What are the different energy stores, and what does it mean to say energy is conserved?
The main energy stores and the ways energy is transferred, the principle of conservation of energy, and dissipation of energy to the surroundings.
A CCEA GCSE Physics answer on the main energy stores and the ways energy is transferred between them, the principle of conservation of energy, and how energy is dissipated as heat to the surroundings.
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What this dot point is asking
CCEA wants you to name the main energy stores, describe how energy is transferred between them, state the principle of conservation of energy, and explain how energy is dissipated to the surroundings. This underpins every other energy calculation.
The answer
Energy stores
Energy is stored in different ways. The main stores you need are:
Energy transfers
Energy is moved between stores by different pathways: mechanically (by a force doing work), electrically (by a current), by heating, and by radiation (light, sound, infrared).
For example, in a falling ball, gravity does work and transfers energy from the gravitational potential store to the kinetic store.
Conservation of energy
This is why, in calculations, the energy you start with must all be accounted for at the end, even if some of it ends up in less useful forms.
Dissipation
In real systems, some energy is always transferred to the surroundings in a way that is no longer useful.
Worked example: a bouncing ball
Examples in context
Example 1. A car braking. The kinetic energy of the car is transferred mainly to thermal energy in the brakes and tyres by friction, dissipating to the surroundings as heat. The energy is conserved but no longer useful for driving.
Example 2. A torch. Chemical energy in the battery is transferred electrically to the bulb, which transfers it by radiation as light (the useful output) and by heating (the dissipated part). Total energy is conserved.
Try this
Q1. Name three energy stores. [3 marks]
- Cue. Any three of: kinetic, gravitational potential, elastic potential, chemical, thermal, nuclear.
Q2. State the principle of conservation of energy. [1 mark]
- Cue. Energy cannot be created or destroyed, only transferred from one store to another.
Q3. What usually happens to dissipated energy? [1 mark]
- Cue. It is spread to the surroundings as heat and is no longer useful.
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA style4 marksA child slides down a playground slide and reaches the bottom moving slowly. Describe the energy transfers that take place and explain why the child's kinetic energy at the bottom is less than the gravitational potential energy lost.Show worked answer →
At the top the child has gravitational potential energy. As they slide down, this is transferred to a kinetic energy store, so they speed up.
Friction between the child and the slide transfers some energy to heat (and a little sound), warming the slide and surroundings.
By conservation of energy, the kinetic energy gained is less than the gravitational potential energy lost because some energy has been dissipated as heat by friction.
Markers reward: gravitational potential to kinetic; friction transfers energy to heat; total energy conserved so kinetic energy less than potential energy lost.
CCEA style3 marksState the principle of conservation of energy and explain what is meant by energy being dissipated.Show worked answer →
The principle of conservation of energy states that energy cannot be created or destroyed, only transferred from one store to another (or the total energy stays constant).
Energy being dissipated means it is spread out to the surroundings, usually as heat, in a form that is no longer useful for the intended job.
Markers reward a correct statement of conservation of energy and a correct meaning of dissipation (spread to surroundings, usually as heat, and less useful).
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A CCEA GCSE Physics answer on calculating kinetic energy with half m v squared and gravitational potential energy with mgh, and using conservation of energy to link the two stores in falling objects.
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Sources & how we know this
- CCEA GCSE Physics specification — CCEA (2017)