2nd Law of
Thermodynamics
• Reversible and
irreversible processes
• Order and disorder
• Engines
• Ordered energy
• Heat death of the
universe
2nd Law of
Thermodynamics
• Reversible and
irreversible processes
• Order and disorder
• Engines
• Ordered energy
• Heat death of the
universe
Why does time
have an arrow?
TIME
R.I.P.
Irreversible
Processes
• Processes that don’t go
backwards on their
own.
– Ice cube melting in a
glass of hot water
– Mixing and un-mixing
Why does time
have an arrow?
TIME
R.I.P.
Irreversible
Processes
• Processes that don’t go
backwards on their
own.
– Ice cube melting in a
glass of hot water
– Mixing and un-mixing
Reversible
Processes
• Processes that do go
backwards on their
own.
• Almost never happen
Examples of irreversible processes
(in-class demonstrations)
Order
• Putting similar
molecules in the same
place
– White and blue balls
– Refining ores
• Separating into energy
classes
– Ice cube in water
– Refrigerator
Reversible
Processes
• Processes that do go
backwards on their
own.
• Almost never happen
Order
• Putting similar
molecules in the same
place
Disorder
•
Changes in nature
happen in two ways:
1.
The total disorder in the
universe is unchanged
(reversible processes)
The total disorder in the
universe goes up
(irreversible processes)
– White and blue balls
– Refining ores
• Separating into energy
classes
– Ice cube in water
– Refrigerator
2.
2nd law of thermodynamics
But….
But I can reverse
some processes
on my own. What
happens to the
disorder then?
Disorder
•
Changes in nature
happen in two ways:
1.
The total disorder in the
universe is unchanged
(reversible processes)
The total disorder in the
universe goes up
(irreversible processes)
2.
Well, disorder
increases
somewhere else
in the universe.
2nd law of thermodynamics
But….
But I can reverse
some processes
on my own. What
happens to the
disorder then?
Well, disorder
increases
somewhere else
in the universe.
Disorder
•
•
“Entropy”
S = k log W
Boltzmann
Gibbs
To the tune of
“Give said the little stream”
Gibbs says there's entropy
Entropy, entropy
Gibbs says there's entropy
All around the universe
It grows, you know
So wherever you go
Things just keep getting worse
Growing, growing, all day long,
Entropy, oh entropy
Even while we sing this song,
Things keep getting worse.
Entropy is also…
• the tendency for
concentrated energy to
disperse
– The sun shines
– Heat dissipates
– Mixing hot and cold
things puts them at a
medium temperature
Disorder
•
•
“Entropy”
S = k log W
Boltzmann
Gibbs
Order List for
Energy
Entropy is also…
• the tendency for
concentrated energy to
disperse
– The sun shines
– Heat dissipates
– Mixing hot and cold
things puts them at a
medium temperature
•
Macroscopic K.E.
Gravitational P.E.
.
A bunch of other stuff
(nuc,elect,chem) p.220
.
• Ambient
Temperature
Thermal Energy
ORDERED
DISORDERED
Order List for
Energy
Energy Conversion
• You can go down the
energy list with 100%
efficiency.
• You can go up the
energy list, but it
requires energy to do it!
Another form of the 2nd
Law of Thermodynamics
•
Macroscopic K.E.
Gravitational P.E.
.
A bunch of other stuff
(nuc,elect,chem) p.220
.
• Ambient
Temperature
Thermal Energy
ORDERED
DISORDERED
Energy Conversion
Engines
• You can go down the
energy list with 100%
efficiency.
• You can go up the
energy list, but it
requires energy to do it!
Energy in (ordered)
Machine
Another form of the 2nd
Law of Thermodynamics
Heat out (disordered)
Work
Energy Conversion
• You can go down the
energy list with 100%
efficiency.
• You can go up the
energy list, but it
requires energy to do it!
Engines
Energy out (ordered)
Machine
Another form of the 2nd
Law of Thermodynamics
Heat out (disordered)
Work
A refrigerator can turn warm water into
ice, which increases the order in the ice.
What kind of process is this?
A. Reversible
B. Irreversible
What happens to the energy when warm
water is turned into ice in a refrigerator?
A. The electrical power
goes into the ice.
B. The water/ice process
is reversible so no
input energy is needed
C. The fridge requires
energy to run, and this
comes from the wall
plug.
What happens to the disorder when a
fridge turns warm water into ice?
1. The ice freezes and the total
energy in the universe
decreases.
2. Disorder goes down in the ice,
but goes up the same amount
in the room so that the total
disorder in the universe is
constant.
3. Disorder goes down in the ice,
but goes up MORE in the
room, so that the total energy
in the universe increases.
Water runs over a waterfall. What kind
of process is this?
A. Reversible
B. Irreversible
What happens to the gravitational potential
energy when water runs over a waterfall?
A. It evaporates.
B. Water running over
a waterfall is
reversible, so
nothing happens to
the energy.
C. It is converted into
kinetic energy.
What happens to the disorder when
water runs over a waterfall?
1.
2.
3.
The water splashes into the lake
below and the total energy in the
universe decreases.
Disorder goes down in the falling
water, but goes up the same amount
in the lake below so that the total
disorder in the universe is constant.
The water splashes into the lake
below the falls, and the GPE is lost
to the random motion of the water
molecules in the lake. The total
disorder in the universe increases.
Perpetual motion
machines
• These machines violate
either
Engines
Energy out (ordered)
– Conservation of Energy
– 2nd Law of
Thermodynamics
Machine
Heat out (disordered)
Work
Perpetual motion
machines
My first million $
• These machines violate
either
• What is wrong with this
picture?
– Conservation of Energy
– 2nd Law of
Thermodynamics
Generator
Battery
Another kind of
disorder
HOT
COLD
My first million $
• What is wrong with this
picture?
Ordered
Generator
Battery
WARM
Disordered
Another kind of
disorder
HOT
COLD
But…
What makes the
universe want to
be disordered?
Ordered
WARM
Disordered
It is a statistical
consequence of a
molecular world,
and it is real.
Heat Death of the
Universe
• Use energy to do work
– Cars, Bicycles, Farming
• Produce useless heat
• Eventually all energy
sources will be
exhausted
• All stars will die out
• The universe will all be
at the same temperature
But…
What makes the
universe want to
be disordered?
It is a statistical
consequence of a
molecular world,
and it is real.
Heat Death of the
Universe
• Use energy to do work
– Cars, Bicycles, Farming
• Produce useless heat
• Eventually all energy
sources will be
exhausted
• All stars will die out
• The universe will all be
at the same temperature
Equilibrium
• When left alone, all
systems change to the
arrangements with
maximum disorder,
called “Thermodynamic
Equilibrium.”
– No further macroscopic
changes
– No temperature changes
– No living creatures