Key Questions

  • Energy is a measure of the capability of doing work (=transform energy) and is measured in Joules. Power is the work per unit time and is measured in Watts (Joules/seconds).

    Consider a pile of bricks. You can take them up to the second floor of a building by hand using energy (you do work transforming chemical energy of your muscles into potential energy of the bricks that are now at the second floor). The same energy is used by a fork lift (it transforms chemical energy in the fuel to do the same as you) BUT it takes 1/10 of the time to do that! The power measures the “efficiency” in terms of time performance in transforming energy doing work.

  • Answer:

    Power is the rate at which Work is performed.


    In general we can write:


    basically it tells us how "fast" we transfer Energy.
    Consider an example:
    You need to take one truckload of bricks to the third floor of a building.
    You can take the bricks by hand or using a lifting crane; at the end of the day the Work done (against gravity) will be the same in both cases BUT the crane will have done the Work more rapidly than by hand!!!