Lectures on Physics has been derived from Benjamin Crowell's Light and Matter series of free introductory textbooks on physics. See the editorial for more information.... 
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See also: Converting kilowatthours to joules, Human wattage, You take the high road and I’ll take the low road  
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PowerA car may have plenty of energy in its gas tank, but still may not be able to increase its kinetic energy rapidly. A Porsche doesn't necessarily have more energy in its gas tank than a Hyundai, it is just able to transfer it more quickly. The rate of transferring energy from one form to another is called power. The definition can be written as an equation
where the use of the delta notation in the symbol ΔE has the usual interpretation: the final amount of energy in a certain form minus the initial amount that was present in that form. Power has units of J/s, which are abbreviated as watts, W (rhymes with lots).
If the rate of energy transfer is not constant, the power at any instant can be defined as the slope of the tangent line on a graph of E versus t. Likewise ΔE can be extracted from the area under the Pversust curve.


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