1. 1
   A student builds a simple circuit containing a cell, a switch, a fixed resistor and a lamp, all connected one after another in a single loop, to investigate how each component affects the circuit.

   (a)

   A student describes a circuit containing a cell, a switch, a fixed resistor and a lamp, all connected one after another in a single loop. What term describes a component, such as the switch in this circuit, that is used to open or close the circuit and stop or allow current to flow?

   [1 mark]

   • AA switch
   • BA relay
   • CA transformer
   • DA thermistor

   (b)

   The student replaces the fixed resistor with a variable resistor. What is the purpose of a variable resistor in a circuit?

   [1 mark]

   • ATo measure the current flowing in the circuit
   • BTo allow the resistance, and therefore the current, in the circuit to be adjusted
   • CTo store electrical charge for later use
   • DTo convert alternating current into direct current

   (c)

   State the difference in how an ammeter and a voltmeter must each be connected within this circuit, and state the electrical quantity each instrument measures.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A greenhouse heating system uses a small control circuit containing a thermistor (NTC type) connected to a relay, which switches on a much larger separate heater circuit automatically when the greenhouse becomes cold.

   (a)

   How does the resistance of an NTC thermistor change as its temperature falls?

   [1 mark]

   • AIts resistance decreases as temperature falls
   • BIts resistance stays exactly constant regardless of temperature
   • CIts resistance increases as temperature falls
   • DIts resistance becomes zero at any temperature below freezing

   (b)

   Why does the system use a relay to connect the small thermistor control circuit to the much larger heater circuit, rather than connecting the heater directly in series with the thermistor?

   [1 mark]

   • AA relay stores charge to power the heater when the control circuit is switched off
   • BA relay increases the resistance of the thermistor
   • CA relay converts direct current into alternating current
   • DA relay allows a small current in the control circuit to switch a much larger current in a separate circuit

   (c)

   Explain how the combination of the NTC thermistor and the relay allows the heater to switch on automatically as the greenhouse cools, describing the sequence of events in the control circuit.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A security company designs a control circuit for an outdoor light that must turn on automatically at dusk and off again at dawn, using a light-dependent resistor (LDR), a fixed resistor, a power supply and a lamp.

   (a)

   A security company designs a circuit for an outdoor light that must turn on automatically at dusk. The circuit contains an LDR (light-dependent resistor), a fixed resistor, a power supply, and a lamp, arranged so that the p.d. across the fixed resistor controls when the lamp switches on. Describe how the resistance of the LDR changes between daylight and darkness, and explain why this behaviour makes the LDR suitable for this application.

   [3 marks]

   (b)

   Explain how the LDR and the fixed resistor could be arranged as a potential divider in this circuit, and describe how the potential difference across the fixed resistor changes between daylight and darkness as a result.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A local council is designing an automatic streetlamp control system, in which a generator supplies a.c. to a step-down transformer, a diode converts the transformer's output to d.c., a fuse protects the circuit, an LDR-based potential divider detects darkness, and a relay switches the main lamp circuit on and off.

   (a)

   Explain the role of each of the transformer, the diode, the fuse and the relay in this system, and explain why a potential divider using an LDR is a suitable method of detecting when it becomes dark enough to switch the lamp on.

   [6 marks]

   (b)

   A councillor suggests removing the relay from the streetlamp system and instead connecting the LDR-based potential divider circuit directly in series with the main streetlamp itself, to simplify the design. Explain, using ideas about the current-carrying capability of small electronic components compared with a mains lamp circuit, and the purpose of a relay, why this suggestion is likely to cause the control circuit to fail or be damaged, and explain what would need to change for the LDR circuit to safely control the lamp without a relay.

   [6 marks]

   Total for question 4: 12 marks

End of questions