Packet 3.4
Thermodynamics
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Internal Energy
W = PΔV
1st Law of Thermodynamics: ΔU = Q – W
Define: Adiabatic, isothermal, isobaric &
isochoric and show these on a P-V diagram
Irreversibility & disorder
Entropy is a measure of disorder
State 2nd Law of Thermodynamics
Heat engine efficiency, η = W/Qh
Carnot Engine
Energy Degradation
Physics 213: Lecture 3, Pg 1
Internal Energy
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Is the total potential and kinetic energy of the molecules in
a substance.
Potential energy is associated with intermolecular
forces.
Kinetic energy includes both translational and rotational
motion.
When we consider an ideal gas, the intermolecular forces
are assumed to be zero!
Internal energy of a gas comes only from the random
kinetic energy of the atom of the gas.
Physics 213: Lecture 3, Pg 2
Ideal Gas & Internal Energy
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Ek = 1/2mv2 = 3/2kT
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U =3/2nRT
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Δ U = 3/2 nR Δ T
so,
U = 3/2 NkT
Physics 213: Lecture 3, Pg 3
Ideal Gas & Internal Energy
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What is the internal energy of 30 moles of
oxygen gas at room temperature?
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U =3/2nRT
Physics 213: Lecture 3, Pg 4
Ideal Gas & Internal Energy
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If the room were moving at a high velocity would
that mean the internal energy of the gas would
be greater?
Physics 213: Lecture 3, Pg 5
Work done on or by a gas
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Imagine compressing a gas by exerting a force on the
piston from the outside.
Consider heating the piston and it expands to perform
work.
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W = F x D & F = PA
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W = P (A x D)
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W=PΔV
Physics 213: Lecture 3, Pg 6
Example
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A gas is compressed at constant pressure
2.00 x 105 Pa from a volume of 2.00 m3 to
a volume of 0.500 m3.
What is the work done on the gas.
If the temperature initially was 40˚ C what is
the final temperature of the gas?
Physics 213: Lecture 3, Pg 7
Thermodynamic Processes
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CLICK HERE – TO GO OVER EACH PROCESS!!
Isochoric
Const. Volume
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Isobaric
Const. Pressure
Isothermal
Const. Temp
Remember! Area under curve is Work Done!
Physics 213: Lecture 3, Pg 8
Adiabatic Expansion
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Rapid expansion or
compression of a
gas.
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No Heat (Q) can flow
in or out of the
system. ΔQ = 0
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Any work done
equals a direct
change in internal
energy. ΔU = ΔW
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Bottle Rockets
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Diesel Engines
Physics 213: Lecture 3, Pg 9
Figure 18-11
Adiabatic Heating
Physics 213: Lecture 3, Pg 10
The First Law of Thermodynamics (FLT)
-- Heat and work are forms of energy transfer
and energy is conserved.
U = Q - Wby
change in
total internal energy
State Function
heat added
to system
work done
by the system
Process Functions
or
U = Q + Won
Physics 213: Lecture 3, Pg 11
"Process Problems"
For which process is W the largest? smallest?
For which process is Q the largest? smallest?
Physics 213: Lecture 3, Pg 12
Physics Joke
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Once all the scientists die and go to heaven.
They decide to play hide-n-seek and Einstein
goes first. Einstein counts up to 100 and then
start searching. Everyone starts hiding except
Newton. Newton just draws a square of 1 meter
and stands in it, right in front of Einstein.
Einsteins counting ....97,98,99,100, opens his
eyes and finds Newton standing in front. Einstein
says "Newtons out, Newtons out." Newton denies
and says I am not out. He claims that he is not
Newton. All the scientists come out and he
proves that he is not Newton. How??
Physics 213: Lecture 3, Pg 13
His proof:
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Newton says: I am standing in a square of
area 1m square.. That means I am
Newton per meter square.. Hence I am
Pascal. Since newton per meter square =
Pascal
Physics 213: Lecture 3, Pg 14
Conceptual Checkpoint 18-2
Which is the adiabatic curve?
Physics 213: Lecture 3, Pg 15
The second law of thermodynamics
When objects of different temperatures are
brought into thermal contact, the spontaneous
flow of heat that results is always from the
high temperature object to the low
temperature object.
Physics 213: Lecture 3, Pg 16
Heat Engines
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Energy goes in
Useful Work taken out
Some gets wasted
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Max Efficiency:
TH – Tc/ Th= x 100
Physics 213: Lecture 3, Pg 17
The 2nd Law of Thermodynamics
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The second law of thermodynamics deals
with the limitations imposed on heat
engines: that is on devices whose aim is
to covert heat (disordered energy) into
mechanical energy (ordered energy).
Physics 213: Lecture 3, Pg 18
The 2nd Law of Thermodynamics
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The Entropy of an isolated system never decreases.
It is impossible for heat to (spontaneously) flow from a cold
to a hot object.
It is impossible for a heat engine working in a cycle to
absorb heat and perform an equal amount of work.
The most efficient heat engine operating between two
given temperatures is the Carnot Engine.
Physics 213: Lecture 3, Pg 19
Entropy
Entropy like Internal energy is a State
Function!
 Entropy Is a measure of the disorder of a
system.
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ΔS = ΔQ/T
If ΔQ > 0 entropy increases
If ΔQ < 0 entropy decreases.
Physics 213: Lecture 3, Pg 20
Philosophy
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Three Laws of Thermodynamics
(paraphrased): First Law: You can't get
anything without working for it. Second
Law: The most you can accomplish by
work is to break even. Third Law: You
can't break even.
Physics 213: Lecture 3, Pg 21