LET’S TRY ONE
A motor receives 100 J and transfers 40 J to useful lifting. What is its efficiency for lifting?
40%
40 ÷ 100 × 100% = 40%.
Learning steps & teaching tips
Use a stated system boundary to calculate efficiency and account for energy transferred to other forms.
- Define the useful output for the stated purpose.
- Compare that output with the entire measured input.
- Account for other energy transfers without describing energy as destroyed.
Try it together
Draw an energy-flow diagram for an invented lamp or motor. Give every arrow a value and check that outputs account for the input.
Efficiency = useful output ÷ total input × 100%; energy not counted as useful is still transferred somewhere.