The principle of conservation of energy
- The principle of conservation of energy states that:
Energy cannot be created or destroyed, it can only transferred from one energy store to another
- This means the total amount of energy in a closed system remains constant
- Common examples of energy transfers are:
- A falling object (in a vacuum): gravitational potential ➝ kinetic
- Horizontal mass on a spring: elastic potential ➝ kinetic
Table of energy stores
| Energy store | Description |
| Kinetic | Moving objects have energy in their kinetic store |
| Gravitational potential | Objects gain energy in their gravitational potential store when they are raised through a gravitational field |
| Elastic | Objects have energy in their elastic potential store if they are stretched or compressed |
| Electrostatic | Objects with charge interacting with one another have energy in their electrostatic store |
| Magnetic | Magnets interacting with each other have energy in their magnetic store |
| Chemical | Objects with energy in their chemical store can release energy in chemical reactions |
| Nuclear | Atomic nuclei release energy from their nuclear store during nuclear reactions |
| Thermal | All objects have energy in their thermal store; the hotter an object is, the more energy it has in this store |
Table of energy transfers
| Energy transfer | Description |
| Mechanical | When a force acts on an object e.g. pulling, pushing, stretching, squashing |
| Electrical | A charge moving through a potential difference e.g. electrons flowing around a circuit |
| By heating | Energy is transferred from a hotter object to a colder one |
| By radiation | Energy is transferred by electromagnetic radiation |
Energy dissipation
- When energy is transferred from one store to another, not all the energy will end up in the desired store
- Dissipation is used to describe ways in which energy is wasted
- Any energy not transferred to useful energy stores is wasted because it is lost to the surroundings by heating, light or sound
- What counts as wasted energy depends on the system
- For example, in a television:
- Energy is transferred electrically from the mains supply to the thermal store of the television
- Energy is usefully transferred by radiation (light) and by heating (sound vibrations) to the surroundings
- Energy is dissipated (wasted) by heating to the thermal energy store of the surrounding air
- Another example, in a heater:
- Energy is transferred electrically from the mains supply to the thermal store of the heater
- Energy is usefully transferred by heating to the surroundings
- Energy is dissipated (wasted) by radiation (light) to the thermal energy store of the surrounding air
Worked example
The diagram shows a rollercoaster going down a track.
The rollercoaster takes the path A → B → C → D.
Which statement is true about the energy transfers that occur for the roller coaster down this track?
A. KE - GPE - GPE - KE
B. KE - GPE - KE - GPE
C. GPE - KE - KE - GPE
D. GPE - KE - GPE - KE
Answer: D
- At point A:
- The roller coaster is raised to a height, therefore it has energy in its GPE store
- As it travels down the track, GPE is transferred to KE and the roller coaster speeds up
- At point B:
- KE is transferred to GPE as the roller coaster rises up the loop
- At point C:
- This GPE is transferred back into KE as the roller coaster travels back down the loop
- At point D:
-
- The flat terrain means the roller coaster only has energy in its KE store
-
Examiner Tip
You may not always be given the energy transfers happening in the system in exam questions. By familiarising yourself with the transfers and stores of energy, you will be expected to relate these to the situation in question. For example, a ball rolling down a hill is transferring gravitational potential energy to kinetic energy whilst a spring transfers energy from the elastic potential store to kinetic store.
