Thermal energy transferAQA A-Level Physics: Subtopic test

1. 1
   An electric kettle has a heating element of power 2.2 kW. It contains 2.0 kg of water at 20 °C, which is heated to 100 °C. The specific heat capacity of water is 4180 J kg⁻¹ K⁻¹.

   (a)

   What is the minimum energy that must be transferred to the water?

   [1 mark]

   • A3.3 × 10⁵ J
   • B1.7 × 10⁵ J
   • C6.7 × 10⁵ J
   • D8.4 × 10⁵ J

   (b)

   What is the shortest time the kettle takes to bring the water to 100 °C?

   [1 mark]

   • A3.0 × 10² s
   • B1.5 × 10² s
   • C6.1 × 10² s
   • D3.3 × 10⁻³ s

   (c)

   In practice the kettle takes longer than the time calculated in part (b). Suggest two reasons why.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A gas is trapped in a cylinder by a piston. The piston is pushed in quickly, so that 450 J of work is done on the gas, and during the process 120 J of energy is transferred from the gas to its surroundings by heating.

   (a)

   What is the change in internal energy of the gas?

   [1 mark]

   • A+570 J
   • B+330 J
   • C−330 J
   • D−570 J

   (b)

   Which statement gives the correct general definition of the internal energy of a substance?

   [1 mark]

   • AThe kinetic energy of the gas as a whole as it moves
   • BThe total of the kinetic energy of the molecules and the gravitational potential energy of the whole cylinder
   • CThe sum of the kinetic energies of the molecules only
   • DThe sum of the randomly distributed kinetic and potential energies of the molecules

   (c)

   Use the idea of internal energy to explain why the temperature of the gas rises when it is compressed quickly.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A beaker contains 0.250 kg of ice at 0 °C. It is heated by a 150 W immersion heater, and all of the energy from the heater is transferred to the ice. The specific latent heat of fusion of ice is 3.34 × 10⁵ J kg⁻¹.

   (a)

   The temperature of the ice remains at 0 °C while it melts, even though energy is being supplied. Explain this in terms of the energies of the molecules.

   [3 marks]

   (b)

   Calculate the time taken for all the ice to melt.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A continuous-flow electric water heater has a power of 8.5 kW. Water enters it at 12 °C. The specific heat capacity of water is 4180 J kg⁻¹ K⁻¹. In part (a) the heater is 100 % efficient. In part (b) a laboratory version of the same arrangement is used to measure the specific heat capacity of water.

   (a)

   The water is to leave the heater at 42 °C. Calculate the mass of water that flows through the heater each second. Then calculate the temperature of the water leaving the heater when the flow rate is increased to 0.100 kg s⁻¹.

   [6 marks]

   (b)

   Explain how the continuous-flow method can be used to find the specific heat capacity of a liquid so that the energy transferred to the surroundings does not have to be measured.

   [6 marks]

   Total for question 4: 12 marks

End of questions