Energy is Matter in Motion
Kinetic energy is
the energy contained in a moving mass or in a moving particle (a particle is a
tiny bit of matter). I think it might be fun to call this "happening energy"
(hoping I don't get in trouble with any teachers). If you could look close enough,
or small enough, you would see that when anything is "happening", work
is being done, or has been done, and something (mass), or a whole lot of little
"somethings", are moving.
So this page is the nit-picky page. Here is where
those of us that want to really "get it straight" - or really get confused
- can come to ponder and argue, and hopefully understand a little more about energy.
Read on, McDuff....
and Heat are Processes (and Forms of Energy?)
There are just two ways that energy
is transferred - by work or by heat. We often use the words work and heat
as if they are forms of energy. I do it all the time and I'm not sorry. But some
thermodynamic text books say that work and heat are processes or methods of energy
transfer, not forms of energy.
When describing energy transfers in this way, we
should say something like, "energy transferred during a work process".
Or when talking about a heat process, we would be more correct to say, "energy
transferred by heat flow". Most
of us don't because it gets old fast. But that would be the clearer way to refer
to those two processes.
Really. Think about it for a bit. It is energy that
is transferred or changed. The process by which this occurs is either by work
or by heat (not by hook or by crook). In this context, they are processes or methods
of energy change, not types of energy.
A man is floating in water after falling out of
a boat. He is wearing a life jacket, of course. Anyone who goes boating without
a life jacket is a fool. The water temperature is 0 degC (32 degF). Because the
water temperature is much lower than the man's body temperature of 38 degC (98.6
degF), and because water is such a good conductor for heat flow, energy as heat
will flow rapidly from his body into the water. In just a few miserable minutes
the man's body temperature will start to drop, and the water around him will be
warmed up a little bit. Energy is being transferred, by the process of heat
flow, from his body to the surrounding water. It was internal energy that
was transferred, not heat. We usually say he is losing heat. But
what he is really losing is internal energy.
Internal energy is the word that is more correctly used in thermodynamics to describe
the microscopic forces and movements that are often called heat.
(Note: see new comments on the term "thermal energy"
Heat is Work when Things are Teesy-Weensy
energy is transferred through a work process, kinetic energy is involved. Energy
is transferred by work to an object or system
when a force is exerted through a distance on that object or system. For a force
to travel through a distance there has to be movement. Something has to move and
push against something else. That something else that is pushed on is then also
moved. If nothing moves, no work is done, and no energy is transferred. Anytime
a small or large chunk of matter is moving it contains kinetic energy.
that's not all. Our nit-pickiness has just begun. To be really nit-picky, we have
to realize that down on the teensy-weensy scale ("microscopic or atomic"
scale, if you must be formal), heat is work. Yup, the concept of heat is sort
of an illusion resulting from our hugeness of size and the impossibility of keeping
track of unimaginably large numbers of unimaginably small atoms and molecules.
What we call heat flow is really the average result
of trillions of atoms and molecules transferring their kinetic energy, through
the work process, to other atoms and molecules.
and internal energy are two quite different things; the energy associated with
microscopic motions and forces is internal energy, not heat."
from Thermo text book:
by William Reynolds and Henry Perkins
McGraw-Hill Book Company
"See, I told you so."
from small minded know-it-all author David
In hot water, or your body, or a shrew's body,
or a whale's body, or a mosquito's body, or in the air they all breathe, or the
rocks they sit on (except for whales), molecules and atoms are spinning, vibrating,
binding together with nuclear forces, storing energy as mass according to Einstein'
equation, transferring electrons, or in the case of a gas, just generally flying
around and banging into each other. When these tiny bits of matter bang into each
other, or transfer electrons or electromagnetic energy, or vibrate or spin against
each other, they transfer energy by doing work on each other. Yup, work is being
done, one tiny atomic particle at a time.
I'll say it again. Because they are so really really
(two really's) small, and because there are many trillions of them, and because
things happen so fast, we cannot even begin to think about measuring the energy
of each atomic particle. However, we can measure some things that indicate the
average energy of all of them combined. When we measure temperature or pressure
we are measuring the average energy of all of the atoms and molecules.
