Changes

'''Energy''' is the ability to do work.<ref>Wile, Dr. Jay L. ''Exploring Creation With [[Chemistry]]''. Apologia Educational Ministries, Inc. 1998</ref>  In [[physics]], energy is a conserved quantity, and a closed system of interacting particles is known to always have the same value for its energy; this is known as the law of conservation of energy.<ref>Halliday, Resnick, Walker: '' Fundamentals of Physics'', 6th Edition</ref> The SI unit of energy is [[joule]]. Energy can be converted into various forms like [[Potential energy]], [[Kinetic energy]] and Thermal energy (see below for more).  

'''Energy''' is the ability to do work.<ref>Wile, Dr. Jay L. ''Exploring Creation With [[Chemistry]]''. Apologia Educational Ministries, Inc. 1998</ref>  In [[physics]], energy is a conserved quantity, and a closed system of interacting particles is known to always have the same value for its energy; this is known as the law of conservation of energy.<ref>Halliday, Resnick, Walker: '' Fundamentals of Physics'', 6th Edition</ref> The SI unit of energy is [[joule]]. Energy can be converted into various forms like [[Potential energy]], [[Kinetic energy]] and Thermal energy (see below for more).  

More generally, energy is a source of usable heat or power.

More generally, energy is a source of usable heat or power. [[Nobel Prize]]-winning [[Economics|economist]] [[Milton Friedman]] observed that energy is the “lifeblood of our economic system.”

There are a number of formulas in physics that relate to energy, such as "E=(1/2)mv&sup2;", "E=Fd", "E=QV" (charge times voltage), "E=CV&sup2;", "E=LI&sup2;", and the very famous but not often used in ordinary physics "[[E=mc²]]".

There are a number of formulas in physics that relate to energy, such as "E=(1/2)mv&sup2;", "E=Fd", "E=QV" (charge times voltage), "E=CV&sup2;", "E=LI&sup2;", and the very famous but not often used in ordinary physics "[[E=mc²]]".