4.1.1.1a - Energy Stores and System Changes
Energy is a way of tracking changes in a system. In this lesson, you will practise describing where energy is stored before and after a change, without doing calculations. The key skill is to choose the system clearly, then describe which energy stores increase and which decrease.
Systems and energy stores
In physics, a system is the object or group of objects you choose to study. A system might be one ball, a car and the road, or the kettle, heating element, and water.
System
An object or group of objects chosen for study.
Energy is stored in different ways. For this lesson, the most useful stores are:
| Energy store | Useful clue |
|---|---|
| Kinetic energy store | Something is moving. |
| Gravitational potential energy store | Something has been raised in a gravitational field. |
| Elastic potential energy store | Something is stretched, squashed, or bent. |
| Thermal energy store | Something is at a higher temperature. |
| Chemical energy store | Chemicals can react, such as in fuels, food, cells, or batteries. |
A careful answer does not say that energy is stored as light energy, sound energy, or electrical energy. Those are better treated as ways energy is transferred. The energy usually ends up in a named store, such as a thermal store.
Describing store changes
When a system changes, the way energy is stored changes. The best way to describe this is to compare two snapshots:
| Step | What to decide | Example prompt |
|---|---|---|
| 1 | What is the system? | "I am studying the ball after it leaves the hand." |
| 2 | What is the starting snapshot? | "The ball is moving upwards just after release." |
| 3 | What is the finishing snapshot? | "The ball is higher and moving more slowly." |
| 4 | Which stores change? | "The kinetic energy store decreases; the gravitational potential energy store increases." |
If a force acts through a distance, work is done. In GCSE Physics, work done is energy transferred. In this lesson, you only need that idea qualitatively: a force can transfer energy from one store to another.
For example, as a ball travels upwards after being thrown, its kinetic energy store decreases because it slows down. Its gravitational potential energy store increases because it is higher above the ground. If your chosen system includes the thrower, the thrower's chemical energy store decreased earlier as energy was transferred to the ball's kinetic energy store. If air resistance is included, some energy is also transferred to thermal stores of the ball and surrounding air.
Start with the system and the two snapshots; then name the stores that increase and decrease.
Forces, acceleration and collisions
A force can change how energy is stored in a system. If a constant force accelerates an object, the object speeds up, so its kinetic energy store increases. The store that decreases depends on what provides the force: it might be a chemical store in a person or fuel, an elastic potential store in a stretched band, or another store in the wider system.
A collision is different because the moving object slows down very quickly. Its kinetic energy store decreases. Energy is transferred to thermal stores of the object, obstacle, and surroundings. If the object or obstacle changes shape, energy may also be stored temporarily in elastic potential stores. Sound can transfer energy away during the collision, but sound is not the final store named in the answer.
| Situation | Main store decrease | Main store increase |
|---|---|---|
| Object accelerated by a constant force | Store supplying the force, such as chemical or elastic potential | Kinetic energy store of the object |
| Moving object hits an obstacle | Kinetic energy store of the moving object | Thermal stores, and sometimes elastic potential stores during deformation |
A vehicle slowing down
When a vehicle slows down, its kinetic energy store decreases. Brakes, tyres, the road, and the surrounding air all exert forces that oppose the motion. These forces transfer energy away from the vehicle's kinetic energy store.
Most of this energy is transferred to thermal stores. The brakes become warmer, and the tyres, road, and surrounding air may also warm. Some energy may be transferred by sound, but a complete store answer should still name the final stores, especially thermal stores.
This lesson does not need stopping-distance calculations or braking-force equations. The important qualitative description is:
kinetic energy store of the vehicle decreases -> thermal energy stores of brakes, tyres, road and surroundings increase
Bringing water to a boil
An electric kettle is a useful example because the system has several objects: the electrical supply, the heating element, the kettle, and the water. For a simple GCSE description, focus on the heating element and the water.
When the kettle is switched on, energy is transferred electrically to the heating element. The thermal energy store of the heating element increases, so the element becomes hot. Energy is then transferred by heating from the element to the water. The thermal energy store of the water increases until the water reaches boiling point.
Do not write that electricity is stored in the water. Also do not calculate the energy needed or discuss latent heat here; those belong to later lessons.
For any system change, name the system, compare before and after, then describe which stores increase and decrease.