Specific heat capacity and latent heatEdexcel A-Level Physics: Subtopic test

1. 1
   A 2.0 kW electric kettle contains 1.5 kg of water at 20 °C. The water is heated to 100 °C and the kettle is then switched off. The specific heat capacity of water is 4180 J kg⁻¹ K⁻¹. In practice the kettle takes 280 s to do this.

   (a)

   What is the energy needed to raise the temperature of the water from 20 °C to 100 °C?

   [1 mark]

   • A6.3 × 10⁵ J
   • B1.3 × 10⁵ J
   • C3.3 × 10⁵ J
   • D5.0 × 10⁵ J

   (b)

   What is the efficiency of the kettle in heating the water?

   [1 mark]

   • A112 %
   • B90 %
   • C10 %
   • D50 %

   (c)

   Calculate the energy transferred to the surroundings and the kettle itself while the water is being heated.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student determines the specific latent heat of fusion of ice. A 60 W heater is placed in crushed ice at 0 °C in a funnel above a beaker. An identical funnel of ice without a heater is set up beside it. After 300 s the heated funnel has produced 62.0 g of melted water and the other funnel has produced 8.0 g.

   (a)

   Why is the second funnel, without a heater, used?

   [1 mark]

   • ATo check that the heater power is 60 W
   • BTo ensure that the ice stays at 0 °C
   • CTo measure how much ice melts because of energy from the room, so it can be subtracted
   • DTo measure the specific heat capacity of the ice

   (b)

   The ice stays at 0 °C while it is melting, even though the heater supplies energy. What is the explanation?

   [1 mark]

   • AThe energy increases the potential energy of the molecules as the bonds between them are broken, not their mean kinetic energy
   • BThe energy increases the mean kinetic energy of the molecules, but the water carries it away immediately
   • CThe energy is all transferred to the surroundings
   • DThe energy increases the mass of the ice

   (c)

   Calculate the specific latent heat of fusion of ice from these results.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student calibrates an NTC thermistor so that it can be used as the sensor in a thermostat for a greenhouse heater. The thermistor is in series with a 4.0 kΩ fixed resistor across a 6.0 V supply, and the output p.d. is taken across the fixed resistor. A relay switches the heater off when the output p.d. rises to a set value.

   (a)

   Describe how the student could calibrate the thermistor circuit.

   [3 marks]

   (b)

   At 20 °C the thermistor has a resistance of 12 kΩ. Calculate the output p.d. at 20 °C. The heater is to switch off when the output p.d. reaches 3.0 V. Calculate the resistance of the thermistor when this happens.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A physics class carries out electrical heating experiments using a 50 W immersion heater. The accepted specific heat capacity of aluminium is 900 J kg⁻¹ K⁻¹ and the accepted specific latent heat of vaporisation of water is 2.26 × 10⁶ J kg⁻¹.

   (a)

   A 1.00 kg block of aluminium is heated by the immersion heater for 300 s, and its temperature rises from 20.0 °C to 35.0 °C. Calculate the specific heat capacity of aluminium from this result, and evaluate the procedure, suggesting improvements.

   [6 marks]

   (b)

   The heater is now placed in a beaker of boiling water on a balance. In 600 s the reading on the balance falls by 11.5 g. Calculate the specific latent heat of vaporisation of water from this result, and evaluate the method, suggesting how the effect of energy losses could be reduced.

   [6 marks]

   Total for question 4: 12 marks

End of questions