Efficiency and Energy Dissipation

This lesson covers energy efficiency and dissipation as required by the Edexcel GCSE Physics specification (1PH0), Topic 3: Conservation of Energy. You need to understand how to calculate efficiency, interpret Sankey diagrams, and explain how wasted energy can be reduced.

---

What Is Efficiency?

Efficiency is a measure of how much of the input energy is usefully transferred. It tells you what fraction (or percentage) of the total energy input is converted to useful output.

The Equations

As a decimal (fraction):

$\text{Efficiency} = \frac{\text{useful energy output}}{\text{total energy input}}$Efficiency=total energy inputuseful energy output​

As a percentage:

$\text{Efficiency (\%)} = \frac{\text{useful energy output}}{\text{total energy input}} \times 100\%$Efficiency (%)=total energy inputuseful energy output​×100%

You can also use power instead of energy:

$\text{Efficiency} = \frac{\text{useful power output}}{\text{total power input}}$Efficiency=total power inputuseful power output​

Key Points

• Efficiency is always less than 1 (or less than 100%) — no device is 100% efficient.
• Efficiency has no units — it is a ratio.
• An efficiency of 0.75 means 75% of the input energy is usefully transferred, and 25% is wasted.