Define efficiency as: (a) (%) efficiency = (useful energy output) / (total energy input) (× 100%) (b) (%) efficiency = (useful power output) / (total power input) (× 100%) recall and use these equations

1.7.3 Energy Resources – Efficiency

What is Efficiency?

Efficiency tells us how good a device is at turning the energy we give it into useful work or power. Think of it like a car engine: the more of the fuel’s energy that actually moves the car, the more efficient the engine is. 🚗

Key Equations

Energy Efficiency:

\$\eta_{\text{energy}} = \frac{\text{useful energy output}}{\text{total energy input}}\times 100\%\$

Power Efficiency:

\$\eta_{\text{power}} = \frac{\text{useful power output}}{\text{total power input}}\times 100\%\$

Analogy: The Light Bulb

A 60 W incandescent bulb uses 60 W of electrical power. But only about 10 % of that energy becomes visible light; the rest turns into heat. So the bulb’s efficiency is roughly 10 %. 💡

Example Calculation

Suppose a battery supplies 100 J of chemical energy to a motor, and the motor produces 70 J of mechanical work.

Using the energy efficiency formula:

\$\eta_{\text{energy}} = \frac{70\,\text{J}}{100\,\text{J}}\times 100\% = 70\%\$

Quick Reference Table

Exam Tips

• Always state the efficiency formula you use.
• Show all steps and units; the examiner checks your work.
• Remember that efficiency is a percentage, so multiply by 100 %.
• When given power values, use the power efficiency formula.
• Check if the question asks for “useful energy” or “useful power” – they are different.
• Use the analogy of a car engine or light bulb to explain why some energy is lost.