Efficiency Physics Example 4

Follow the full solution, then compare it with the other examples linked below.

Example 4

hard

A car engine has an efficiency of

25\%

and a petrol pump has an efficiency of

90\%

. The pump delivers

40 \text{ litres}

of fuel containing

1.4 \times 10^9 \text{ J}

of chemical energy. How much useful kinetic energy does the car ultimately produce from this fuel?

Solution

1
Energy delivered by pump: $E_{\text{pump}} = 0.90 \times 1.4 \times 10^9 = 1.26 \times 10^9 \text{ J}$ .
2
Useful energy from engine: $E_{\text{useful}} = 0.25 \times 1.26 \times 10^9 = 3.15 \times 10^8 \text{ J}$ .
3
Overall efficiency: $0.90 \times 0.25 = 0.225 = 22.5\%$ .

Answer

E_{\text{useful}} = 3.15 \times 10^8 \text{ J} \quad (\text{overall efficiency } 22.5\%)

When energy passes through multiple stages, the overall efficiency is the product of each stage's efficiency. Each stage loses some energy, so the overall efficiency is always lower than any individual stage.

About Efficiency

The ratio of useful output energy (or power) to total input energy, expressed as a percentage — always less than 100% due to energy losses.

Learn more about Efficiency →

More Efficiency Examples

Example 1 easy

A machine uses [formula] of input energy and produces [formula] of useful output. What is its effici

Example 2 medium

An electric motor with [formula] efficiency lifts a [formula] load by [formula]. How much electrical

Example 3 medium

A coal power plant burns fuel releasing [formula] and generates [formula] of electrical energy. What

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