The principle of Energy Conservation is one of the great unifying
principles of physics. There is no quick and simple way to grasp the
idea of energy or to appreciate the importance of the idea and of the
conservation principle. The appreciation comes slowly with use and
practice, much like learning a language comes with listening and
speaking. 
The
one sentence definition: "Energy is the ability to do work" is next
to useless unless one has a clear understanding of what "work" is.
Energy is what we buy from the electrical company, energy is what we
eat food for, energy can be extracted from gasoline in the tank and
from waterfalls and the wind. Energy is what reaches us from the Sun
and makes life on Earth possible. Energy is what gives us cancer if
we get exposed to radiation. In all the variety of change around us,
energy is converted from one form to another but is never lost. The
appreciation of the idea comes when you realize the importance of any
conservation principle. A conservation principle states that
confusing as change may be, some quantities stay the same. All
through human history people tried to make sense of things by looking
for these invariant, conserved, quantities.
Glimpses
of the idea of energy predate Newton and his laws of mechanics. In
1638 the Italian scientist
Galileo published a book in which he noted that the speed of an
object descending under gravity depends only on the height from which
it is released and not on the path it takes. This is the germ of the
idea of potential energy. 
If
you know the height from which an object is released, you can predict
the speed it will attain. Or conversely, if you know the speed, you
can predict the height it can attain. There are many different paths
but something doesn't change. The sum total of potential and kinetic
energies. Of course, friction can spoil the result. But then you get
heat. And if you account for heat, the total energy still stays the
same.
In
a paper in 1686 the German scientist
Leibniz (he also invented calculus independently of Newton)
argued that the true measure of the effect of force is "living force"
as exhibited in motion and not "dead force" as exhibited in statics.
Buildings and bridges stay up by "dead force" whereas moving object
possess "living force". The "living force" of Leibniz was the product
of mass and velocity squared, (almost) what we now call kinetic
energy. By the way, Leibniz wrote his paper to argue against the
claim by another scientist that the product of mass and velocity was
the universally conserved quantity. They were both right in some
sense. In the weeks that follow we will have to sort this out.
During
the nineteenth century the term "physics" stopped being a synonym for
general science . Physics meant the study of mechanics, electricity,
and optics employing mathematical and experimental methodology. The
unifying concept was energy. The nineteenth century saw a rapid
development of engineering and technology based on mathematics and
physics. The invention of the steam engine led people to ponder the
relation between motion and heat. Experiments with electricity and
electric motors led people to ponder the relation between motion and
electricity. The ideas of energy helped to destroy the notion that
heat and electricity are strange forms of matter that has no mass and
occupies no space. Such materials were called imponderables. In the
1840's Benjamin Thompson, a weird American scientist-engineer who
became a German nobleman Count Rumford after he sided with the
British in the War of Independence, proposed that heat and
temperature are related to motion.
Subsequently James
Prescott Joule measured the mechanical equivalent of heat via an
electrical. In 1847 he suggested that the motion of particles of
matter is related to temperature. Ultimately these ideas have rise to
atomic physics and modern chemistry. Again the unifying idea was
energy. Heat energy was now added to mechanical forms of energy. The
notion of work, first as the product of force and distance and then
more formally as the integral of force with respect to distance was
introduced at about the same time.
In
the early years of the twentieth century physicists gave up the idea
that energy must be "property" of matter and accepted the notion that
energy packets exist in their own right. Light is "pure" energy which
is emitted by atoms can travel thorough space and can be reabsorbed
by atoms. The ultimate demonstration of these ideas was the first
nuclear explosion. And yet, as far as we know, energy in all its
forms is conserved. It is constantly being transformed but it is
never lost or created.
The links below will take you to sites with more detailed information on this story and on related stories.
1.
Who originated the idea of a horsepower?
2. What is the relation of the horsepower to a watt?
3. How is a BTU related to the joule?
To get the extra credit send an email message with your answers to
Prof. Nick
