Efficiency, Power and Energy Resources
Efficiency
Efficiency measures how much of the energy supplied to a device is transferred usefully (not wasted). It is always a fraction or percentage — no device is 100% efficient (some energy is always dissipated, usually as heat).
Equation:
efficiency = useful energy output ÷ total energy inputMultiply by 100 for a percentage. You can also use power instead of energy:
efficiency = useful power output ÷ total power inputPower
Power is the rate of energy transfer — how much energy is transferred per second. Measured in watts (W), where 1 watt = 1 joule per second.
Equations:
power = energy transferred ÷ time (P = E ÷ t)
power = work done ÷ timeA more powerful device transfers the same energy in less time.
Reducing wasted energy
- Lubrication reduces friction (less heat wasted).
- Thermal insulation reduces heat loss. Materials with low thermal conductivity and thicker walls keep heat in for longer (important for house heating bills).
Energy resources
Resources are renewable (won't run out) or non-renewable (finite).
| Renewable | Non-renewable |
|---|---|
| Solar, wind, hydroelectric | Coal, oil, gas (fossil fuels) |
| Tidal, wave, geothermal | Nuclear (uranium) |
| Bio-fuel |
Fossil fuels: reliable and high energy output, but finite and release CO₂ (contributing to climate change) and other pollutants.
Renewables: cleaner and won't run out, but often depend on conditions (no wind/sun) and some have high set-up costs or environmental/visual impact.
Comparing resources — the trade-offs
When choosing an energy resource, consider: reliability, cost (build and running), environmental impact, and power output. There is no perfect resource — it's about balancing these factors.
Worked example
A motor takes in 200 J and usefully transfers 150 J to kinetic energy. What is its efficiency?
efficiency = 150 ÷ 200 = 0.75 = 75%The other 50 J is wasted (mostly as heat). ✓
Common mistakes
- Writing efficiency greater than 100% — it can never exceed 1 (or 100%).
- Confusing power (per second) with total energy.
- Calling nuclear a renewable — uranium is non-renewable.
Exam tips
- Learn both efficiency forms (energy and power).
- Remember P = E ÷ t and the unit: watt = joule/second.
- For "compare energy resources", give balanced points (reliability, cost, environment, output).
Key facts to remember
- Efficiency = useful output ÷ total input (×100 for %); never over 100%.
- Power = energy ÷ time (P = E ÷ t), measured in watts (1 W = 1 J/s).
- Renewables (solar, wind, hydro, tidal, geothermal, bio) won't run out; non-renewables (fossil fuels, nuclear) are finite — choice balances reliability, cost, environment and output.