AQA GCSE Combined Science (8464), Higher tier · Physics › Energy › Conservation and dissipation of energy
Practise Energy transfers in a system. 19 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
Energy can be transferred usefully, stored or dissipated, but never created or destroyed. You need to explain where energy is wasted, how to reduce unwanted energy transfers (lubrication and insulation), and how the thickness and thermal conductivity of its walls affect how fast a building cools. GCSE Physics students also need the required practical on thermal insulators. Expect 2 to 4 mark 'describe' and 'explain' questions, and practical questions of up to 6 marks.
Grade by grade
What you need to be able to do, from the first marks up to the top grade.
3
State the law of conservation of energyEnergy can be transferred usefully, stored or dissipated, but cannot be created or destroyed.
4
Give examples of energy being dissipatedFriction heats moving parts, air resistance heats the air, and a motor makes sound.
5
Explain how lubrication reduces wasted energyOil reduces friction between moving parts, so less energy is dissipated by heating.
5
Explain how insulation reduces energy transferThermal insulation lowers the rate of energy transfer by heating, e.g. loft insulation in a house.
6
Link thermal conductivity to rate of transferThe higher the thermal conductivity, the higher the rate of energy transfer by conduction through a material.
6
Describe how walls affect a building's coolingThicker walls made of a material with a lower thermal conductivity make a building cool more slowly.
Notes
Conservation of energy
Energy can be transferred usefully, stored or dissipated, but it cannot be created or destroyed.
In a closed system no energy enters or leaves, so there is no net change to the total energy. Energy just moves between stores.
Example: a swinging pendulum with no air resistance. Energy moves back and forth between its kinetic and gravitational potential stores, but the total stays the same.
Dissipation (wasted energy)
In all system changes some energy is dissipated: it spreads out and ends up stored in less useful ways, usually in the thermal store of the surroundings. This is often called 'wasted' energy.
Examples: friction heats the moving parts of a machine; air resistance heats the air around a moving car; a lamp heats the room as well as lighting it; a motor makes sound.
Once it has spread out, dissipated energy is very hard to use again.
Reducing unwanted energy transfers
Lubrication: oil or grease between moving parts reduces friction, so less work is done against friction and less energy is dissipated by heating.
Thermal insulation reduces the rate of energy transfer by heating, e.g. loft insulation, cavity wall insulation and double glazing in a house.
Thermal conductivity and buildings
The higher the thermal conductivity of a material, the higher the rate of energy transfer by conduction across it.
Metals have a high thermal conductivity. Materials that trap air, such as foam, wool and fibreglass, have a low thermal conductivity, so they are good insulators.
A building cools more slowly if its walls are thicker and made of a material with a lower thermal conductivity.
Cheatsheet
Conservation of energy: energy can be transferred usefully, stored or dissipated, but cannot be created or destroyed
Closed system: no net change to the total energy
Dissipated (wasted) energy usually ends up in the thermal store of the surroundings
Lubrication → less friction → less energy dissipated
Thermal insulation → lower rate of energy transfer by heating
Higher thermal conductivity → higher rate of energy transfer by conduction
Thicker walls + lower thermal conductivity → building cools more slowly
How to answer each type of question
Describe how energy is wasted
2 to 3 marks4
Identify the useful energy transfer first, e.g. to the kinetic store of the fan blades.
Name the cause of the waste: friction, air resistance, the current heating wires, or sound.
Say where the energy ends up: the thermal store of the surroundings.
Example. An electric fan is designed to transfer energy to the kinetic store of its blades. Describe how some of the energy supplied to the fan is wasted.
Show the model answer
Friction in the motor and bearings heats them (1), so energy is dissipated to the thermal store of the surroundings (1). Some energy is transferred to the surroundings as sound (1).
Explain how a change reduces wasted energy
2 marks5
Say what the change does: reduces friction, or reduces the rate of energy transfer by heating.
Say the result: less energy is dissipated to the surroundings.
Example. A cyclist puts oil on the chain of her bicycle. Explain how this reduces the energy wasted as she cycles.
Show the model answer
The oil lubricates the chain, reducing the friction between the chain and the gears (1). So less work is done against friction, and less energy is dissipated to the thermal store of the chain and the surroundings (1).
Explain how walls affect the rate of cooling
3 marks6
Compare thermal conductivity: lower conductivity means a lower rate of energy transfer.
Compare thickness: thicker walls lower the rate of energy transfer.
Conclude which building cools more slowly, using the word 'rate'.
Example. Two houses are the same except for their outside walls. House A has walls of material X that are 20 cm thick. House B has walls of material Y that are 30 cm thick. Material Y has a lower thermal conductivity than material X. Explain which house will cool down more slowly on a cold night.
