Efficiency & Energy Resources

Efficiency

• Recall both equations:

$$\text{efficiency} = \frac{\text{useful output energy transfer}}{\text{total input energy transfer}} \qquad \text{efficiency} = \frac{\text{useful power output}}{\text{total power input}}$$

• Efficiency has no unit. It is a decimal between 0 and 1, or a percentage (multiply by 100). It can never be more than 1 (100%)
• Worked example: a motor takes in 500 J and does 350 J of useful work. Efficiency $= \dfrac{350}{500} = 0.7 = 70\%$. The other 150 J is dissipated to the thermal store
• Rearranged: useful output $=$ efficiency $\times$ input
• (HT) To increase the efficiency of an intended transfer: lubricate moving parts, insulate against heating losses, streamline to reduce air resistance, use low-resistance wires to reduce heating in circuits, and recover wasted energy (a heat exchanger)