Physics · Grades 8–11
Power and efficiency without unit confusion · Revision sheet
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.
Explain without looking
- A system takes in 80 J and has 25% efficiency. Find useful output energy.
- A calculation gives 140% efficiency for a simple device with no extra energy source. What should be checked?
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.