Fig. 1 shows the temperature graph made when a 0.080 kg sample of candle wax is heated at a constant power of 120 W. At first the wax is solid. Between poin...
Question 1 Report
Fig. 1 shows the temperature graph made when a 0.080 kg sample of candle wax is heated at a constant power of 120 W. At first the wax is solid. Between points B and C the graph is horizontal. The student records time using a stopwatch and keeps the heater power constant.
(a) State the change of state taking place between B and C. [1]
(b) Calculate the energy supplied to the wax between B and C. [2]
(c) Explain why the temperature is constant between B and C although energy is supplied. [2]
(d) State the unit of energy used in your answer to part (b). [1]
Answer Details
(a) The horizontal section is where solid wax changes to liquid wax, so the change of state is melting. [1 mark]
(b) Read the times at B and C from the graph:
\[t=280-100=180\text{ s}\]
At constant power, \(E=Pt\):
\[E=120\times180=21\,600\text{ J}\]
21 600 J. [2 marks]
(c) During melting, supplied energy is used to overcome forces or bonds between wax particles. It increases their potential energy rather than their kinetic energy. Since temperature depends on average kinetic energy, the temperature stays constant. [2 marks]
(d) The unit of energy is the joule, J. [1 mark]
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