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The
Second Law of Thermodynamics
Or Energy
is Forever, but Not Exactly
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Keeping
It Simple (and Clear)
Teachers and Learners:
The Second Law of Thermodynamics is the most misunderstood, abused, and needlessly
confused principle of physics.
Because of the confusion and misinformation regarding this principle, I've dragged
my feet shamelessly when it came to dealing with it in this website. Finally,
I came to realize that I cannot undo all the confusion with one little web site.
So I'm not going to try. My suggestion is that you start here, and stay here.
Unless you are going to be an engineer or scientist, this is probably all you'll
ever need.
Yes, I do know about statistical thermodynamics and probability. It is ultimately
all about atoms and molecules. I know that. I also know about entropy.
But believe it or not, we don't need to even mention entropy to understand
the basic concept of the 2nd law. If you understand what I am describing in these
pages, then entropy will make a lot more sense to you (unless you are being wrongly
taught it is a measure of disorder).
We don't need the silly (and wrong) examples of messy desks. We don't need math.
We don't need to talk about isolated systems or the direction of time. Isolated
systems can be useful for engineers when solving problems, but they needlessly
confuse us when used to define the 2nd Law of Thermodynamics. Think big picture!
We don't need to discuss probability and measurements of disorder. We never need
disorder. What are those units of disorder anyway?
Disorderites? Messyisms?
Gobbledygooks?
All of those concepts (except disorder), when presented
correcly, and the many definitions presented in text books,
lead to the basic energy change results that I describe
in these pages. It is only about energy.
It is only about energy changes.
It is only about energy changes and the condition of the
energy before and after the change. That's all.
To be sure, there are useful concepts about organizational
disorder and things getting messy (and the propagation of
misinformation about entropy). But those concepts do not
belong in a discussion of energy and thermodynamics. They
will not help you understand the thermodynamic property
of matter called entropy.
The Second Law of Thermodynamics
does NOT say everything tends toward disorder (and/or decay)!
Maybe everything does tend toward disorder, many people
say so.
But the second law doesn't say so.
It is only about energy.
In the words of Forest Gump,
"That's all I'm going to say about that"....for
a while.
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Picture
1: The pressure in the volume on the left is higher than the pressure
on the right. The pressure energy in the left side can be thought of as more "concentrated"
than the pressure energy on the right side. Both sides take up the exact same
amount of space (or volume), but there is more pressure energy in the left side.
More pressure energy in the same space means it is more concentrated.
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Picture
2: The
valve has just been opened. Immediately, air on the higher pressure left side
starts to flow to the right side, because the pressure is lower there. Just like
air escaping from a baloon.
Energy is flowing from more concentrated to less concentrated. |
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Picture
3: The air kept flowing through the opening until the pressure on both
sides was equal. See the pressure guages? They show the same pressure on both
sides. The pressures are now equal - no difference in concentration levels.
There is no more air flow through the opening.
We have reached equilibrium.
The total energy hasn't changed (First Law), but it is more "spread out"
or less concentrated now (2nd Law). |
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Picture
1: Two tanks of water. The water on the
left side is hotter than the water in the tank on the right side. There is only
a thin piece of sheet metal, or maybe some glass, separating the water, so heat
(thermal energy) can easily flow from one side to the other. Thermal Energy is
more concentrated in the hotter water. A cubic inch of water on the left side,
has more thermal energy in it than a cubic inch of water on the right side.
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Picture
2: The thermal energy continued to flow from the left side into the
right side. The temperature on the right steadily increased, while the temperature
on the left side got steadily cooler. Eventually the temperatures on both sides
became the same, as shown by the cartoon thermometers above. Equilibrium
has been reached. The concentration of thermal energy is the same on both sides,
so there is no more energy flow. |
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How
Everything Happens
 They
are the "Laws of Energy". We attach the big word "thermodynamics",
but it is about Energy - only Energy. Energy makes everything
happen. There are two important natural "laws" that describe how energy
always works. We call them "laws" because countless observations and
thousands of experiments have shown them to be always true.
