1.5.1Dynamics
Conservation of Energy
5 min read 6 parts Lesson 16 of 22
Section 1 of 6The Principle
Energy is the ability to do work, and it is always accounted for even when it seems to disappear.
Statement
- Energy cannot be created or destroyed; it can only be transferred from one form to another.
Total energy
- The total energy in a closed system stays constant, so energy into a system equals energy out.
Unit
- Energy is measured in joules (J), and 1 J is the energy transferred when a force of 1 N moves an object 1 m.
Wasted energy
- Some energy is always transferred to the surroundings as heat, and this dissipated energy is not useful.
Section 2 of 6Forms of Energy
Forms of Energy
Kinetic
- The energy of a moving object, depending on its mass and speed.
Gravitational potential
- The energy an object has because of its height above the ground.
Heat
- The energy transferred because of a temperature difference, also called thermal energy.
Others
- Chemical, electrical, light, sound and nuclear energy are other forms you must recognise.
| Device | Input energy | Useful output |
|---|---|---|
| Electric motor | Electrical | Kinetic |
| Lamp | Electrical | Light |
| Falling ball | Gravitational potential | Kinetic |
| Loudspeaker | Electrical | Sound |
Section 3 of 6Energy Transfers
1
A ball held high stores gravitational potential energy.
2
When released the potential energy is transferred to kinetic energy as the ball speeds up.
3
On hitting the ground some energy becomes sound and heat in the ball and surroundings.
4
If the ball bounces it does not reach its original height, because the energy transferred to the surroundings can not be recovered.
5
The total energy at the end is still equal to the total at the start.
Section 4 of 6Energy Stores and Efficiency Ideas
Energy Stores and Efficiency Ideas
Chemical store
- Food, fuel and batteries store chemical energy, which is transferred when they react or are used.
Electrical to heat
- In a kettle nearly all the electrical energy becomes heat, which is useful here as it warms the water.
Heat as waste
- In a filament lamp most electrical energy becomes heat rather than light, so it is mostly wasted.
Reducing waste
- Lubrication, insulation and streamlining reduce the amount of energy dissipated as unwanted heat.
Section 5 of 6Worked Example
Question
- An electric motor receives 12 000 J of electrical energy and transfers 9000 J as useful kinetic energy.
- How much energy is wasted and in what form?
- Total energy in = 12 000 J.
- Useful energy out = 9000 J.
- Wasted energy = 12 000 − 9000 = 3000 J, mostly as heat (and some sound) in the surroundings.
Answer 3000 J wasted, mainly as heat.
Section 6 of 6Exam Checklist & Tips
1.5.1 – Key Points for the Exam
- Conservation of energy Energy cannot be created or destroyed, only transferred.
- Joule The SI unit of energy.
- Dissipate To spread energy out into the surroundings where it is no longer useful.
- Closed system A system that does not exchange energy with the outside.
Common Mistakes
- Saying energy is 'used up'; it is transferred, not destroyed.
- Forgetting that wasted energy still counts in the energy total.
- Confusing heat energy with temperature.
- Giving the unit of energy as watts rather than joules.