Electrical power and energyIB MYP Physics: Subtopic test

1. 1
   A café owner in Istanbul buys a new electric kettle. The label on the base states that it is designed for the 230 V mains supply. While testing it, she finds that the kettle draws a steady current of 8.0 A when switched on.

   (a)

   What is the power of the kettle?

   [1 mark]

   • A29 W
   • B1840 W
   • C238 W
   • D0.035 W

   (b)

   Which expression gives the power of a component in terms of its current I and resistance R?

   [1 mark]

   • AP = I²R
   • BP = IR²
   • CP = I ÷ R
   • DP = R ÷ I²

   (c)

   The kettle has a power of 1840 W. Calculate the energy it transfers in 3.0 minutes.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A family in Cairo has a 2.0 kW air conditioner. They run it for 6.0 hours every day. Their electricity supplier charges 10 cents for each kilowatt-hour (kWh) of energy.

   (a)

   How much energy does the air conditioner transfer in one day?

   [1 mark]

   • A8.0 kWh
   • B0.33 kWh
   • C3.0 kWh
   • D12 kWh

   (b)

   What is a kilowatt-hour?

   [1 mark]

   • AA unit of power equal to 1000 W
   • BA unit of current
   • CThe energy transferred by a 1 kW device running for 1 hour
   • DThe cost of 1 unit of electricity

   (c)

   Calculate the cost of running the air conditioner for 30 days.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student investigates how efficiently an electric heater warms water. She connects a 12 V heater, which carries a current of 5.0 A, to a beaker of water and switches it on for 5.0 minutes. She then measures how much thermal energy the water has gained.

   (a)

   Calculate the power of the heater and the electrical energy transferred to it in 5.0 minutes.

   [3 marks]

   (b)

   The water gained 13 500 J of thermal energy. Calculate the efficiency of the heater. Suggest why it is not 100%, and suggest one change to the method that would improve the efficiency.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A school hall in Lagos has 40 filament lamps, each rated at 100 W. The school is considering replacing them with 40 LED lamps, each rated at 12 W, that give the same amount of light. The lamps are used for 5.0 hours a day for 200 days a year. Electricity costs 20 cents per kWh, and each LED lamp costs 6 dollars to buy.

   (a)

   Calculate the money the school would save on electricity in one year by replacing the filament lamps with the LED lamps.

   [6 marks]

   (b)

   Evaluate whether the school should replace the filament lamps with the LED lamps, using your answer to (a).

   [6 marks]

   Total for question 4: 12 marks

End of questions