1. 1
   An electric kettle in a family kitchen is labelled as drawing a current of 10 A from the 230 V mains supply, and a student calculates how much electrical energy it transfers when boiling water.

   (a)

   An electric kettle is labelled as drawing a current of 10 A from the 230 V mains supply. Which equation should be used to calculate the electrical power of the kettle?

   [1 mark]

   • A$P = \frac{V}{I}$
   • B$P = IV$
   • C$P = \frac{I}{V}$
   • D$P = I + V$

   (b)

   Using $P = IV$ with I = 10 A and V = 230 V, what is the power of the kettle?

   [1 mark]

   • A23 W
   • B240 W
   • C2300 W
   • D2.3 W

   (c)

   Explain, in terms of energy transfer, what happens to the electrical energy supplied to the kettle's heating element while it is switched on.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A factory installs a large stamping machine that operates from a 400 V supply and has a power rating of 8000 W, and the plant's engineer needs to check that the machine's current draw is within the wiring's safe limit.

   (a)

   Which equation should be rearranged to find the current drawn by the machine?

   [1 mark]

   • A$P = IV$
   • B$E = IVt$
   • C$R = \frac{V}{I}$
   • D$Q = It$

   (b)

   Using $I = \frac{P}{V}$ with P = 8000 W and V = 400 V, what current does the stamping machine draw?

   [1 mark]

   • A3 200 000 A
   • B8400 A
   • C0.05 A
   • D20 A

   (c)

   Explain why the engineer needs to know the current drawn by the stamping machine, not just its power rating, when choosing suitable cabling and a fuse for the machine's circuit.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A household electric heater has a power rating of 2000 W and is used for several hours on a cold evening, and the household wants to work out how much this costs on their electricity bill.

   (a)

   The heater is switched on for 3 hours. Calculate the energy transferred by the heater during this time, in kilowatt-hours (kWh).

   [3 marks]

   (b)

   The electricity supplier charges 0.28 dollars per kWh. Using your answer to part (a), calculate the cost of running the heater for the 3 hours, and explain why electricity bills are usually calculated using the kilowatt-hour rather than the joule as the unit of energy.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A homeowner is deciding whether to replace ten old incandescent light bulbs, each rated at 60 W, with ten LED bulbs, each rated at 8 W, that produce a similar brightness, with each bulb used for an average of 5 hours per day.

   (a)

   Explain, using the equations for electrical energy and power, and the concept of the kilowatt-hour, how to work out the yearly energy saving from switching to LED bulbs, and discuss why this could still make the LED bulbs worthwhile even though they cost more to buy initially.

   [6 marks]

   (b)

   A neighbour argues that because the LED bulbs have a much lower power rating, they must also transfer much less total energy over their lifetime and so cannot possibly be as bright as the incandescent bulbs for the same length of use. Using the equations for electrical power and electrical energy, explain why the neighbour's reasoning about brightness is flawed, and explain how a lower power rating combined with equal or greater efficiency of converting electrical energy to light can still give the same brightness while transferring less total energy.

   [6 marks]

   Total for question 4: 12 marks

End of questions