 Ponder
that for a moment - how everything happens. It means we don't understand much,
if we don't understand both laws of Energy.
These next two pages will give you an overview of the famous, but generally misunderstood,
2nd Law.
Beyond the First Law
The First Law of Thermodynamics tells
us energy is conserved. The total amount never changes. But something does change.
I will call it "re-usability", for now. It's not an official text book word, but
pretty good for communicating the basic idea.
Remember that there has to be an energy
transfer for something to happen; energy changes form or moves from place
to place (heat flow, for example).
As energy moves and changes, the total amount of energy stays the same, constant
forever as far as we know.
That sounds good doesn't it?
Energy is forever.
But wait! If it's forever, why are all these do-gooders telling us we need to
conserve energy by using less? Can't we just keep using it over and over? Why
shouldn't everyone drive himself or herself to work alone in a 300 horsepower
car?
The Rest of the Story...
Alas, my friends, there is a rub, and it is described by the Second Law of Thermodynamics.
The first law would be quite happy to let us re-use energy over and over. The
first law is happy as long as energy is conserved. It says, "why not"?
The second law says, "because, that's why". No, just kidding. It's not
a rude law. There is a reason. The second law says that each time energy gets
transferred or transformed, some of it, and eventually all of it, gets less useful.
That's the truth. It gets less useful, until finally, it becomes simply useless.
All of the energy we use ends up, sooner or later, as what we engineers like to
call "low-grade" energy. This low-grade energy is only good for warming the air
around us a little bit. Inevitably, it gets radiated out into the vast cold universe,
lost to us forever.
The 2nd Law is, as Paul Harvey used to say, "the rest of the story".
To understand this, it is helpful to start with another aspect of the Second Law.
Let's call it "the direction energy moves" aspect.
The Direction Energy Always Goes
The second law tells us which way energy flows. It says energy has an absolute
tendency to go from "more concentrated" to "less concentrated". It sort of "spreads
out" and gets "diluted". Those are not very technical terms, but I think it is
the best way for beginners to think about it.
Energy flows from a higher temperature to a lower temperature (heat flow).
Energy flows from a higher pressure to a lower pressure (expansion).
Energy flows from a higher voltage potential to a lower voltage potential (electric
current).
Energy flows from a higher gravitational potential to a lower gravitational potential
(falling objects).
Marbles and trucks roll downhill.
Water flows and falls from higher elevation to a lower elevation (downhill).
And last, but not least, chemical reactions proceed from higher concentrations
of molecular bond energy to lower bond energies.
In each of those cases, we can think of the energy in the higher level as being
more concentrated. Energy inevitably moves to a less and less concentrated condition.
Less concentrated = less useful.
For anything to happen, energy has to move or flow or change. Energy will keep
flowing or changing from a higher concentration to a lower concentration until
the concentrations are equal (not necessarily more disordered). We call that condition,
cleverly enough, equilibrium.
So that's one part of the Second Law of Thermodynamics. Energy will flow to a
more "spread out" or "less concentrated" condition. It stops flowing when there
is no longer a difference in concentration levels - when things have reached the
great state of equilibrium.
Simple enough, eh? Who needs a law for that?
The Unstoppable Tendency of Energy
Unless something is put in its way to stop it, energy always flows that way. It
is an unstoppable tendency.
We can slow it down, or even stop it by blocking the more concentrated energy
from reaching or connecting with the less concentrated energy. But the potential
is still there. If we close the switch, open the valve, start the chemical reaction,
or break the levee, look out.
Potential Energy
That's what potential energy is all about, concentrated energy waiting
for its chance to flow to a less concentrated level. Things happen when energy
is allowed to move from high potential to low potential. Things stop happening,
or don't start, when there is equilibrium.
Examples
In the pressure tanks in the green column on the right, we can keep the valve
closed. As long as the high pressure side is blocked from the low pressure side,
nothing will happen. But the potential is there, waiting for its chance
to move, which in this case means opening the valve.
In the water tanks shown in the pictures to the right above, the thermal energy
in the hotter water will flow into the cooler water until the temperature in both
tanks is the same - equilibrium!
