Energy stores and systems — AQA GCSE Physics
Test yourself on Energy stores and systems with AQA GCSE practice questions.
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Energy stores and systems explained
A system in physics is whatever you choose to draw a boundary around: one object, such as a stretched spring, or a group of objects, such as a bumper car and the barrier it hits.
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The boundary matters because energy is only counted inside it. Energy sits in stores - kinetic, gravitational potential, elastic potential, thermal, chemical, magnetic, electrostatic and nuclear - and moves between them by mechanical working, electrical working, heating or radiation. A closed system is one that nothing crosses, so the total energy inside stays constant however it is shuffled between stores. Name the objects in the system before describing any change, or a transfer to the surroundings looks like energy vanishing.
There are changes in the way energy is stored when a system changes.
Every change in a system is a redistribution between stores, never a creation or destruction of energy. Lift a 100 kg mass of water 500 m and about 490 kJ moves from the chemical store of the fuel powering the pump to the gravitational potential store; heat the same water by 80 °C and roughly 33 600 kJ moves into its thermal store. Comparisons like that are common: state where energy started, where it ended, and how much. Storing energy by heating holds far more per kilogram but leaks steadily through the insulation, while pumping uphill holds less yet wastes a smaller percentage and needs mountains. So the useful question is always which store, how much, and how much of it is recoverable.
Students should be able to describe all the changes involved in the way energy is stored when a system changes, for common situations. For example: an object projected upwards a moving object hitting an obstacle an object accelerated by a constant force a vehicle slowing down bringing water to a boil in an electric kettle.
Five standard situations come up again and again. An object projected upwards: kinetic store empties as the gravitational potential store fills, with some energy passing to the thermal store of the surroundings through air resistance. A moving object hitting an obstacle: kinetic store empties, mostly into thermal stores, with some transferred by sound waves. An object accelerated by a constant force: a force does work, filling the kinetic store. A vehicle slowing down: brakes do work against friction, filling the thermal store of the brakes, which is why they get hot. Water boiling in a kettle: the mains does electrical work, filling the thermal store of the water and of the kettle itself.
Throughout this section on Energy students should be able to calculate the changes in energy involved when a system is changed by: heating work done by forces work done when a current flows use calculations to show on a common scale how the overall energy in a system is redistributed when the system is changed.
Three routes change a system, and each has its own equation. Heating uses ΔE = m c Δθ, so 0.5 kg of water warmed 20 °C gains 0.5 × 4200 × 20 = 42 000 J. Work done by a force uses W = F s, so a 200 N force over 3 m transfers 600 J. Work done when a current flows uses E = P t or E = Q V, so a 2 kW heater running 300 s transfers 600 000 J. Once the numbers are in the same unit, usually joules, put them on a single scale or a Sankey style sketch to show the redistribution: the input on the left equals the useful output plus the energy dissipated, because nothing disappears. Data questions usually supply figures in mixed units, so convert everything to joules before comparing and say which store each figure belongs to.
Your focus
- Name the objects that make up the system in a given scenario, such as a bumper car together with the barrier it strikes.
- Distinguish an energy store, such as the kinetic store, from a pathway such as heating or electrical working.
- Explain why the total energy inside a closed system stays constant even though the individual stores change.
Show all 14 objectives
- Identify the starting and finishing stores for a change such as pumping water uphill or heating it in a tank.
- Calculate the useful energy recovered when a percentage wasted figure is given alongside the energy stored.
- Compare two methods of storing energy using the size of the energy stored, the percentage wasted and where each can be installed.
- Describe the store-to-store transfer for a ball thrown upwards, including where energy goes through air resistance.
- Explain why the brakes of a vehicle get hot when it slows, in terms of work done against friction.
- Trace the transfers in an electric kettle from the mains supply to the thermal store of the water.
- Identify the pathway responsible in each of the five common situations, distinguishing working from heating.
- Calculate the energy transferred by heating, by a force and by a current using the appropriate equation for each.
- Convert masses, times and energies into consistent units before substituting them into an equation.
- Construct a scale diagram showing input energy split into useful output and dissipated energy for a given device.
- Verify an energy balance by checking that the outputs on a diagram add back to the input.
Energy stores and systems exam tips
Marking Points
- one mark for naming the objects that make up the system in the situation described
- one mark for stating that the total energy of a closed system stays constant
- one mark for saying that no energy is transferred to or from the surroundings
- one mark for identifying which store fills and which store empties inside the system
- one mark for identifying the store energy is transferred from and the store it is transferred to
- one mark for quantifying the change, for example the energy stored per 100 kg by each method
- one mark for using the percentage wasted to work out the useful energy that could be recovered
- one mark for a genuine comparison that links the two methods rather than describing them separately
- one mark for naming the store that empties and the store that fills for the situation given
- one mark for naming the pathway, such as work done by a force, electrical working or heating
- one mark for including the transfer to the thermal store of the surroundings where friction or air resistance acts
- one mark for a statement that the total energy is unchanged across the change described
- one mark for selecting the correct equation for heating, for work done by a force or for electrical working
- one mark for a correct substitution with all quantities converted to kilograms, metres, seconds and joules
- one mark for the calculated value with the unit joules given
- one mark for showing the input equal to the useful output plus the energy dissipated on a common scale
Examiner Tips
- 💡Open the answer by saying what is inside the system; the boundary has to be clear before the energy argument means anything.
- 💡For a closed system, the examiner credits stating that there is no net energy transfer to or from the surroundings, which means the total energy remains constant.
- 💡In a compare question, write sentences that hold both options together: method A stores more per kilogram but wastes 40% against method B at 25%.
- 💡Give the magnitude of any difference when the data allows it; the top level asks for the size of the difference, not just its direction.
- 💡Write the change as a chain: store, pathway, store, then add where energy is dissipated.
- 💡Say which object holds the store, because the thermal store of the brakes is not the thermal store of the road.
- 💡A one-mark answer still needs both ends of the transfer, so never name a single store on its own.
- 💡Write the equation, then the substitution, then the answer; the first two marks survive even when the arithmetic slips.
- 💡Convert kJ to J before any comparison, because the scale you draw has to be a common one.
- 💡Check the total across your diagram adds back to the input before you move on.
Common Mistakes
- treating the system as one object only, leaving out the barrier, spring or surroundings that the question named
- listing electrical, light or sound as energy stores when they are pathways between stores
- writing that energy is lost or used up when it has been transferred to the thermal store of the surroundings
- writing that energy is created by the pump or used up by the heater instead of transferred between stores
- naming heating, a pathway, as the final store rather than naming the thermal store
- listing features of each method in turn, which caps a compare question at the lower level
- saying the kinetic energy is destroyed in a collision rather than transferred to thermal stores and sound
- forgetting that a kettle also heats its own element and body, not only the water
- using the phrase movement energy or heat energy instead of the kinetic store and the thermal store
- leaving out air resistance for a projected object, so the gravitational store is said to gain everything the kinetic store lost
- leaving mass in grams or time in minutes, so the answer is out by a factor of 1000 or 60
- adding energies calculated in kilojoules to energies in joules without converting first
- using the distance moved by the whole vehicle rather than the distance moved in the direction of the force
- drawing a redistribution diagram whose output arrows total more than the input arrow