Physics · Grades 8–11
Power and efficiency without unit confusion
Compare rates and fractions rather than mixing energy and power.
Learn the idea
Power
Power is energy transferred divided by time. One watt is one joule per second.
Efficiency
Useful output divided by total input is a fraction; multiply by 100 for a percentage.
Equal tasks
The same energy transfer in less time means greater average power, not necessarily greater total energy.
System boundary
State what counts as useful output. A lamp and a heater can judge the same thermal transfer differently.
Worked examples
A machine transfers 600 J in 15 s. Find average power.
600/15 = 40 W.
40 W
A device receives 500 J and delivers 350 J usefully. Find efficiency as a percentage.
350/500 × 100 = 70%.
70 %
Check your understanding
Two motors do the same work; one takes half the time. Its average power is what?
- Unchanged
- Twice as large
- Half as large
A 25 W model device runs for 12 s. Find energy transferred.
Why should a question define useful output before asking for efficiency?
Answers and marking notes
Answers and marking notes
- Twice as large
At the same work, halving time doubles power.
- 300 J
E = Pt = 25 × 12 = 300 J.
- Usefulness depends on the intended purpose and chosen system boundary.
Credit the goal of the device and consistent input/output accounting.
Further reading and review references
These are reference links, not copied exercises, institutional endorsement or proof of independent review.
Independent educational review required. Grade bands are approximate; these are not officially aligned curricula.