1.1
1
Energy
As this book is on solar energy, it is good to start the discussion with some general
thoughts on energy. We begin with a quote from The Feynman Lectures on Physics [1].
There is a fact, or if you wish, a law, governing all natural phenomena that are known to date. There is no
known exception to this law—it is exact so far as we know. The law is called the conservation of energy.
It states that there is a certain quantity, which we call energy, that does not change in the manifold
changes which nature undergoes. That is a most abstract idea, because it is a mathematical principle; it
says that there is a numerical quantity which does not change when something happens. It is not a
description of a mechanism, or anything concrete; it is just a strange fact that we can calculate some
number and when we finish watching nature go through her tricks and calculate the number again, it is
the same.
…
Energy has a large number of different forms, and there is a formula for each one. These are: gravitational
energy, kinetic energy, heat energy, elastic energy, electrical energy, chemical energy, radiant energy,
nuclear energy, mass energy. If we total up the formulas for each of these contributions, it will not change
except for energy going in and out.
It is important to realize that in physics today, we have no knowledge of what energy is. We do not have a
picture that energy comes in little blobs of a definite amount. It is not that way. However, there are
formulas for calculating some numerical quantity, and when we add it all together it gives … always the
same number. It is an abstract thing in that it does not tell us the mechanism or the reasons for the various
formulas.
Some definitions
We will now state some basic physical connections between the three very important
physical quantities of energy, force, and power. These connections are taken from classical
mechanics but are generally valid. We start with the force F, which is any influence on an
object that changes its motion. According to Newton’s second law, the force is related to
the acceleration a of a body via
where m is the mass of the body. The bold characters denote that F and a are vectors. The
unit of force is newton (N), named after Sir Isaac Newton (1642–1727). It is defined as the
force required to accelerate the mass of 1 kg at an acceleration rate of 1 m s
−2
, hence 1 N =
1 kg m s
−2 .
1
Energy
As this book is on solar energy, it is good to start the discussion with some general
thoughts on energy. We begin with a quote from The Feynman Lectures on Physics [1].
There is a fact, or if you wish, a law, governing all natural phenomena that are known to date. There is no
known exception to this law—it is exact so far as we know. The law is called the conservation of energy.
It states that there is a certain quantity, which we call energy, that does not change in the manifold
changes which nature undergoes. That is a most abstract idea, because it is a mathematical principle; it
says that there is a numerical quantity which does not change when something happens. It is not a
description of a mechanism, or anything concrete; it is just a strange fact that we can calculate some
number and when we finish watching nature go through her tricks and calculate the number again, it is
the same.
…
Energy has a large number of different forms, and there is a formula for each one. These are: gravitational
energy, kinetic energy, heat energy, elastic energy, electrical energy, chemical energy, radiant energy,
nuclear energy, mass energy. If we total up the formulas for each of these contributions, it will not change
except for energy going in and out.
It is important to realize that in physics today, we have no knowledge of what energy is. We do not have a
picture that energy comes in little blobs of a definite amount. It is not that way. However, there are
formulas for calculating some numerical quantity, and when we add it all together it gives … always the
same number. It is an abstract thing in that it does not tell us the mechanism or the reasons for the various
formulas.
Some definitions
We will now state some basic physical connections between the three very important
physical quantities of energy, force, and power. These connections are taken from classical
mechanics but are generally valid. We start with the force F, which is any influence on an
object that changes its motion. According to Newton’s second law, the force is related to
the acceleration a of a body via
where m is the mass of the body. The bold characters denote that F and a are vectors. The
unit of force is newton (N), named after Sir Isaac Newton (1642–1727). It is defined as the
force required to accelerate the mass of 1 kg at an acceleration rate of 1 m s
−2
, hence 1 N =
1 kg m s
−2 .