Energy Transfers and Efficiency
Energy Transfers and Efficiency
Energy cannot be created or destroyed — it can only be transferred, stored, or dissipated. This is the law of conservation of energy, and it underpins everything in this topic.
Energy Stores and Transfers
Energy exists in different stores:
- Kinetic — energy of a moving object
- Gravitational potential — energy due to height in a gravitational field
- Elastic potential — energy stored in a stretched or compressed object
- Thermal — energy related to the temperature of an object
- Chemical — energy stored in chemical bonds (fuels, food, batteries)
- Nuclear — energy stored in the nuclei of atoms
- Electrostatic — energy due to charges
- Magnetic — energy stored in magnetic fields
Energy is transferred between stores by four pathways:
| Pathway | Example |
|---|---|
| Mechanically (by a force) | Pushing a trolley |
| Electrically (by current) | A heater warming a room |
| By heating (conduction, convection, radiation) | A hot drink cooling down |
| By radiation (light, sound, other waves) | A lamp lighting a room |
Efficiency
No energy transfer is 100% useful. Some energy is always dissipated — spread out to the surroundings, usually as thermal energy. This wasted energy is no longer useful.
Efficiency tells us what fraction of the input energy is usefully transferred:
efficiency = useful output energy transfer / total input energy transfer
This can also be written using power:
efficiency = useful power output / total power input
Efficiency is a ratio with no units. It is often expressed as a percentage by multiplying by 100.
Example: A motor uses 500 J of electrical energy. It transfers 350 J as kinetic energy and 150 J as thermal energy (waste). Its efficiency is:
efficiency = 350 / 500 = 0.70 = 70%
Reducing Unwanted Energy Transfers
Engineers design systems to reduce wasted energy:
- Lubrication reduces friction between moving parts, reducing thermal energy waste
- Thermal insulation reduces energy transfer by heating (e.g. loft insulation, cavity wall insulation, double glazing)
- Streamlining reduces air resistance on moving vehicles
Sankey Diagrams
A Sankey diagram represents energy transfers visually. The width of each arrow is proportional to the amount of energy it represents.
- The input arrow enters from the left
- Useful output goes straight across (right)
- Wasted energy branches off downwards
- All arrows must add up to the input (conservation of energy)
Exam Tips
- Always state what store energy starts in AND what store it ends in — do not just say "energy is transferred"
- When calculating efficiency, check whether the question asks for a decimal or a percentage
- In Sankey diagrams, you can find waste energy by subtracting useful output from total input