AQA GCSE Physics (8463), Higher tier · Energy › Energy stores and changes
Practise Energy changes in systems. 19 exam-style questions plus unlimited generated ones on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
A student heats some water in a beaker.
(a) What is meant by the specific heat capacity of a substance? Tick (✓) one box.[1]
The energy needed to change the state of 1 kg of the substance.
The energy needed to raise the temperature of 1 kg of the substance by 1 °C.
The energy needed to raise the temperature of the substance by 1 °C.
The temperature rise of 1 kg of the substance when 1 J of energy is supplied.
(b) The mass of the water is 2.0 kg. The temperature of the water increases by 15 °C. The specific heat capacity of water is 4200 J/kg °C. Calculate the energy transferred to the water. Use the equation: change in thermal energy = mass × specific heat capacity × temperature change[2]
(c) The student then transfers the same amount of energy to 2.0 kg of cooking oil. Cooking oil has a lower specific heat capacity than water. How does the temperature rise of the oil compare with the temperature rise of the water? Tick (✓) one box.[1]
The temperature rise of the oil is greater.
The temperature rise of the oil is the same.
The temperature rise of the oil is smaller.
Show the answer and mark scheme
(a)Answer: The energy needed to raise the temperature of 1 kg of the substance by 1 °C.
(b)Answer: 126 000 J
ΔE = 2.0 × 4200 × 15
ΔE = 126 000 (J)
(c)Answer: The temperature rise of the oil is greater.
Question 2Medium7 marks
A hot-water bottle contains 1.5 kg of water. Specific heat capacity of water = 4200 J/kg °C
(a) The water cools from 60 °C to 25 °C. Calculate the energy transferred from the water. Use the Physics Equations Sheet.[2]
(b) A wheat bag can be used instead of a hot-water bottle. It is heated in a microwave oven. The wheat bag has a mass of 0.80 kg. When 57 600 J of energy is transferred to the wheat bag, its temperature rises from 20 °C to 60 °C. Calculate the specific heat capacity of the wheat bag.[3]
(c) Explain why water is a good substance to use in a hot-water bottle.[2]
Show the answer and mark scheme
(a)Answer: 220 500 J
ΔE = 1.5 × 4200 × 35
ΔE = 220 500 (J)
(b)Answer: 1800 J/kg °C
57 600 = 0.80 × c × 40
c = 57 600 ÷ 32
c = 1800 (J/kg °C)
(c)Answer: Water has a high specific heat capacity, so a lot of energy is transferred to the surroundings for each degree it cools.
water has a high specific heat capacity
so a lot of energy is released / transferred for each °C fall in temperature (so it stays warm for a long time)
Question 3Hard8 marks
An electric kettle has a power of 2.8 kW. It contains 1.2 kg of water at 18 °C. Specific heat capacity of water = 4200 J/kg °C
(a) Calculate the energy needed to raise the temperature of the water to 100 °C. Use the Physics Equations Sheet.[3]
(b) Calculate the shortest possible time the kettle could take to heat the water to 100 °C.[2]
(c) The kettle actually takes 170 s to heat the water to 100 °C. Calculate the efficiency of the kettle. Give your answer as a percentage.[3]