2.1. The Big Idea http://www.ck12.org
2.1 The Big Idea
Energy, a scalar quantity measured in Joules, is a measure of the amount of, or potential for, dynamical activity in
something. The total amount of energy in the universe is constant. This kind of symmetry is called a conservation
law. Conservation of energy is actually just one of five conservation laws that have been identified by scientists.
The concept of energy conservation is one of the most important in physics, since according to our best model of
the world, everything in the universe — even mass — is a form of energy. Any group of things (we’ll use the word
systemfor this concept in the book) has a certain amount of energy. Energy can be added to a system: when chemical
bonds in a burning log break, they release heat. A system can also lose energy: when a spacecraft “burns up” its
energy of motion during re-entry, it releases energy to the surrounding atmospher in the form of heat. Aclosed
system is one for which the energy is constant, orconserved. In this chapter, we will often consider closed systems;
although the total amount of energy stays the same, it can transform from one kind to another. We will consider
transfers of energybetweensystems – known aswork– in more detail in Chapter 8. Needless to say, the universe as
a whole is a closed system in this sense.