In fact temperature in a gas is often described (not
defined!) as the average kinetic energy of all the atoms and molecules in the
Pressure is a force acting on a certain area. Pressure
in a gas like air, or a liquid like water, is the average force of trillions of
atoms and/or molecules pushing on a certain area. We measure pressure in units
like Newtons of force per square meter (called Pascals) or pounds of force per
square inch (psi). The higher the average "pushing" energy of all the
microscopic particles, the higher the pressure.
If the temperature of the air in your house goes
up, it is because the average kinetic energy of the air molecules in your house
has increased. At any given moment some molecules are going fast and some are
going slow. Some are banging into others, transferring energy by work from one
to another. But the average energy of all of them combined has increased. If you
start to feel hot, it is because the higher energy air
molecules have started to transfer more of this energy by the mechanism
of heat flow into the molecules of your body, and less internal energy, created
by your metabolism, is able to flow out of
There are Two Types of Energy - in Many Forms
Despite all my nit-picking in the words above, most
scientists and engineers find it acceptable and useful to talk about chemical
and electrical and mechanical and magnetic energy and others. They also frequently
describe internal energy as heat or "heat
content" or "quantity of heat" as some of the older texts call
But it is helpful to understand why some people
say that all those other forms of energy are really types of kinetic energy or
potential energy being "expressed" in different ways.
for example, is the flow of charged particles called electrons or ions. When electrons
are flowing through a wire or through hundreds of feet of air (an event we call
lightning) it is because they are being "pushed" or forced by an electrical
field. This field is caused by a difference in electrical charge. A force is exerted
on the electrons and they move. Work is done on the charged particles. A force
is pushing them through a distance. Actually, they are hopping from atom to atom,
being pushed by an electromotive force. While the electrons are moving they contain
kinetic energy. So at the itsy-bitsy atomic level electricity is a form of kinetic
is the useful way we sometimes refer to things like gears, engines, locomotives
pulling trains, canon balls flying through the air, or other examples of energy
in mechanical devices. But, of course, by now you see that all these moving parts
contain kinetic energy. They are really just different modes of kinetic energy
- the energy contained in a moving mass. In order to get these various objects
spinning or rolling, a force has to be exerted. Work is a force acting through
a distance, so the way they get moving and keep moving is by having work done
on them. Work is an energy transfer process.
is another term we use a lot. This is more vague. We say things like, "during
combustion, chemical energy is released". Hmmm. The term chemical energy
refers to energy that is stored in molecular bonds, the forces that hold molecules
together. So releasing chemical energy must mean the energy is finally free from
its molecular bonds. In the more general sense, of course, it is potential energy.
Stored energy, or energy that is "waiting to happen", or that has the
"potential" to happen, or that can happen but hasn't yet, is rather
sensibly called potential energy.
As described in the photosynthesis
section, carbohydrate molecules, used by living organisms for food (and other
things), store energy in their atomic bonds. Living cells release this stored
energy relatively slowly by a process called respiration. Some of the stored potential
energy becomes the kinetic energy of cell processes and muscle movement and some
of it becomes internal energy (often called heat). But now you know I should have
said, "some of stored energy is transferred by the heat process into the
internal energy of the cell. The cell is "warmed up" by increasing the
average energy of the cell molecules. Eventually, of course, all of it becomes
internal energy and then flows by heat transfer into the air and objects around
Enough examples. We get the idea. Maybe you can
think about some other forms. Unless you are writing a thermodynamics text book
it is probably okay to say there are more than two forms of energy and to use
the terms heat and work as if they are a type of energy. So when you find
me doing it in this web site, don't write me a nit-picky e-mail telling me heat
is a process, not a form of energy. I know. I know.
Thermal Energy (side note
Bopping around in various text books and resources is the fairly common term "thermal
energy". This seems generally to be used as another term for
internal energy or heat content. One of my thermo books does outright propose
this term as a way to avoid using heat as a type of energy. According to this
book, it was tried in the past but never quite caught on. I still like the idea,
and so have started using the term interchangeably in new pages on this site.
So in this web site when you see the term thermal energy it means the type of
internal energy that is usually called heat as described above. It the short run
it may add to your confusion, but in the long run you'll be the wiser for it.
Do you buy that?
||Written by David Watson
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