2.1. The Big Idea http://www.ck12.org
2.1 The Big Idea
Energy 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 symmetry is called a conservation law. Physicists have identified five conservation
laws that govern our universe.
A group of things (we’ll use the wordsystem) 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 and the surrounding atmosphere absorbs it in
the form of heat. Aclosedsystem 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 energy between systems — known aswork-–in more detail in Chapter 8.
Key Definitions
Kinetic energy
K=
1
2
mv^2{
m mass (in kilograms, kg)
v speed (in meters per second, m/s)Gravitational potential energy
Ug=mgh
h height above the ground in meters(m)
g acceleration due to gravity, 9.8 m/s^2
E energy (in Joules; 1 J=1kg m^2 /s^2 )Key Concepts
- Any object in motion has kinetic energy. Kinetic energy increases as the square of the velocity, so faster
objects have much more kinetic energy than slower ones. - The energy associated with gravity is called gravitational potential energy. Near the surface of the earth, an
object’s gravitational potential energy increases linearly with its height. - Molecules store chemical potential energy in the bonds between electrons; when these bonds are broken the
released energy can be transferred into kinetic and/or potential energy. 1 KCal (1 food Calorie) is equal to
4180 Joules of stored chemical potential energy. - Energy can be transformed from one kind into another and exchanged between systems; if there appears to be
less total energy in a system at the end of a process then at the beginning, the “lost” energy has been transferred
to another system, often by heat or sound waves.