Key Questions for Chapter 18
2nd Law of Thermodynamics
Chapter 18: The law of increasing
disorder
1. Can nature’s processes run backwards?
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 ever be increased?
5. Is there an energy crisis?
PS 110A Hatch Ch. 18 - 3
C.P. Snow
PS 110A Hatch Ch. 18 - 4
Irreversible processes
Once or twice I have been provoked and have
asked the company [around me] 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
Qualitatively first.
If you run the process backward and it
doesn’t look like a normal process, then it is
irreversible.
Eraser sliding along table and stopping. KE
goes into heat. The reverse process is, heat
collects from table and eraser, which causes
the eraser to slide backward.
This doesn’t look right.
PS 110A Hatch Ch. 18 - 5
The arrow of time ~ irreversibility
irreversible
PS 110A Hatch Ch. 18 - 6
Almost all spontaneous events in
nature are irreversible
An irreversible process is one that occurs spontaneously in
one way, but not in the exactly reversed way.
Examples: an ice cube melting on the counter, water
flowing over Niagara falls, balloon popping, beans
stirred into rice, food coloring spreading, eraser sliding
on table, getting older.
Which direction is the
bullet traveling
Video: egg drop
Is the animation of the diver
correct with physical laws?
PS 110A Hatch Ch. 18 - 7
PS 110A Hatch Ch. 18 - 8
Reversible processes can occur with equal ease
in the forward and backward direction
If a glass of ice water is held at
exactly zero degrees Celsius
and placed in an environment of
the same temperature, the total
amount of ice and water will
remain constant. However, the
shape of the ice cubes will
change as water molecules
become ice, and ice molecules
become water.
If the water or the environment is
at a different temperature the
process becomes irreversible
Reversible processes occur under
special circumstances, but not
usually in nature.
? perfectly elastic collision
Changes occurring in natural systems always
proceed in such a way that the total amount
of “disorder” in the universe either is
unchanged or increased. If total “disorder”
is increased, the process is irreversiblereversible when entropy doesn’t change
Entropy is the technical name for disorder,
and it does not always have the same
connotations that we associate with disorder,
so we will study it by example.
PS 110A Hatch Ch. 18 - 9
PS 110A Hatch Ch. 18 - 10
Getting a feel for entropy by
example
A $100 word
Entropy
The second law of
Thermodynamics
(disorder)
Does order spontaneously increase (entropy
Allowing gas
molecules to mix
decrease)?
–food coloring in water coalesce to a point
–stirred beans separate out from rice
–water freezes into ice cubes, rest of water gets warmer
–fragrance becomes concentrated
–a firm apple forms from rotten fruit
–your room becomes clean
Example of Dye in water
PS 110A Hatch Ch. 18 - 11
P1: In a natural system,
which came first, I or II?
PS 110A Hatch Ch. 18 - 12
Ice in warm water
I.
Temperature difference destroyed
Ice molecules mixed throughout the water
Nature will not, by itself, recreate the blue cube separate from the
frozen water with the temperature difference.
To recover the original “system” we must grab each original ice
molecule, return it to the ice cube tray, and refreeze it.
Demo: puff balls
II.
vacuum
PS 110A Hatch Ch. 18 - 13
PS 110A Hatch Ch. 18 - 14
Molecules arranged in crystals have more
“order”
order” ------ Ice crystals
= Oxygen
= Hydrogen
P2: When water freezes into ice
a) The entropy of the
universe goes
down.
b) The entropy of the
water/ice system
goes down
c) The entropy of the
water/ice system
goes up
d) None of the above
PS 110A
Ch. 18 - 15
Pictures
fromHatch
snowcrystals.com
2nd Law is a statement of
probabilty
P3: Likelihood of two molecules being
completely disordered vrs. being on the
left (ordered).
1
1
2
1
2
2
2
PS 110A Hatch Ch. 18 - 16
Why does disorder increase?
