Edexcel iGCSE Physics -4.4 Efficiency- Study Notes- New Syllabus

Efficiency, Useful Energy Output and Total Energy Output

Efficiency is a measure of how much of the input energy is transferred usefully by a device or system. Not all input energy is converted into useful output energy — some energy is always transferred to wasted energy stores, usually as thermal energy.

Definition of Efficiency

Efficiency compares the useful energy output with the total energy input.

\( \mathrm{efficiency = \dfrac{useful\ energy\ output}{total\ energy\ input}} \)

Efficiency has no units.

It is often written as:

• A decimal (e.g. 0.75)
• A percentage (e.g. 75%)

\( \mathrm{efficiency(\%) = \dfrac{useful\ energy\ output}{total\ energy\ input} \times 100} \)

Understanding Energy Outputs

• Useful energy: energy transferred to the desired energy store.
• Wasted energy: energy transferred to unwanted stores (often thermal or sound).
• Total energy input = useful energy + wasted energy

Efficiency can never be greater than 1 (or 100%).

Sankey Diagrams

• Used to show energy transfers visually.
• Arrow width represents the amount of energy.
• Useful energy continues straight.
• Wasted energy branches off.

Key Idea

• Higher efficiency means less energy is wasted.
• Some energy is always wasted due to friction or resistance.
• Improving efficiency reduces energy loss.

Important Points to Remember

• Use energy values in joules (J).
• Do not confuse energy with power.
• Check whether the answer is required as a decimal or percentage.