AQA GCSE Combined Science (8464), Higher tier · Physics › Energy › Energy changes in a system, and the ways energy is stored
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.
How much energy it takes to change the temperature of a material. You need to know what specific heat capacity means, use ΔE = m c Δθ, and know the required practical in which you measure the specific heat capacity of a material. Expect 2 to 4 mark calculations and practical questions, including 6-mark method questions.
Grade by grade
What you need to be able to do, from the first marks up to the top grade.
3
Name what affects the energy needed to heatThe energy needed depends on the mass, the material and the temperature rise.
4
Define specific heat capacityThe amount of energy needed to raise the temperature of 1 kg of a substance by 1 °C.
5
Calculate energy using ΔE = m c ΔθFind the temperature change first, then multiply mass × specific heat capacity × temperature change.
6
Rearrange to find c, m or ΔθE.g. c = ΔE ÷ (m × Δθ) or Δθ = ΔE ÷ (m × c).
6
Describe the specific heat capacity practicalHeat a metal block of known mass with an electric heater, measuring the energy supplied and the temperature rise.
7
Explain why the measured value is too highEnergy is dissipated to the surroundings, so more energy is supplied than the block gains.
8
Find c from a temperature–time graphWith a heater of constant power P, the gradient is P ÷ (m × c), so c = P ÷ (m × gradient).
Notes
Specific heat capacity
When you heat a substance, energy goes into its thermal (internal) store and its temperature rises. When it cools, energy is released.
The specific heat capacity (c) of a substance is the amount of energy needed to raise the temperature of 1 kg of the substance by 1 °C.
Water has a high specific heat capacity (about 4200 J/kg °C), so it takes a lot of energy to heat up and releases a lot as it cools. Most metals have much lower values, so they heat up quickly.
The equation
ΔE = m c Δθ (on the equations sheet)
ΔE = change in thermal energy (J), m = mass (kg), c = specific heat capacity (J/kg °C), Δθ = temperature change (°C).
Δθ = final temperature − starting temperature.
Rearranged: c = ΔE ÷ (m × Δθ) and Δθ = ΔE ÷ (m × c).
The energy often comes from an electric heater: E = P t, with P in watts and t in seconds.
Required practical: measuring specific heat capacity
Measure the mass of a metal block with a balance.
Put an electric heater and a thermometer into the holes in the block. Wrap the block in insulation.
Record the starting temperature, then switch on the heater. Measure the energy supplied with a joulemeter (or record the power and the time and use E = P t).
After a set time (e.g. 10 minutes), switch off. Record the energy supplied and the highest temperature the block reaches.
Calculate c = energy supplied ÷ (mass × temperature rise).
Your value is usually higher than the true value, because some energy is dissipated to the surroundings instead of heating the block. Insulation reduces this. grade 7+
With a heater of constant power P, a graph of temperature against time is a straight line with gradient P ÷ (m × c), so c = P ÷ (m × gradient). grade 8+
Cheatsheet
ΔE = m c Δθ (on the equations sheet)
Specific heat capacity: the energy needed to raise the temperature of 1 kg of a substance by 1 °C
Units: ΔE in J, m in kg, c in J/kg °C, Δθ in °C
Δθ = final temperature − starting temperature
Water: c ≈ 4200 J/kg °C (given when you need it)
Energy from a heater: E = P t (t in seconds)
Practical: c = energy supplied ÷ (mass × temperature rise)
Measured c too high: energy was dissipated to the surroundings grade 7+
How to answer each type of question
Calculate the energy needed to heat something
2 marks5
Work out the temperature change first.
Substitute into ΔE = m c Δθ.
Give the answer in J (or in kJ if the question asks).
Example. A saucepan contains 1.5 kg of water at 18 °C. The water is heated to 78 °C. specific heat capacity of water = 4200 J/kg °C Calculate the energy transferred to the water.
Calculate the specific heat capacity from practical results
3 to 4 marks7
Find the energy supplied: E = P t with t in seconds (or read the joulemeter).
Find the temperature rise.
Rearrange: c = ΔE ÷ (m × Δθ).
Give the unit J/kg °C.
Example. A student heats a 1.0 kg aluminium block with a 50 W heater for 6.0 minutes. The temperature of the block rises from 21 °C to 41 °C. Calculate the specific heat capacity of aluminium given by these results.
Show the model answer
E = 50 × 360 (1) E = 18 000 J (1) c = 18 000 ÷ (1.0 × 20) (1) c = 900 J/kg °C (1)
6-mark: describe a method to find a specific heat capacity
6 marks6
Say what you measure and the instrument for each: balance, thermometer, joulemeter.
Put the steps in order: mass, set-up, starting temperature, heating, energy supplied, final temperature.
Say how you reduce energy dissipated to the surroundings (insulation).
Finish with how you calculate c from your measurements.
Example. Describe a method a student could use to determine the specific heat capacity of a metal block. The student has an electric heater, a joulemeter, a thermometer and any other equipment they need.
Show the model answer
Marked by levels. A full answer could include: measure the mass of the block with a balance put the heater and the thermometer into the holes in the block wrap the block in insulation record the starting temperature connect the heater through the joulemeter and switch it on after about 10 minutes switch off, then record the joulemeter reading and the highest temperature reached work out the temperature rise c = energy supplied ÷ (mass × temperature rise) Top level (5 to 6 marks): a logical method that would work, including how c is calculated.
Explain why the result is not accurate
2 to 3 marks7
Say where the energy goes: it is dissipated to the surroundings.
Link this to the result: the block gains less energy than the heater supplies, so c comes out too high.
Give an improvement, e.g. more insulation.
Example. A student's value for the specific heat capacity of copper is higher than the accepted value. Explain why, and suggest one improvement to the method.
Show the model answer
Some energy from the heater is dissipated to the surroundings (1). So the energy supplied is more than the energy gained by the block, and the calculated value of c is too high (1). Improvement: wrap the block in (more or thicker) insulation (1).
Shortcuts and memory tricks
The definition has three parts: energy needed, 1 kg, 1 °C.
Sense check: 1 kg of water needs about 4200 J for each 1 °C rise, so heating a kettle of water takes hundreds of thousands of joules.
Calculator: for c = ΔE ÷ (m × Δθ) use brackets, or divide by m and then by Δθ.
In this practical, a value that is too high is almost always due to energy dissipated to the surroundings.
Where marks are lost
Using the final temperature instead of the temperature change.
Forgetting to change minutes to seconds in E = P t.
Using the mass in grams instead of kilograms.
Confusing specific heat capacity (temperature change) with specific latent heat (change of state, no temperature change).
Writing that 'heat is lost' without saying it is dissipated to the surroundings or how this affects the value of c.
Exam technique
In method questions, name the equipment: balance, thermometer, electric heater, joulemeter (or power supply and stopwatch), insulation.
Give c the unit J/kg °C.
If a question gives a power and a time, you almost always need E = P t first.
Sample questions
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]