1. 1
   A string of 20 identical old-style decorative fairy light bulbs is wired with all the bulbs connected in series to a single 24 V power supply. One bulb in the middle of the string burns out and its filament breaks completely.

   (a)

   What happens to the rest of the bulbs in the string?

   [1 mark]

   • AAll the other bulbs stay lit as normal
   • BAll the other bulbs go out, since the circuit is broken
   • COnly the bulbs before the broken one stay lit
   • DOnly the bulbs after the broken one stay lit

   (b)

   Before the bulb burned out, how did the potential difference across each identical bulb compare with the potential difference of the power supply, in this series string of 20 identical bulbs?

   [1 mark]

   • AEach bulb had the full supply potential difference across it
   • BThe potential difference across each bulb depended only on the total current
   • COnly the first bulb had any potential difference across it
   • DEach bulb had a share of the total supply potential difference

   (c)

   The fairy light string has 20 identical bulbs connected in series to a 24 V supply, and the bulbs share the potential difference equally. Calculate the potential difference across each bulb when the string is working normally.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A homeowner rewires their house lighting so that each room's lamp is connected in parallel to the 230 V mains supply. One lamp in the living room blows and its filament breaks.

   (a)

   What happens to the other lamps in different rooms?

   [1 mark]

   • AAll the other lamps also go out
   • BThe other lamps become dimmer but stay on
   • CThe other lamps continue to work normally
   • DThe other lamps become brighter

   (b)

   How does the potential difference across each lamp compare with the mains supply potential difference, given all the lamps are connected in parallel?

   [1 mark]

   • AEach lamp has the full supply potential difference across it
   • BEach lamp has a fraction of the supply potential difference, shared equally
   • COnly the lamp nearest the supply has the full potential difference
   • DThe potential difference across each lamp decreases as more lamps are added

   (c)

   Two identical lamps are connected in parallel across the 230 V mains supply, and each lamp draws a current of 0.4 A when working normally. Calculate the total current drawn from the mains supply when both lamps are switched on.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   An electronics student builds a circuit with two resistors, of resistance 10 ohms and 15 ohms, connected in series with a 12.5 V battery, and then reconnects the same two resistors in parallel with each other using the same battery.

   (a)

   Calculate the total resistance of the circuit and the current supplied by the battery.

   [3 marks]

   (b)

   The student then reconnects the same two resistors (10 ohms and 15 ohms) in parallel with each other, still using the same 12.5 V battery. Explain, without doing a full calculation of total resistance, why the total resistance of this parallel combination must be less than 10 ohms, and describe how the total current drawn from the battery will compare with the series arrangement.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   An engineer is designing the wiring for a small workshop with several power tools that may be used at the same time, and is deciding whether to wire the sockets in series or in parallel. The engineer also wants a single master emergency-stop switch that cuts power to all the tools at once.

   (a)

   Explain the practical and safety reasons why household and workshop circuits are always wired in parallel rather than in series, referring to potential difference, current, and the effect of switching individual appliances on and off.

   [6 marks]

   (b)

   The engineer also wants to include a single master emergency-stop switch that cuts power to all the tools in the workshop at once, while keeping the individual sockets wired in parallel to each other for normal use. Evaluate how the engineer could design the circuit to achieve both requirements, and explain why this arrangement combines features of both series and parallel wiring.

   [6 marks]

   Total for question 4: 12 marks

End of questions