Example
Probability to find molecules disordered vrs. in
left hand side of box (the ordered case)
4 food coloring molecules 1/16 for ordered and
6/16 for evenly disordered – 6/1 ratio
(transparencies of possibilities)
6
10
20
50
1
20/1
252/1
184,756/1
1.2 million billion/1
WOW!
PS 110A Hatch Ch. 18 - 17
Why does disorder increase?
There are many more ways for things to be in
disorder!
For natural random changes the disordered is
more likely, thus disorder tends to
increase.
2nd Law of Thermodynamics
PS 110A Hatch Ch. 18 - 19
PS 110A Hatch Ch. 18 - 18
Boltzmann:
Think of it this way -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?
PS 110A Hatch Ch. 18 - 20
Application:
The Pollution Problem
Heat always naturally flows from Hot to Cold
Before
- Faster moving molecules
Most environmental problems
are 2nd Law Problems:
1. litter
2. air pollution
3. water pollution
4. thermal pollution
5. toxic waste
6. recycling
After
PS 110A Hatch Ch. 18 - 21
Pollution and
Thermodynamics
PS 110A Hatch Ch. 18 - 22
Reality
You will spend most of your life
trying to create order, that is,
fighting the 2nd law of
thermodynamics.
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
PS 110A Hatch Ch. 18 - 23
Can we ever reduce
disorder?
Getting a feel for entropy by
example
Examples:
•
•
•
•
PS 110A Hatch Ch. 18 - 24
Have the food coloring coalesce into a single drop
Separate the beans from the rice
Freeze ice cubes
Concentrate fragrance
A freezer performs the ordering process in making
ice cubes (removes heat from water)
Extra energy (electricity) is needed to perform this
ordering.
Room Temp
We can create local order but a price must be
paid. Disorder must increase somewhere
else!! (2nd law still holds)
Electricity
Cool Spot
PS 110A Hatch Ch. 18 - 25
PS 110A Hatch Ch. 18 - 26
Take home lessons from the
fridge
The second law of thermodynamics says you
have to plug in the refrigerator for it to
work.
The second law does not say that you can’t
locally create order. It says that when you
do create order, you have to create a
corresponding (larger) amount of disorder.
Increasing order
P4: Can you cool a room by leaving the
refrigerator door open? Why?
PS 110A Hatch Ch. 18 - 27
PS 110A Hatch Ch. 18 - 28
Classification of the “order”
of a form of energy
Order and Energy
Macroscopic Kinetic /
Gravitational
Increasing Disorder
Order in a system is related to the accessibility
of its energy.
(accessibility - how easy is it to use the energy to do work?)
The more ordered the system,
the more accessible is its energy.
NOTE: Energy is conserved - but it may be
easier or harder to make use of it.
Nuclear Potential
Electrical Potential
(household)
Chemical Potential
Ambient thermal
Example: Ball on track
(temperature or heat)
PS 110A Hatch Ch. 18 - 29
Transforming Energy
PS 110A Hatch Ch. 18 - 30
Limits to efficiency
Energy (work) can be
completely converted
into heat
The Industrial Revolution
was driven by the
invention of the steam
engine.
Most all kinetic energy
ends up as heat
eventually.
They changed chemical
potential energy to kinetic
energy via heat.
But how efficient could
they be?
But heat cannot be
completely converted
into kinetic energy.
Some heat is always left
over.
PS 110A Hatch Ch. 18 - 31
PS 110A Hatch Ch. 18 - 32
Car Engine
Trace the Order
Heat cylinder walls,
exhaust heated gas
fuel
Burn and
release energy
PS 110A Hatch Ch. 18 - 33
Automobile
PS 110A Hatch Ch. 18 - 34
Why is that?
Chemical potential energy in fuel
Macroscopic K.E. and
Hot gases from combustion
Efficiency = % of input energy which is
converted to useful, desired energy
Auto ~25% efficient
For every $1 you spend on gas, you get 25 cents of
motion
No heat engine is 100% efficient
When we want to get work out of a machine,
energy must pass from one material to the
next and from one form of energy to the
next.
