Knowledge Organiser: Efficiency
Key Equations
- Efficiency = Useful output ÷ Total input
- Efficiency (%) = (Useful ÷ Total) × 100
- Wasted = Total input − Useful output
- Same formula works with power (W)
Key Terms
- Efficiency — ratio of useful output to total input (no unit)
- Useful output — energy in intended form
- Wasted energy — unintended outputs (mainly heat)
- Sankey diagram — width = energy amount
- Dissipated — spread into surroundings as heat
Key Rules
- Efficiency is always less than 100%
- Energy is conserved (never lost)
- Wasted energy goes to thermal store
- Efficiency has NO unit
Improving Efficiency
- Lubrication — reduces friction
- Insulation — reduces heat loss
- Streamlining — reduces air resistance
- Superconductors — reduce electrical resistance
Common Mistakes
- Efficiency greater than 1 (or 100%): Efficiency can never exceed 1.0 (100%) — if your answer is above this, check you divided useful by total, not the other way round
- Using the wrong form of the equation: Both forms are correct — useful energy ÷ total energy, OR useful power ÷ total power — choose based on what data is given
- Saying wasted energy disappears: Wasted energy is transferred to the thermal store of the surroundings — it is not destroyed
- Forgetting to multiply by 100 for percentage: The decimal form (e.g. 0.75) and percentage form (75%) are both valid — make sure you give the correct form
- Assuming more efficient = more powerful: Efficiency is a ratio — a low-power device can be 100% efficient while a high-power device is only 50% efficient
Memory Aid
- Efficiency = Useful Output ÷ Total Input (remember: U.O.T.I.)