We can slow down the heat flow by adding a thick layer of insulation between the
two tanks. This will slow down the heat flow, but not stop it. Energy flowing
from hotter to cooler is unstoppable. Eventually, both sides will still reach
equilibrium and the temperatures will be equal.
If we keep the light switch open, the electric current can't flow from higher
voltage potential to lower potential. But the tendency is always there as long
as the potential difference is there. Close the switch and electricity
flows from high voltage to low voltage, making the light bulb glow. The potential
energy is converted to thermal energy and light.
Hold a rock over your head. The gravitational potential energy is there. Let go
of the rock and it gets pulled down to the ground (or your head if you're not
careful). The rock's potential energy is converted to kinetic energy as
it rushes to get to the lower energy level of the ground. When it hits the ground
the kinetic energy is converted to sound waves and low-grade thermal energy, heating
up the rock and the ground.
We Can Catch Some Energy as it Flows from High to Low
Concentration
Nature and human engineers have learned to control and use energy flows. We can
catch some of it as it flows by, maybe with a turbine, or piston and crankshaft,
maybe even a sail or wind turbine
blades. We can transform some of it (but not all) into useful work.
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Catching
Some of the Energy:
The picture above shows how we might be able to convert some of the concentrated
pressure energy into mechanical energy. The air has to push against the turbine
blades as it flows from higher pressure to lower pressure (expansion). If the
turbine shaft were attached to a little generator, we could make some electricity.
The air will only flow as long as there is a difference in pressure between the
two spaces. |
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The
Big Picture and Perpetual Motion
In our turbine example to the right, we can "re-concentrate" some of
the energy. We could take a motor and an air pump and push most of the air back
to the left tank. So now energy is concentrated again in the form of pressure
- high pressure in the left tank, low pressure in the right. Open the valve and
let the air expand into the right side and once again spin our little turbine-generator
until the pressures in both tanks are equal.
If you are just thinking about the left tank, it will look like we got all the
concentrated energy back. Does this mean we can re-use energy over and over?
No.
You must look at the big picture.
You must think about all the energy processes involved. Because of air friction
and turbulence, and friction in the turbine bearings, and friction losses in the
generator, the amount of electrical energy produced by our turbine-generator system
will always be less than the concentrated pressure energy that was available in
the left tank.
Also, because of friction and other losses, our electric motor and air pump will
always use more energy than they can put into the compressed air.
At each step of our process of expansion-power generation-recompression, some
energy is lost. It "leaks" away into lower-concentration. It becomes
useless.
So even though we are able to keep re-concentrating energy in one area, in the
big picture we always lose. The electricity made by our little turbine isn't enough
to keep the system going. It will eventually "wind down" and stop, because
all of the energy will eventually become low-grade and useless. The only way to
keep it going is to keep adding concentrated energy from outside the system.
The above describes why we can't have perpetual motion machines. That's
another one of the many ways the 2nd Law is sometimes defined. "You can't
have perpetual motion machines" we say, because some energy in every process
becomes low-grade and useless. You have to keep adding fresh concentrated energy
from outside the machine, or the machine will eventually come to a stop.
On earth our cycle of energy conversion is pretty much
this:
 Earth
is not a perpetual motion machine either. Each day concentrated energy from the
sun shines down onto the earth.
This light energy powers almost all life through the mechanism of photosynthesis.
The solar energy also causes wind to blow, rain to fall, and snow to melt. During
each of these activities, some of the solar energy is converted to low-grade thermal
energy...
Until, finally, all of the once mighty concentrated solar energy becomes too "spread
out" and diluted to use. In the end, it meekly radiates back out into the
cold vastness of space.
From space it comes, to space it returns - in a much less concentrated form.
Without a new blast of high-concentration high-grade solar energy everyday, the
earth would quickly become lifeless and frozen.
 The
process of using energy always converts some of it into less and less useful form.
It must be replenished, as fast as we use it, with new sources of concentrated
energy.
And that,
my fellow humans and friends
living on
and sort of sharing the energy on
our spaceship earth,
is the inescapable truth defined by the 2nd Law of Thermodynamics.
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