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The
Conservation of Energy and the First Law of Thermodynamics
Our changeable
friend energy, with its multiple "personalities",
is sometimes hard to keep track of and even harder to define. But it does always
behave in predictable ways. This means that if you know the rules you, like engineers
and scientists, can often predict what will happen and what won't happen.
Smart scientists and engineers, through clever experiments and even cleverer thinking,
figured them out in days gone by. It took some open-minded creative thinking.
It was not obvious to anyone back then. And much of what was obvious back
then about energy was wrong.
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This section is on the First Law of Thermodynamics, or the Conservation of Energy
as it is often called. To start, let's remind ourselves of a few things.
A Few things to remember:
Energy can be stored.
Energy can move from one bunch or piece of matter to another bunch or piece of
matter.
Energy can be transformed from one
type of energy to another type of energy. |
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A
wind turbine converts some, not
all, of the kinetic energy in the
wind into mechanical and electrical energy. The First Law of Thermodynamics says
the sum of the energy put into the wind turbine plus the remaining energy in the
air after it passes through the turbine, must exactly equal the energy in the
wind before it entered the turbine. |
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The First Law:
During all this moving and transforming the total amount of energy never changes.
That's it! I've done it!
That's the first law of thermodynamics! Ta da!
Energy changes form and moves from place to place
but the total amount doesn't change.
Simple, eh?
 Other
ways to say it:
It's conservation of energy. Energy is conserved during any and every process
or transformation or "happening".
Energy doesn't pop into existence from nowhere.
Energy doesn't pop out of existence into nowhere.
Energy is neither created nor destroyed (the old fuddy duddy way of saying it).
"Energy in" equals "energy stored" plus "energy out"
(if no energy is stored, then "Energy in" equals "Energy out").
You may have already heard that the 1st Law says energy is "always conserved".
So why are we always saying we need to conserve more energy if it is "always
conserved" already? Darn old english language, same word but different
meanings. When we say energy is always conserved we mean the total amount of energy
in any process or reaction never changes. When we tell ourselves to conserve energy
we mean to use less of it by doing things like driving more fuel efficient cars,
or insulating our houses better.
 But
hold on, if energy never goes away, if the total amount always stays the same,
why do we have to worry about not wasting it? Can't we just keep reusing it? We
don't really "use" energy, we convert it, and it never "get's used
up".
It's not a stupid question. It's a very good question. Unfortunately, the First
Law of Thermo doesn't really answer it. The popular first law just says the total
amount of energy doesn't change.
Scientists and Engineers had to discover a second law
of thermodynamics to answer questions like those above. This section is only on
the first law but you can jump to the Second
Law page now for a very good explanation of the gist of that law, or continue
on to the fascinating next page for some good examples to further clarify the
First Law. I hope you will read all the pages, for the sake of my fragile ego.
 On
page two of this section we'll give some examples of the results of the conservation
of energy. Click on the Next Page link below.
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<<
Previous Page The First Law
of Thermodynamics Next
Page >>
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The
Second Law of Thermodynamics |
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©Copyright
2005, 2007, David Watson. All rights reserved. Everything in the FT Exploring
web site is copyrighted, either by us or by someone else. For information concerning
use of this material, click on the word Copyright
to go to our legal page. |
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Digressions
& Further Explanations Section
(Late breaking news: The
Second Law pages are finally here!!)
Energy is Conserved but Not Its Usefulness
As we said above the
first law of thermodynamics just says the total amount of energy never changes,
even after many changes. It doesn't answer questions like "why can't we re-use
the energy over and over again?" or "where do my son's guitar picks
go?"
Alas, the not so popular second law does answer the questions about re-using energy
(not the one's about missing guitar picks and socks though, those mysteries are
still beyond the realm of science).
The Second Law is not as easy to say as the first law.
It can't be described in one or two sentences. We can't cover it all here, but
we can give a brief answer to the questions above. The second law deals with the
availability or useability of energy. The unhappy truth is that as energy gets
used or transformed in machines or living animals it gets converted into a form
that is less useful than before. Something in the energy does get "used up"
or even "destroyed". That something is what you might call "usefulness"
or as it is often called in thermodynamics, "availability". The energy
becomes less available for us to use.
In engines like those in cars, trucks, locomotives, and airplanes, the energy
in the fuel is highly concentrated, meaning there is a lot of energy in a fairly
small light package. Each gallon of gasoline (petrol for those of you in other
lands) or diesel fuel or jet fuel contains a lot of concentrated easy to use energy.
Once that fuel is burned in the engines we are all so dependant on, it gets converted
into highly concentrated thermal energy (high temperature) and mechanical energy.
The mechanical energy is used to do useful work for us like moving people or freight
from one place to another. But eventually all of the fuel's concentrated useful
energy get's changed into "low grade" thermal energy. Most is converted
immediately into "heat" and
goes out of the engine in the cooling water or the engine's hot exhaust gas. The
useful mechanical energy gets turned into heat through friction of moving parts
or by pushing air molecules out of the way. The molecules that get pushed out
of the way absorb the energy of the car or train as increased kinetic energy of
the air molecules.
All of the fuel's formerly concentrated "high grade" energy, becomes
what we engineer's call "low grade" energy or "waste" heat.
We call it that because it is no longer useful to us. It's there, but it's sort
of messy and all spread out now, to "gather it up" would take more energy
than it's worth. To reuse that low grade energy is simply impossible or would
be too expensive and impractical to contemplate. So there it goes, all absorbed
into the air and the objects around us as low temperature low grade energy which
will finally radiate into outer space to help warm up the universe a tiny little
bit. We have to replenish it by burning more fuel. If the fuel is what we call
fossil fuel it will eventually all be gone, along with all of its cheap easy to
use energy.
Since we haven't yet figured out how to use other forms of energy or alternative
fuels as well yet, and those alternatives are likely to be more expensive, most
of us think it should not be wasted. That's why we talk about the need to conserve
energy. The first law tells us that energy is forever and the total amount stays
the same. The second law tells us that "high grade" useful energy always
gets turned into "low grade" less useful energy that is difficult (really
really expensive) or impossible to re-use.
The same is true of the energy conversions in living organisms. The food you eat
eventually gets converted into heat (thermal energy) and mechanical work. The
heat is useful for a little while to help keep your body temperature constant.
The mechanical work is used to make more cells, to breathe, think, run, grow,
etc. But in the end, just like with our human made machines, all of the concentrated
high grade food energy gets converted into low grade energy we can't reuse. This
also ends up getting radiated into space. And, as you know if you've read the
photosynthesis pages,
we would run out of useful energy for life very quickly if it didn't get replenished
every day by a fresh dose of concentrated energy from the sun.
Back to Where You Were
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