Practice Efficiency in Physics
Use these practice problems to test your method after reviewing the concept explanation and worked examples.
Quick Recap
The ratio of useful output energy to total input energy, expressed as a percentage showing how little is wasted.
What fraction of the energy you put in actually goes where you want it to go, rather than being wasted as heat.
Example 1
easyA machine uses 500 \text{ J} of input energy and produces 350 \text{ J} of useful output. What is its efficiency?
Example 2
mediumAn electric motor with 85\% efficiency lifts a 200 \text{ kg} load by 10 \text{ m}. How much electrical energy is consumed? Use g = 10 \text{ m/s}^2.
Example 3
mediumA coal power plant burns fuel releasing 1{,}000{,}000 \text{ J} and generates 350{,}000 \text{ J} of electrical energy. What is the efficiency?
Example 4
hardA 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?