Editing Power (physics) (section)

==Average power and instantaneous power==
As a simple example, burning one kilogram of [[coal]] releases more energy than detonating a kilogram of [[TNT]],<ref>Burning coal produces around 15-30 [[megajoule]]s per kilogram, while detonating TNT produces about 4.7 megajoules per kilogram. For the coal value, see {{cite web | last = Fisher | first = Juliya | title = Energy Density of Coal | work = The Physics Factbook | url = http://hypertextbook.com/facts/2003/JuliyaFisher.shtml|year=2003|access-date =30 May 2011}} For the TNT value, see the article [[TNT equivalent]]. Neither value includes the weight of oxygen from the air used during combustion.</ref> but because the TNT reaction releases energy more quickly, it delivers more power than the coal.
If {{math|Δ''W''}}  is the amount of [[mechanical work|work]] performed during a period of [[time]] of duration {{math|Δ''t''}}, the average power {{math|''P''<sub>avg</sub>}} over that period is given by the formula
<math display="block">P_\mathrm{avg} = \frac{\Delta W}{\Delta t}.</math>
It is the average amount of work done or energy converted per unit of time. Average power is often called "power" when the context makes it clear.

Instantaneous power is the limiting value of the average power as the time interval {{math|Δ''t''}} approaches zero.
<math display="block">P = \lim_{\Delta t \to 0} P_\mathrm{avg} = \lim_{\Delta t \to 0} \frac{\Delta W}{\Delta t} = \frac{dW}{dt}.</math>

When power {{math|''P''}} is constant, the amount of work performed in time period {{mvar|t}} can be calculated as
<math display="block">W = Pt.</math>

In the context of energy conversion, it is more customary to use the symbol {{mvar|E}} rather than {{mvar|W}}.