Every time energy changes hands, some of
the used energy is lost to random energy
of motion of molecules.
PS 110A Hatch Ch. 18 - 35
PS 110A Hatch Ch. 18 - 36
2nd Law of thermodynamics
Disorder! Entropy!
(another way of stating it)
Increasing disorder (entropy)
An increase in random motion.
A greater physical mixing of types.
A reduction of temperature differences.
Conversion of energy to a lower order
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.
No engine can be 100% efficient.
PS 110A Hatch Ch. 18 - 37
PS 110A Hatch Ch. 18 - 38
The second law of thermodynamics
and energy policy.
Trace the Order
U.S. daily energy use/capita = 109 Joules or:
A pole vault.
A car accelerates to 60 mph then coasts to a stop.
A refrigerator freezes an ice cube.
The sun warms the earth causing living things to
grow.
90 lbs. Coal
125 lbs. Wood
8 gal Gasoline
10 Therms Natural Gas
100 kWh
4m x 4m sq of solar radiation in a day
106 BTU
PS 110A Hatch Ch. 18 - 39
2004
15 TW (1.5x1013 W) ~ (4x US and 7x China)
Dude, why not run the refrigerator
using the microscopic energy in the air?
Air energy is already quite disordered. Can we
make it “flow” spontaneously to create a more
disordered system?
How about heat flow from a hotter air pocket to a
nearby colder… or hotter water to nearby
colder?
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as
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%
PS 110A Hatch Ch. 18 - 40
Lots of energy in air
molecules. So ...
World Total Energy Data
50
45
40
35
30
25
20
15
10
5
0
1/6 Barrel of Crude Oil
Source: BP & IEA
Demo: heat flow motor
PS 110A Hatch Ch. 18 - 41
Kinetic Energy of Water
Macro Kinetic Energy of Water
- waterfall
concerted (coordinated) motion
- use it to drive an electric generator
Rest goes to micro Kinetic Energy of Water
- stagnant pond
random motion
Can you run a generator with pond water?
PS 110A Hatch Ch. 18 - 43
PS 110A Hatch Ch. 18 - 42
Gravitational.Potential.Energy.
Highly ordered G.P.E. (at Hoover Dam) is
converted to electrical current.
Toaster uses current to burn toast.
Eventually, all energy becomes thermal
(micro K.E.) energy which is not useful.
Energy of sun produces rain in mountains to
regenerate G.P.E.
What happens when the sun dies?
PS 110A Hatch Ch. 18 - 44
The Energy
Crisis
Experiment
A drop of ink is released into a
large flask of water.
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
a. Describe what happens to the system.
(Demo: ink in water)
P5: What is the fundamental principle that explains the
observations?
PS 110A Hatch Ch. 18 - 45
Quizlet
PS 110A Hatch Ch. 18 - 46
Another
P6: In a reversible process, the total amount of
disorder
a. is zero
b. becomes infinite
c. stays the same
d. decreases
e. increases
P7: Of the quantities listed below, the one
which is associated with the most disorder
is:
a. sunlight
b. ambient temperature thermal energy
c. macroscopic kinetic energy
d. internal energy of stars
e. chemical potential energy
PS 110A Hatch Ch. 18 - 47
PS 110A Hatch Ch. 18 - 48
The Gospel Gives a More
Optimistic View
Equilibrium
When left to themselves, all systems change to the
arrangement with maximum disorder. No further
macroscopic changes occur once the system is in
equilibrium.
As far as we can tell, the universe is a closed system
…
Video: increasing
disorder
PS 110A Hatch Ch. 18 - 49
“If you picked up a watch far from
human habitation and found it
running, you would ask not only ‘Who
made this watch?’
watch?’ but ‘Who wound it
up?’
up?’ So it is with the universe. The
universe is running down…
down… In a very
real sense, then, the universe is like a
clock that has been wound up. If it is
selfself-winding, it is unique in scientific
experience.”
experience.”
-Henry Eyring
Reflections of a Scientist, pp. 75-76
PS 110A Hatch Ch. 18 - 50