to flow easily in one direction only
   • CIts resistance decreases as temperature increases
   • DIts resistance decreases as light intensity increases

   (b)

   If the battery is connected in the correct direction, what happens to the current in the circuit containing the diode?

   [1 mark]

   • ACurrent flows normally, since the diode is forward biased
   • BNo current flows at all, since the diode always blocks current
   • CCurrent flows but at a much lower value than without the diode
   • DCurrent only flows briefly before stopping

   (c)

   Explain what would happen to the current flowing to the electronic control unit if the battery were accidentally connected with reversed polarity, and explain why this protects the control unit.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A greenhouse automation system uses a thermistor connected in a circuit to control an electric heater, switching the heater on when the greenhouse gets too cold. The sensing circuit has a constant potential difference of 4 V across it.

   (a)

   As the temperature inside the greenhouse falls overnight, what happens to the resistance of the thermistor?

   [1 mark]

   • AThe resistance becomes negative
   • BThe resistance decreases
   • CThe resistance stays exactly the same
   • DThe resistance increases

   (b)

   As the thermistor's resistance changes overnight, what happens to the current flowing in its sensing circuit, assuming the potential difference across the circuit stays constant?

   [1 mark]

   • AThe current increases as resistance increases
   • BThe current stays the same regardless of resistance
   • CThe current decreases as resistance increases
   • DThe current becomes zero as soon as temperature falls

   (c)

   At a certain overnight temperature, the thermistor's resistance is 800 ohms, and the sensing circuit has a constant potential difference of 4 V across it. Calculate the current flowing in the sensing circuit at this temperature.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A designer is choosing components for an automatic street lighting system that must switch the lights on as it gets dark and off again at dawn. The designer is also deciding between fitting a separate light sensor to each individual street lamp, or using one shared sensing circuit to control the entire street of lamps.

   (a)

   Explain why a light-dependent resistor (LDR) is a suitable component for this system, describing how its resistance changes with light intensity and how this could be used to control a switch.

   [3 marks]

   (b)

   The designer is deciding between using a single LDR sensing circuit for an entire street of lamps, or fitting a separate LDR to each individual lamp. Evaluate one advantage and one disadvantage of fitting a separate LDR to each lamp, compared with using one shared LDR circuit for the whole street.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   An appliance manufacturer is designing a smart kettle that must switch off automatically at boiling point and include reverse-polarity protection for its circuit board, choosing between a thermistor, a diode, an LDR and a fixed resistor for these two functions.

   (a)

   Evaluate which components should be used for each function, explaining the reasoning based on how each component's resistance behaves.

   [6 marks]

   (b)

   During testing, the manufacturer finds that the kettle's thermistor-based switch-off circuit sometimes triggers slightly earlier or later than the true boiling point, depending on how quickly the water is heated. Evaluate possible reasons for this inconsistency, and suggest how the design team could improve the reliability of the switch-off function while still using a thermistor as the sensing component.

   [6 marks]

   Total for question 4: 12 marks

End of questions