Physics 8.1 - 8.4 - Energy stores and transfers
A falling ball, a warming motor and a cooling drink look unrelated, but the same energy model can describe all three. This lesson shows how to choose a system, track changes in its energy stores and represent transfers with diagrams. It also explains why the total energy of a closed system stays unchanged even while every visible part of the system is changing.
Systems and energy stores
Imagine releasing a ball from above the floor. The ball speeds up, so something measurable has changed. To describe that change with energy, first choose the system: the object or group of interacting objects being studied. For the falling ball, a useful system contains the ball and the Earth because gravity is an interaction between them.
The system boundary is an imaginary line around everything included. Naming the boundary matters: an account that includes only the ball is different from one that includes the ball, Earth and surroundings.
Energy store
An energy store is a way of keeping track of energy associated with a system at one instant. Energy is measured in joules (J). A store can increase or decrease when the system changes.
These eight store labels cover the situations normally met at GCSE:
| Energy store | Physical clue that its amount is changing | Example owner or system |
|---|---|---|
| chemical | chemical composition changes | a battery, or fuel and oxygen |
| kinetic | speed changes | a moving object |
| gravitational | relative height changes in a gravitational field | an object-Earth system |
| elastic | an object is stretched or compressed | a spring or elastic band |
| thermal | temperature or physical state changes | an object |
| magnetic | the relative positions of magnets change | interacting magnets |
| electrostatic | the relative positions of charges change | interacting charged objects |
| nuclear | nuclei change | nuclear fuel or an unstable nucleus |
The store label must match the physical change. A car moving at constant speed has a kinetic store, but that store is not increasing just because the car is moving. A book sliding horizontally does not change its gravitational store if its height stays constant.
A store is part of a model, not a container filled with a material called energy. It is usually clearest to say both whose store and how it changes: "the kinetic store of the ball increases" or "the gravitational store of the ball-Earth system decreases".
Ways systems change
When energy moves between stores, it is transferred. A store describes the energy at a snapshot in time; a transfer describes the process taking place between snapshots.
Physics 8.4 names three ways that the energy of a system can be changed:
| Way the system changes | What is happening | Example energy account |
|---|---|---|
| work done by forces | a force moves or deforms something | gravity does work as a ball falls, so its kinetic store increases |
| in electrical equipment | a current passes through a working electrical device | a cell's chemical store decreases while a motor's kinetic store increases |
| in heating | a temperature difference causes energy to pass from a hotter object to a cooler one | the thermal store of hot water decreases while the thermal store of a cooler cup increases |
These transfer processes are not extra stores. For example, an electrical transfer is an arrow between stores, not a box labelled "electrical store". Similarly, heating is a process; "heat" is not a store. Light and sound can carry energy between places, but neither is an energy store.
Drawing and interpreting transfer diagrams
An energy-transfer diagram is a model. Its boxes name stores at the start and end; its arrows show the direction and process of transfer. It does not show energy as a substance flowing through a pipe.
Use this method:
- State the system and what is inside its boundary.
- Compare the starting and final conditions. Name only stores that change.
- Put a decreasing store on the left and an increasing store on the right.
- Draw an arrow in the direction of transfer and label it with the process: work done by a force, transfer in electrical equipment, or heating.
- Check that every label matches a physical change in the situation.
Worked example: a falling ball
The system is the ball and Earth, and air resistance is ignored. The ball begins above the floor and speeds up as it falls.
gravitational store of ball-Earth system decreases
|
| gravity does work on the ball
v
kinetic store of ball increases
The arrow points from the decreasing store to the increasing store. "Gravity" alone would not be a complete arrow label: the force does work because the ball moves in the force's direction.
Continue this lesson.
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