In physics, power is the rate of doing work or transferring heat. It is measured in joules per second (J/s) or watts (W). In SI units, power takes the unit watt (W), equal to one joule per second. Other units for power include ergs per second (e/s) and calories per second (cal/s).
Power has several different forms that can be classified according to their causes:
-mechanical, e.g., windmill, waterwheel
-electrical, e.g., batteries, generators
-chemical, e.g., gasoline engine
-nuclear fission and fusion reactors
Philosophers and scientists since antiquity have studied the concept of power. The early Greek philosopher Aristotle distinguished between two types of cause: formal cause and efficient cause. The former corresponds to what we would now call “power,” while the latter corresponds to modern notions of “energy” or “work.”