AQA GCSE Combined Science (8464), Higher tier · Physics › Energy › Energy changes in a system, and the ways energy is stored
Practise Energy stores and systems. 12 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
A system is an object or a group of objects, and when a system changes, the way its energy is stored changes too. You need to describe these changes for everyday situations, such as a ball thrown upwards or a kettle boiling water. Expect 1-mark recall questions and 2 to 4 mark 'describe the changes in energy stores' questions on Paper 1, often at the start of a longer calculation question.
Grade by grade
What you need to be able to do, from the first marks up to the top grade.
3
Define a systemA system is an object or a group of objects that you choose to study, e.g. a ball and the Earth.
3
Name the main energy storesKinetic, gravitational potential, elastic potential, thermal (internal), chemical, magnetic, electrostatic and nuclear.
4
Name the ways energy is transferredMechanically (a force doing work), electrically (a current doing work), by heating and by radiation such as light.
5
Describe energy changes in common situationsSay which store decreases and which increases, e.g. a braking car: kinetic store down, thermal store of the brakes up.
6
Include wasted energy in your descriptionsSay that friction or air resistance dissipates some energy to the thermal store of the surroundings.
7
Use calculations to compare energy storesCalculate the energy in each store before and after a change to show how the total energy is shared out.
Notes
Systems and energy stores
A system is an object or a group of objects. When a system changes, the way its energy is stored changes.
Energy stores: kinetic (moving objects), gravitational potential (raised objects), elastic potential (stretched or squashed objects), thermal or internal (hot objects), chemical (fuels, food, batteries), magnetic, electrostatic and nuclear.
When a system changes, energy leaves some stores and goes into others. The total amount of energy stays the same.
How energy is transferred
Mechanically: a force does work, e.g. pushing a trolley, or friction.
Electrically: a current does work, e.g. a battery driving a current through a motor.
By heating: energy goes from a hotter object to a cooler one.
By radiation: e.g. light and infrared from the Sun or a lamp. Sound waves also carry energy.
Light, sound and electricity are ways of transferring energy. They are not energy stores.
Describing common energy changes
Ball thrown upwards: the kinetic store of the ball decreases and its gravitational potential store increases.
Moving object hits an obstacle (e.g. a car hits a wall): the kinetic store of the car decreases; the thermal stores of the car, the wall and the surroundings increase, and some energy is carried away by sound.
Object accelerated by a constant force: the force does work, so the kinetic store of the object increases. The energy comes from another store, e.g. the chemical store of the person pushing.
Vehicle slowing down: the kinetic store of the vehicle decreases. Friction in the brakes does work, so the thermal store of the brakes increases; this energy then spreads to the surroundings.
Electric kettle: energy is transferred electrically to the heating element, then by heating to the water, so the thermal store of the water increases. Some energy heats the kettle and the air around it.
Showing energy changes with numbers grade 7+
You can calculate the energy in each store before and after a change, using the equations in 'Changes in energy'. The energy lost from one store equals the total energy gained by the others.
Example: a falling ball loses 3.0 J from its gravitational potential store. If its kinetic store gains 2.7 J, then 0.3 J has been dissipated by air resistance.
Transfers: mechanically (force doing work), electrically (current doing work), by heating, by radiation
Energy is measured in joules (J)
Ball thrown up: kinetic store → gravitational potential store
Vehicle braking: kinetic store → thermal store of the brakes → surroundings
Kettle: electrical work → thermal store of the water (some to the surroundings)
Energy lost from one store = total energy gained by the other stores grade 7+
How to answer each type of question
Name the energy store or the energy transfer
1 mark each3
Use the proper name of the store: kinetic, gravitational potential, elastic potential, thermal, chemical and so on.
Say whose store it is if you can (the spring, the water).
Remember that light, sound and electricity are ways of transferring energy, not stores.
Example. (a) A student compresses a spring. Name the energy store of the spring that increases. (b) Name the energy store in the food that a person eats. (c) A battery is connected to a lamp. State how energy is transferred from the battery to the lamp.
Show the model answer
(a) Elastic potential (energy store) (1) (b) Chemical (energy store) (1) (c) Electrically / by an electric current doing work (1)
Describe the changes in the way energy is stored
2 to 3 marks5
Name the object each time, e.g. 'the kinetic store of the car'.
Say which store decreases and which store increases.
If there is friction, air resistance or heating, add where the wasted energy goes: the thermal store of the surroundings.
Example. A toy car is released from rest at the top of a ramp. It rolls down the ramp. Describe the changes in the way energy is stored as the car rolls down the ramp.
Show the model answer
The gravitational potential energy store of the car decreases (1). The kinetic energy store of the car increases (1). Some energy is dissipated to the thermal store of the car and the surroundings because of friction and air resistance (1).
Calculate: show how the energy is redistributed
3 to 4 marks7
Calculate the energy in the first store, e.g. Ep = m g h.
Calculate the energy in the second store, e.g. Ek = ½ m v2.
Subtract: the difference is the energy dissipated (or transferred to another store).
Give each answer with its unit, J.
