205
C H A P T E R 14
e Structure of Energy and
Matter
14.1 THE NATURE OF ENERGY
Energy
1
can take many forms, and it can change from one form to another (or from one form
to several others), but it does not disappear, or appear from nothing; the total amount remains
constant as stuff happens. is observation is called the Law of Conservation of Energy, and it is
one of the most important principles in physics.
ere is no good one-sentence definition of energy that does not raise more questions
than it answers. As with other physical quantities, to be thorough one must use an operational
definition, describing all of its properties in different circumstances. To do so would require a
book of its own; here I give only a brief overview.
ere are many types of energy, some of which have familiar names—e.g., electrical en-
ergy, solar energy, chemical energy, and nuclear energy. ese different types of energy mani-
fest themselves in different ways and under different circumstances (and so they have different
names). But when it comes down to it, all types of energy can be described as some variety of
four basic types, of which I give only very brief descriptions here.
• Kinetic energy—is is the energy due to an object’s mass and motion, compared to some
other frame of reference.
• Gravitational energy—is is energy associated with the force of gravity. A glass gets
nudged off the table and then falls to the floor and shatters. e energy to make the glass
shatter came at the expense of a decrease in gravitational energy.
• Electromagnetic energy—e energy associated with electric and magnetic forces is the
basis for solar energy and chemical energy, among many other forms. Since light carries
electromagnetic energy, it is of particular importance to astronomy.
• Nuclear energy—is is the energy associated with the enormous forces at work in the
nuclei of atoms.
1
Parts of this chapter appeared, in a somewhat different form, in Beaver [2018b, Chap. 1]
206 14. THE STRUCTURE OF ENERGY AND MATTER
All but the first of these are forms of energy associated with the three fundamental forces
of nature,
2
and I have listed them in order of the relative strength of these forces. is likely fits
with your prior knowledge; compare the power output from an old-fashioned water wheel (grav-
itational energy of the falling water) to typical sources of electrical power. And then compare
these to nuclear energy.
In addition to these explicit forms of energy, there is a direct correspondence between
energy and mass. Albert Einstein discovered, as part of the theory of Special Relativity, that
there is a direct equivalence between mass and energy, embodied in the most famous of all
equations:
E D mc
2
: (14.1)
Here c is the speed of light, and Equation (14.1) says that a tiny bit of mass is equivalent to an
enormous amount of energy. And so matter can manifest itself as energy and vice versa. is
is one of the key discoveries of modern physics, and it is the foundation for many important
phenomena, including the fact that the Sun shines.
e joule (J) is the SI unit for energy. As an example of how much energy a joule repre-
sents, to lift one gallon of milk from the floor to the kitchen counter top requires an increase of
gravitational energy of roughly 40 J. By contrast, the energy content of ordinary matter is nearly
unimaginable. From Equation (14.1) a single kilogram of mass is equivalent to about 9 ˆ 10
16
J
of energy. is is the energy that would be released in the explosion of literally millions of tons
of chemical explosive. ese direct conversions between mass and energy ordinarily occur only
in nuclear reactions. For historical images of examples of such enormous releases of energy, see
the photography of Michael Light [2013].
Although there are four basic forms of energy, there are many other names to describe
common cases of energy transfer that are often complex combinations of the four basic types.
Below are a few examples of particular importance to astronomy.
1. ermal energy: e individual atoms and molecules in a gas, liquid or solid are con-
stantly in random motions, colliding with each other. And so these molecules—each
individually—have kinetic energy. ermal energy is related to the total kinetic energy
of these individual motions. e related concept of temperature refers not to this total in-
ternal energy, but rather to the average kinetic energy per particle.
2. Solar energy: Since light is an electromagnetic wave, it carries electromagnetic energy.
e intensity of light is related to the rate of energy transfer of electromagnetic energy
through a region of space, per square meter.
3. Chemical energy: Atoms are comprised of positively charged nuclei and negatively
charged electrons, and it is the electrical force between opposite charges that holds an
2
e number of fundamental physical forces in nature depends in part on how one counts. ere are, for example, really
two fundamental types of nuclear forces, and thus two fundamental types of nuclear energy. I have lumped these together as
simply “nuclear energy.”
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