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Thermodynamics and Statistical
Mechanics
Spring 2006
Thermodynamics
Thermodynamics is an old science, formulated
empirically without reference to atoms or
molecules. It deals with energy, as does
mechanics, but adds some new quantities,
besides work, potential energy and kinetic
energy.
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Thermodynamic quantities
Internal energy (U): the energy of atoms or
molecules that does not give macroscopic
motion.
Temperature (T): a measure of the internal
energy of a system.
Heat (Q): a way to change internal energy,
besides work. (Energy in transit.)
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Thermodynamic quantities
Entropy (S): a measure of the disorder of the
system. The change of entropy is equal to the
heat added or removed divided by the
temperature.
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Laws of Thermodynamics
First law:
đQ – đW = dU
Energy is conserved
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Laws of Thermodynamics
Second Law: The entropy of an isolated
system increases in any irreversible process
and is unaltered in any reversible process.
This is the principle of increasing entropy.
DS  0
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Laws of Thermodynamics
Third Law: The entropy of a true equilibrium
state of a system at a temperature of absolute
zero is zero.
Equivalent to: It is impossible to reduce the
temperature of a system to absolute zero
using a finite number of processes.
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History
Much of early thermodynamics development
was driven by practical considerations. For
example, building heat engines and
refrigerators. The original statements of the
second law were different.
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Second Law Variations
No series of processes is possible whose sole
result is the absorption of heat from a thermal
reservoir and the complete conversion of this
energy to work.
There are no perfect engines!
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Second Law Variations
No series of processes is possible whose sole
result is the transfer of heat from a reservoir at
a given temperature to a reservoir at a higher
temperature.
There are no perfect refrigerators!
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Third Law
It is impossible to reach a temperature of
absolute zero.
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Popular versions
1st Law: You can’t win.
2nd Law: You can’t break even.
3rd Law: There’s no point in trying.
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Statistical Mechanics
In the late 19th and early 20th century, with the
discovery of atoms, thermodynamics was
treated in terms of atoms and molecules, on a
statistical basis, and the subject of statistical
mechanics came about.
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Zeroth Law
If two systems are separately in thermal
equilibrium with a third system, they are
in thermal equilibrium with each other.
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Zeroth Law
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Constant Volume Gas
Thermometer
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Constant Volume Gas
Thermometer
It is calibrated at the triple point of water, which is
273.16 K on the Kelvin scale.
T (K)
P

273.16 PTP
Even better:
P
T (K)  273.16 lim
P 0 P
TP
TP
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Boiling Point of Water
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