Example. A ball of mass 0.20 kg is dropped from a height of 1.5 m. Just before it hits the ground its speed is 5.2 m/s. gravitational field strength = 9.8 N/kg Calculate the energy dissipated by air resistance as the ball falls.
Show the model answer
Gravitational potential energy lost = 0.20 × 9.8 × 1.5 = 2.94 J (1) Kinetic energy gained = 0.5 × 0.20 × 5.22 = 2.704 J (1) Energy dissipated = 2.94 − 2.704 (1) = 0.24 J (1)
The four transfers: 'My Energy Has Routes': Mechanically, Electrically, by Heating, by Radiation.
Every description needs a store that goes down and a store that goes up.
If anything rubs, moves through air or gets warm, some energy ends up in the thermal store of the surroundings.
Where marks are lost
Using made-up stores such as 'heat energy', 'movement energy' or 'electrical store'. Say thermal and kinetic; electricity is a way of transferring energy.
Calling light or sound an energy store. They are ways energy is transferred.
Not saying whose store it is, e.g. writing 'kinetic energy increases' when there are two objects in the question.
Saying energy is 'used up' or 'lost'. It is transferred or dissipated.
Missing out a step for a kettle: energy goes electrically to the element, then by heating to the water.
Exam technique
'Describe the changes in the way energy is stored' wants store names with the words 'decreases' and 'increases'. You don't need equations.
There is usually one mark for each store named correctly with the right direction of change.
If the question mentions friction, air resistance or something getting warm, there is usually a mark for saying energy is dissipated to the surroundings.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
The student heats some water in an electric kettle. Name the energy store of the water that increases.
Thermal (internal) energy store
Name the energy store of a stretched elastic band.
Elastic potential
Name the energy store of the uranium fuel used in a nuclear power station.
Nuclear
State the energy store that decreases in a torch's battery as it powers a lamp.
Chemical
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 can be stored in different ways.
(a) A student stretches a rubber band. Which energy store of the rubber band increases? Tick (✓) one box.[1]
Chemical
Elastic potential
Gravitational potential
Kinetic
(b) The student heats some water in an electric kettle. Name the energy store of the water that increases.[1]
(c) Energy is transferred to the kettle electrically, by a current. Give two other ways that energy can be transferred from one store to another.[2]
Show the answer and mark scheme
(a)Answer: Elastic potential
(b)Answer: Thermal (internal) energy store
thermal / internal (energy store)
(c)Answer: Any two of: by heating; mechanically (by a force doing work); by radiation (e.g. light or sound waves)
by heating
mechanically / by (a) force(s) doing work
by radiation / (light or sound) waves
Question 2Medium7 marks
A student throws a tennis ball vertically upwards. The ball rises, stops for an instant at its highest point and then falls back down.
(a) Describe the changes in the way energy is stored as the ball moves from the student's hand to its highest point.[2]
(b) Explain why the ball has no kinetic energy at its highest point.[1]
(c) Air resistance acts on the ball as it moves. Explain how air resistance affects the maximum height the ball reaches.[2]
(d) The student catches the ball at the same height it was thrown from. Compare the kinetic energy of the ball just before it is caught with its kinetic energy just after it was thrown. Give a reason for your answer.[2]
Show the answer and mark scheme
(a)Answer: The kinetic energy store of the ball decreases and its gravitational potential energy store increases.
kinetic energy (store of the ball) decreases
gravitational potential energy (store of the ball) increases
(b)Answer: Its speed is zero at the highest point.
the speed / velocity of the ball is zero (at the highest point)
(c)Answer: Work is done against air resistance, so some energy is dissipated to the thermal store of the surroundings and less is transferred to the gravitational potential store — the ball does not rise as high.
(work is done against air resistance so) some energy is dissipated / transferred to the thermal store of the surroundings / air
so less energy is transferred to the gravitational potential energy store (so the maximum height is lower)
(d)Answer: The kinetic energy is less when it is caught, because energy has been dissipated by air resistance on the way up and on the way down.
kinetic energy is less (when it is caught)
(because) energy has been dissipated (to the surroundings) due to air resistance / work done against air resistance
Question 3Hard8 marks
An electric kettle has a power of 2.2 kW. The kettle contains 0.80 kg of water at 10 °C. The kettle is switched on for 150 s, and in this time the water is heated to 100 °C. Specific heat capacity of water = 4200 J/kg °C
(a) Describe the changes in the way energy is stored while the kettle heats the water.[2]
(b) Calculate the increase in the thermal energy store of the water. Use the Physics Equations Sheet.[2]
(c) Calculate the energy transferred to the kettle in 150 s.[2]
(d) Calculate the percentage of the energy transferred to the kettle that ends up in the thermal energy store of the water.[2]
Show the answer and mark scheme
(a)Answer: Energy transferred electrically increases the thermal store of the element, which heats the water so the thermal store of the water increases; some energy is dissipated to the thermal store of the kettle and surroundings.
energy is transferred electrically to (the thermal store of) the heating element
the thermal energy store of the water increases
some energy is dissipated to the thermal store of the kettle / surroundings