Energy
Energy is an abstract quantity of extreme usefulness in physics because it is defined in such a way that the total energy of any closed physical system is always constant (
conservation of energy). It is impossible to overstate the importance of this concept in all branches of physics from elementary mechanics to
general relativity. Energy is measured in units of mass times velocity squared, and the
MKS and
cgs units of energy are the
Joule and
erg, respectively. Other common units of energy include the
Btu,
calorie, and
kilowatt hour.
The important quantity in physics known as
work, which is the product of applied force over a distance, has units of energy. In fact, the notion that
heat is a form of
energy was one of the most important developments in classical physics and thermodynamics. Energy is related to
power P emitted over a time
t by