Energy
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Law of Conservation

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We have often heard phrases like, "Conserve energy; turn off the lights."  To scientists, conservation of energy is something entirely different.  The law of conservation of energy states that the total amount of energy in a system remains constant ("is conserved"), although energy within the system can be changed from one form to another or transferred from one object to another.
 
Energy cannot be created or destroyed, but it can be transformed.  That's really what we mean when we say we are "using" energy.  The law of conservation of energy means that when energy is being used, it is not being used up.  Instead, it is being changed from one form into another.  A car engine burns fuel, converting the fuel's chemical energy into mechanical energy to make the car move.  Windmills change the wind's energy into mechanical energy to turn turbines, which then produce electricity.  Solar cells change sunlight (radiant energy) into electrical energy.
FYI Can Energy Disappear?