Energy stores and systems
Energy is the idea that lets physicists keep track of what happens when anything changes.
A ball slows as it rises, a kettle warms water, a car comes to a stop.
In each case something has changed, and energy is the bookkeeping that links the "before" to the "after".
This first section sets up the words you will use for the rest of the unit: systems, energy stores and energy transfers.
What a system is
A system is an object or group of objects.
You choose the system to suit the question.
It might be one tennis ball, or the ball and the Earth together, or a kettle and the water inside it.
Drawing a boundary round the system tells you what is "inside" (its stores) and what is "outside" (the surroundings).
There are changes in the way energy is stored when a system changes.
Nothing about the total amount of energy changes; what changes is where it is stored.
That is the key idea of the whole unit.
Energy stores
Energy is described as sitting in stores.
Use the names below; they are the ones that score.
Always include the word "potential" in the two potential stores: "gravitational" alone, or "elastic" alone, does not score.
Energy store
Where the energy is
Example
kinetic
a moving object
a cyclist riding along a road
gravitational potential
an object raised above the ground
a book on a high shelf
elastic potential
a stretched or squashed spring or elastic object
a stretched rubber band
thermal (internal)
the particles of a hot object
a radiator full of hot water
chemical
fuels, food and batteries
a charged phone battery
magnetic
two magnets attracting or repelling
a fridge magnet held off the door
electrostatic
two charges attracting or repelling
a charged balloon near a wall
nuclear
the nucleus of an atom
uranium fuel in a reactor
You can use Eₖ, KE, Eₚ and GPE as short forms.
Names that are not stores do not score, such as "hydroelectric energy", "movement energy" or "heat".
Energy transfers
Energy moves from one store to another by a transfer.
A system can be changed in three ways that you must be able to calculate:
- heating: energy is transferred because of a temperature difference (a hob heating a pan);
- work done by forces: a force moves an object (a person pushing a trolley, friction in brakes);
- work done when a current flows: an electric current transfers energy (a battery driving a motor).
Energy can also be carried away by waves such as light and sound.
The stores are the places energy sits; the transfers are the routes between them.
Keep the two lists apart and your descriptions will be clear.
Describing the changes for common situations
You 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.
For each one, say which store decreases, which store increases, and how the energy is transferred.
Situation
Store that decreases
Store that increases
How it is transferred
An object projected upwards (until it stops at the top)
kinetic store of the object
gravitational potential store of the object
work done by the gravitational force (weight)
A moving object hitting an obstacle
kinetic store of the object
thermal stores of the object and the obstacle; some carried away by sound
work done by the contact force between them
An object accelerated by a constant force
the store that supplies the force (for example the chemical store of a battery or a person's muscles)
kinetic store of the object
work done by the force (and by a current, for a motor)
A vehicle slowing down
kinetic store of the vehicle
thermal store of the brakes and the surroundings
work done by the friction force in the brakes
Bringing water to a boil in an electric kettle
no store of the kettle decreases: the energy arrives from the mains supply (at a fossil-fuel power station it came from the chemical store of the fuel)
thermal store of the water (and of the kettle)
work done when a current flows in the heating element, then heating of the water by the element
Why it works: when the projected object rises, its weight acts against its motion.
The weight does work on the object, which slows it down, and the energy taken from the kinetic store goes into the gravitational potential store.
On the way down the reverse happens.
The same "force does work, store changes" picture explains every row of the table.
Connecting the ideas
A vehicle slowing down links straight to Forces (braking, and brakes heating up as work is done against friction) and to work done, which you meet again in the Forces unit.
A stretched spring links to the Forces unit's work on elasticity and springs.
Electrical work links to the Electricity unit, where you calculate energy transferred from current, potential difference and time.
- System: A system is an object or group of objects.
- Energy store: a way energy is held in a system, such as the kinetic, gravitational potential, elastic potential, thermal or chemical store.
- Energy transfer: energy moving from one store to another, by heating, by work done by forces or by work done when a current flows.
A skier starts at rest at the top of a slope and skis to the bottom.
Describe the changes in the way energy is stored. [3]
1. the gravitational potential energy store of the skier decreases (1)
2. the kinetic energy store of the skier increases (1)
3. some energy is transferred by work done against friction and air resistance to the thermal store of the snow, the skis and the air (1)
Answer: gravitational potential to kinetic, with some dissipated to thermal stores.
