1. 1
   A technician connects an oscilloscope to two different household power sockets in a house that has both a battery-powered backup inverter and the normal a.c. mains supply. On the oscilloscope screen, the trace from the battery inverter output is a flat horizontal line, while the trace from the normal mains socket is a smooth wave that repeats above and below the zero line 50 times every second.

   (a)

   Which type of supply produces the flat horizontal trace?

   [1 mark]

   • AAlternating current (a.c.)
   • BInduced current
   • CDirect current (d.c.)
   • DEddy current

   (b)

   What is the frequency, in hertz, of the UK a.c. mains supply shown by the second trace?

   [1 mark]

   • A5 Hz
   • B50 Hz
   • C230 Hz
   • D500 Hz

   (c)

   Describe the difference between direct current and alternating current in terms of the movement of charge.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   An electrician is rewiring a three-pin plug for a domestic kettle. The plug has three wires: one connected to the live pin, one to the neutral pin, and one to the earth pin. The electrician measures the potential difference between each pair of pins with the appliance switched off and unplugged from the wall socket, then compares this with typical UK domestic supply values.

   (a)

   Which statement correctly describes the potential difference of the live wire relative to earth in a working UK mains circuit?

   [1 mark]

   • AThe live wire is at about 230 V relative to earth
   • BThe live wire is at 0 V relative to earth
   • CThe live wire is at about 50 V relative to earth
   • DThe live wire is always at a higher potential than the neutral wire by exactly 12 V

   (b)

   In which wire should the fuse and the switch for the kettle be connected, and why?

   [1 mark]

   • AThe neutral wire, because it carries no current when the appliance is off
   • BEither wire, because it makes no difference to safety
   • CThe earth wire, because it is the safest wire to interrupt
   • DThe live wire, because it becomes dangerously live even when the switch is off if connected elsewhere

   (c)

   Explain the function of the earth wire in the kettle's plug, and describe the danger that would arise if a fault caused the live wire to touch the metal casing of the kettle while there was no earth wire connected.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A maintenance worker accidentally cuts through the insulation on a wire inside a wall socket while drilling, exposing bare copper. The worker cannot tell from the colour of the remaining insulation whether the exposed wire is the live, neutral, or earth wire, and the circuit breaker for that socket has already been switched off at the consumer unit before the drilling began. A colleague suggests touching the bare wire briefly with a finger to check whether it is still live, arguing that if the circuit breaker is off, no current can be flowing.

   (a)

   Explain why the colleague's suggestion is dangerous, referring to the potential difference between the live wire and earth.

   [3 marks]

   (b)

   The maintenance worker safely tests the wire and finds it is not live, then repairs the insulation and reconnects the socket. A heater plugged into this socket is later found to be transferring 2760 J of energy every 12 seconds while operating normally from the 230 V UK mains supply. Calculate the current flowing through the heater, showing your working and stating the equation you use.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   An old caravan is fitted with a domestic-style electric heater designed to run from the 230 V a.c. UK mains supply, connected via a standard three-pin plug and a long extension cable. The heater normally draws only 1 A of current. The caravan owner reports that the heater sometimes runs on a portable battery pack through an inverter when away from a campsite, and at other times is plugged directly into a campsite mains hook-up point.

   (a)

   Explain how energy is transferred to the heater in each case, describing the difference between the direct current supplied by the battery pack and the alternating current supplied by the campsite mains, and explain why the campsite supply must still be treated with the same safety precautions (live, neutral and earth wiring, fuses and switches in the live wire) as a normal household mains supply.

   [6 marks]

   (b)

   A campsite warden notices that several caravan owners have replaced the fuse in their plug with one of a much higher rating than the appliance requires, believing this will stop the fuse from blowing unexpectedly during storms. Evaluate whether this practice is safe, explaining how a correctly rated fuse works together with the earth wire to protect both the appliance and the user, and what could go wrong if the fuse rating is too high for the heater described above, which normally draws only 1 A of current.

   [6 marks]

   Total for question 4: 12 marks

End of questions