Second Law of Thermodynamics
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Changes occurring in natural systems
always proceed in such a way that the total
amount of entropy in the universe is either
unchanged or increased. If total disorder is
increased, the process is irreversible.
Chapter 12The Law of Increasing Disorder
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Student Versions:
Nature Makes Messes
If You Think Things Are Bad
Now, Just Wait a While
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Reading quiz
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What’s Entropy?
State the “law of increasing disorder”
¾
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The moral of the story
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– It takes work (energy) to straighten up
messes.
– If you increase order in one place, disorder
increases someplace else to more than make
up for it.
Another
a.
b.
c.
d.
e.
Of the quantities listed below, the one which is
associated with the most disorder is:
Sunlight
ambient thermal energy
macroscopic kinetic energy
internal energy of stars
chemical potential energy
¾
¾
¾
Total disorder of the universe
increases
or remains the same.
It only remains the same in perfectly
reversible processes.
C.P. Snow
Once or twice I have been provoked and have
asked the company how many of them could
describe the Second Law of thermodynamics
[the law of increasing disorder to us]. The
response was cold; it was also negative. Yet I
was asking something which is the equivalent
of ‘Have you read a work of Shakespeare’s?’
C.P. Snow
Two Cultures
Main Ideas
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Key Questions for Chapter 12
The Law of Increasing Disorder (Entropy)
or the Second Law of Thermodynamics
Reversible/Irreversible Processes
Order and Disorder
1. Can nature run backward in time?
2. What happens to the order of a system over
time?
3. What is the relationship between order and the
accessibility of energy?
4. Can order be increased?
5. Is there an energy crisis?
6. Is it easy to eliminate pollution?
Left alone „
Things tend to fall apart.
z
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Everything is made up of molecules
molecules tend to get mixed up.
The more Mixed, the later the time. “. .
. the arrow of time . . .”
Nature does not run backward
Nothing we’ve seen so far says why.
a. Reversible process - one which works in reverse, one
whose effect you can undo without using energy.
b. Irreversible process - one you cannot undo or run in
reverse unless you add energy to the system.
„ Almost all spontaneous events in nature are
irreversible
irreversible
scrambling eggs
smashing china
real falling balls
reversible (almost)
ice <-> liquid at 0oC
a pendulum swinging
balls falling in a vacuum
before they hit the ground
A $100 word
„Entropy (disorder)
?
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Can order ever increase
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Yes - locally.
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But for the universe as a whole, disorder
increases.
(Entropy decrease)
Boltzmann:
Think of it this way -„
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Any arrangement of particles is equally
probable.
Only a few are what we would call “ordered”.
Most are what we would call “disordered”.
What are the chances that an
explosion in a printing press
would spontaneously generate
the Book of Mormon?
Types of order
„
Does the 2nd law mean it is impossible to
print the Book of Mormon?
How does the 2nd law apply?
You have to “use”
energy to organize the
type into words.
Order and Energy
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Energy hierarchy
Order in a system is related to the accessibility
of its energy.
(accessibility - how easy is it to use the energy to do work?)
Structure
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Type of Energy
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The more ordered the system,
the more accessible is its energy.
energy.
Shape
Function
Arrangement
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Mixed versus sorted
NOTE: Energy is conserved - but may be
easier or harder to use to get somewhere.
Energy hierarchy
Why are KE and GPE more ordered?
Form of Energy
Gravitational Potential /
Macroscopic Kinetic
Nuclear Potential
Electrical (Household)
Chemical Potential
Thermal Energy (heat)
(Microscopic KE)
ordered
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All of the atoms have the same amount of
energy
Why is thermal energy the least
ordered?
disordered
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The atoms are moving in random directions
at random speeds.
Structure
Arrangement
What has more order, a house before or after a
hurricane knocks it over?
What has more order, ‘this set of letters’ or
‘Jsadhjf dl;dfj teoiznt ajsdjfp twe kjdlka’?
„ What has more order, the ink and water
before or after they get mixed?
„ What has more order, a jar of hot water and
a jar of cold water, or 2 jars of warm
water?
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What has more order, the contents of a leftover
container before or after it’s sat in the fridge for a
few months?
What has more order, your computer keyboard
before or after your roommate spills pop on it?
Why is A less ordered?
Which came first?
A.
B.
A
C.
A
Because the ice molecules are
mixed with the water
molecules.
Because the hot molecules are
mixed with the cold molecules
Because the electrical energy
in the ice got turned into
thermal energy
B
B
Completely reversible processes
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Contain frictionless surfaces.
Perfectly insulated systems.
Happen infinitely slowly.
Have odd starting and ending points.
No real process that uses energy is
perfectly reversible.
SO….
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Can you use the 2nd law to prove that it is
impossible to clean your room?
A.
B.
C.
Yes, even though it looks neater, it is really
more disordered after you pick it up
No, it is possible to clean your room, but
doing so is bad for the universe as a whole.
No, the second law doesn’t apply when there
are people involved.
What about life?
„
Life is highly ordered. Does the 2nd law
mean that life couldn’t happen “naturally”?
Increasing order
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You can increase order in part of a system.
When you do, something or some part of the
universe becomes even more disorganized so
that the total disorder is increased.
Examples:
Refrigerator
Car Engine
Example: Refrigerator
Which is more ordered – warm outside and cold
inside or all the same temp.?
Compressor and condenser cool down the air
inside the fridge.
z
z
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an ordering process
use highly organized electrical energy which is
turned into thermal energy that heats up the room
z
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a disordering process
Which is greater?
Example: Automobile
Chemical potential energy in fuel
Thermal Energy
Macroscopic K.E.
+
Thermal energy (Hot gases from combustion)
combustion)
„ Efficiency = % of input energy which is
converted to useful, desired energy.
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Auto ~25% efficient.
For every $1 you spend on gas, get 25 cents of
motion.
No heat engine is 100% efficient. WHY?
Law of increasing disorder
Why is that?
(different form)
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What is highest in the energy hierarchy?
Can you convert 100% of CPE into KE?
Can you convert 100% of CPE into heat?
How does the car get KE out of
thermal energy?
Some of the energy is “wasted” heating up the environment.
Any form of energy can be
totally converted to any other
form which has the same or
more disorder, but it cannot be
totally converted to another form
in which it would have less
disorder.
Lots of energy in air
molecules. So ...
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Dude, why not run the refrigerator
using the energy in the air? Remember the
little dudes are bouncing around at 700mph.
Random thermal energy is not ordered. Can’t
make it “flow” to create a more disordered
system.
What if there is a temperature difference? How
about hotter air to colder… or hotter water to
colder?
Universal Heat Death
“This is the way the world ends, not with a
bang but a whimper.”
T.S. Eliot
The Energy Crisis
The Pollution Problem
Several ways to reduce pollution:
1. Reduce industrial production
2. Eliminate cars
3. Increase efficiency of machines to
their theoretical limit.
4. Collect pollutants after they are
produced
Quizlet
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In a reversible process, the total amount of
disorder
a. is zero
b. becomes infinite
c. stays the same
d. decreases
e. increases
1. Is there a “total” energy crisis?
NO! - total mass-energy is always conserved
2. Is there a “useful” energy crisis?
YES! - burning fossil fuels loses order
irreversibly
- resulting thermal energy is not
very accessible
Equilibrium
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When left to themselves, all systems
change to the arrangement with maximum
disorder. This state of maximum disorder
is called “Equilibrium”.
No further spontaneous macroscopic
changes occur once the system is in
equilibrium.