Conservation of energy and efficiency
Conservation of energy
Energy cannot be created or destroyed, only transferred between stores or dissipated. The total energy of a closed system is constant.
Energy stores at GCSE:
- Kinetic
- Gravitational potential
- Chemical
- Elastic potential
- Thermal (internal)
- Magnetic / electrostatic
- Nuclear
Transfers happen by mechanical work, electrical work, heating, or radiation.
Efficiency
Real devices waste some energy, usually as heat (and sometimes sound). Efficiency measures the fraction usefully transferred.
efficiency = useful energy out / total energy in.
Often given as a percentage (multiply by 100). Efficiency is dimensionless and cannot exceed 1 (or 100%).
✦Worked example
A 60 W lamp transfers 9 J of light per second.
- Total energy per second = 60 J.
- Useful energy per second = 9 J.
- Efficiency = 9 / 60 = 0.15 = 15%.
The remaining 51 J/s is dissipated as heat.
Reducing unwanted transfers
| Unwanted transfer | Reduction technique |
|---|---|
| Heat through walls / loft | Cavity wall insulation, loft insulation (low thermal conductivity) |
| Heat through windows | Double glazing (air gap reduces conduction) |
| Friction in machines | Lubrication / oil |
| Sound / vibration | Sound-absorbing materials |
Lower thermal conductivity = slower rate of heat conduction. Thicker insulation = lower rate too.
WJEC exam tip
Always express efficiency as a value between 0 and 1, OR a percentage 0-100%. If you write "1.5" or "150%" the answer is wrong by definition — go back and check which is "useful" and which is "total".
AI-generated · claude-opus-4-7 · v3-wjec-combined-science-leaves