Saying "potential energy becomes movement energy" is too vague: name each store in full.
A toy car rolls off a table, falls and lands in a cushion, where it stops.
Describe the energy changes from the moment it leaves the table until it stops. [3]
1. as it falls, its gravitational potential store decreases and its kinetic store increases (1)
2. as it hits the cushion, its kinetic store decreases to zero (1)
3. the energy goes to the thermal stores of the car and the cushion, and a little is carried away as sound (1)
Answer: gravitational potential to kinetic, then kinetic to thermal (and sound).
A store that changes must be named together with how it changes: "kinetic and thermal" with no "decreases" or "increases" scores nothing.
An electric motor raises a lift at constant speed.
Describe how the energy stores change. [2]
1. energy is transferred by an electric current to the motor, which does work on the lift (1)
2. the gravitational potential store of the lift increases; its kinetic store stays the same because the speed is constant; some energy goes to the thermal store of the motor and the surroundings (1)
Answer: the gravitational potential store increases at a steady rate.
The kinetic store does not change, which many answers forget.
When a part asks for the energy transfers between two positions, name every store with its full name ("gravitational potential", "elastic potential"), say whether each one increases or decreases, and include any store that is in between.
Naming stores without saying how they change scores nothing.
- "Gravitational energy" or "elastic energy". Both need the word "potential". Write "gravitational potential energy store".
- Naming a transfer or a device as a store. "Hydroelectric energy", "electrical store" and "heat" are not stores. The water behind a dam has a gravitational potential store; a hot object has a thermal store.
- Thinking a rising object gains kinetic energy. As a ball rises it slows, so its kinetic store decreases while its gravitational potential store increases.
- Forgetting the in-between store. A ball dropped onto a spring goes gravitational potential, then kinetic, then elastic potential. Leaving out kinetic loses the mark.
- "The energy is used up". Energy is never used up: it moves to other stores, often thermal stores of the surroundings.
Check your understanding
1. What is a system? [1]
2. A phone is plugged in to charge.
Which energy store of the phone battery increases?
- Chemical ☐
- Elastic potential ☐
- Gravitational potential ☐
- Kinetic ☐
Tick (✓) one box. [1]
3. A skateboarder rolls up a curved ramp, slows down and stops for a moment at the top.
Complete the sentence.
Choose the answers from the box.
chemical
elastic potential
gravitational potential
kinetic
nuclear
As the skateboarder rolls up the ramp, the ______ energy store of the skateboarder decreases and the ______ energy store of the skateboarder increases. [2]
4. A cyclist brakes and comes to a stop.
How does each energy store change?
Tick (✓) one box in each row.
Energy store
Decreases
Stays the same
Increases
kinetic store of the cyclist
☐
☐
☐
thermal store of the brake pads
☐
☐
☐
gravitational potential store of the cyclist (on a flat road)
☐
☐
☐
[3]
5. Draw one line from each situation to the energy store that increases.
Situation
Energy store that increases
a bow string pulled back
thermal
soup warmed on a hob
elastic potential
a crate lifted onto a lorry
gravitational potential
kinetic
[3]
6. A hammer moving at speed hits a nail and stops.
Describe the changes in the way energy is stored. [3]
7. A student says: "When a car brakes to a stop, its energy is used up."
Explain why the student is wrong. [2]
Answers
1. an object or group of objects (1)
2. Chemical (1). Kinetic, elastic potential and gravitational potential stores of the battery do not change when it charges.
3. kinetic (1); gravitational potential (1) this order only
4. one mark for each row:
- kinetic store of the cyclist: decreases (1)
- thermal store of the brake pads: increases (1)
- gravitational potential store on a flat road: stays the same (1)
an extra tick in a row loses the mark for that row
5. - bow string pulled back to elastic potential (1)
- soup warmed on a hob to thermal (1)
- crate lifted onto a lorry to gravitational potential (1)
two lines from one situation lose the mark for that situation; kinetic is not used
6. - the kinetic store of the hammer decreases (1)
- work is done on the nail, pushing it into the wood (1)
- the thermal stores of the hammer, nail and wood increase, and some energy is carried away as sound (1)
7. - energy cannot be used up (or destroyed) (1)
- the kinetic store of the car decreases and the energy is transferred, by work done against friction in the brakes, to the thermal store of the brakes and the surroundings (1)
"it turns into heat" alone scores the first mark only if the thermal store is named