Show the model answer
Material Y has a lower thermal conductivity, so the rate of energy transfer by conduction through house B's walls is lower (1). House B's walls are also thicker, which lowers the rate of energy transfer further (1). So house B cools down more slowly (1).
Shortcuts and memory tricks
Energy never disappears: if it is not useful, it has usually gone to the thermal store of the surroundings.
Low thermal conductivity = good insulator.
'Thick and low': thicker walls and a lower thermal conductivity both slow down cooling.
Most good insulators trap air, which has a very low thermal conductivity.
Where marks are lost
Saying energy is 'lost' or 'used up' without saying that it is dissipated to the surroundings.
Saying insulation 'stops' energy transfer. It reduces the rate of energy transfer.
Writing 'insulation keeps the cold out'. Energy is transferred from the warm inside to the cold outside.
Confusing thermal conductivity (how fast energy passes through a material) with specific heat capacity (the energy needed to warm it up).
In the practical, not keeping the volume, starting temperature and thickness the same.
Exam technique
Use the key words: dissipated, wasted, thermal store of the surroundings, rate of energy transfer.
When comparing materials or walls, use comparative words (higher, lower, thicker, faster) and say which is better.
In a closed-system question, say that the total energy stays the same even though energy moves between stores.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
Complete the sentence. The higher the thermal conductivity of a material, the .................... the rate of energy transfer by conduction through the material.
higher
A student thinks that a thicker layer of bubble wrap around a beaker of hot water will reduce the rate of cooling more than a thinner layer. Write a hypothesis for an investigation into this idea.
The thicker the layer of bubble wrap, the lower the rate of cooling.
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
Energy is transferred when a cyclist rides a bicycle.
(a) Which statement about energy is correct? Tick (✓) one box.[1]
Energy can be created but not destroyed.
Energy can be destroyed but not created.
Energy cannot be created or destroyed.
Energy is used up when it is transferred.
(b) The cyclist oils the bicycle chain regularly. Explain how oiling the chain reduces unwanted energy transfers.[2]
(c) What happens to the energy that is wasted? Tick (✓) one box.[1]
It is destroyed.
It is dissipated to the surroundings.
It is stored in the chemical store of the cyclist.
It is transferred back to the kinetic store of the bicycle.
Show the answer and mark scheme
(a)Answer: Energy cannot be created or destroyed.
(b)Answer: Oil (lubrication) reduces friction between the moving parts, so less energy is dissipated to the thermal store of the chain and surroundings.
(lubrication) reduces friction (between the moving parts)
so less energy is dissipated / wasted / transferred to the thermal store (of the chain or surroundings)
(c)Answer: It is dissipated to the surroundings.
Question 2Medium5 marks
A student thinks that a thicker layer of bubble wrap around a beaker of hot water will reduce the rate of cooling more than a thinner layer.
(a) Write a hypothesis for an investigation into this idea.[1]
(b) State the independent variable and the dependent variable in this investigation.[2]
(c) State two variables the student should control.[2]
Show the answer and mark scheme
(a)Answer: The thicker the layer of bubble wrap, the lower the rate of cooling.
the thicker the (bubble wrap) insulation, the lower the rate of cooling / the smaller the temperature decrease in a given time
(b)Answer: Independent: thickness (number of layers) of bubble wrap. Dependent: rate of cooling (temperature decrease in a fixed time).
independent variable: thickness / number of layers of bubble wrap
dependent variable: rate of cooling / temperature decrease in a fixed time
(c)Answer: Any two from: volume of water; starting temperature; type/size of beaker; room temperature.
volume of water
starting temperature of the water
type / size of beaker
room temperature
Question 3Hard6 marks
A student wants to find out how the thickness of a layer of newspaper wrapped around a beaker affects the rate at which hot water in the beaker cools.
Plan an investigation the student could do. Your plan should give valid results.[6]
Show the answer and mark scheme
Answer: Wrap the beaker in different numbers of layers of newspaper; same volume and starting temperature of water, same lid; measure the temperature at the start and after a fixed time; repeat for each thickness; compare temperature decreases or plot a graph.
wrap the beaker in a known number of layers of newspaper, e.g. 1, 2, 3, 4 and 5 layers (to vary the thickness)
pour the same volume of hot water into the beaker each time, measured with a measuring cylinder
use the same starting temperature each time, e.g. 80 °C, measured with a thermometer
use the same lid / cardboard cover each time
record the temperature after a fixed time, e.g. 10 minutes, timed with a stopwatch (or every minute)
calculate the temperature decrease for each thickness
repeat for each thickness and calculate a mean
plot a graph of temperature decrease against number of layers
take care with hot water, e.g. place the beaker on a heatproof mat
Marked with levels of response: the full level descriptors are in the app.