Average power — Science definition
Transferred energy divided by the stated elapsed time.
Where it is taught
- Power: The Rate of Energy Transfer — Work & Machines, Physical Science: Matter, Forces & Energy, Grade 8
Questions using average power
Problem 1
A device transfers a useful 180 J in 3 s while receiving 240 J in that interval. Find useful average power and efficiency, then explain which changes if the same energy transfers take 6 s.
Answer and explanation
Useful power is 60 W and efficiency 75%. Taking 6 s reduces useful power to 30 W while the same energy ratio stays 75%.
Use energy/time for power and useful/input for efficiency.
Change only the interval in the second comparison.
Useful power is 60 W and efficiency 75%. Taking 6 s reduces useful power to 30 W while the same energy ratio stays 75%.
Response criteria
- Use the named labels, directions or supplied quantities accurately.
- Connect the visual evidence to the scientific mechanism or calculation.
- State the relevant model conditions or limitation.
Problem 2
Independent chapter check: A device transfers a useful 180 J in 3 s while receiving 240 J in that interval. Find useful average power and efficiency, then explain which changes if the same energy transfers take 6 s.
Answer and explanation
Useful power is 60 W and efficiency 75%. Taking 6 s reduces useful power to 30 W while the same energy ratio stays 75%.
Use energy/time for power and useful/input for efficiency.
Change only the interval in the second comparison.
Useful power is 60 W and efficiency 75%. Taking 6 s reduces useful power to 30 W while the same energy ratio stays 75%.
Response criteria
- Use the named labels, directions or supplied quantities accurately.
- Connect the visual evidence to the scientific mechanism or calculation.
- State the relevant model conditions or limitation.