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The Big Ideas—Chapter 21
(Serway and Beichner, Physics for Scientists and Engineers, 5
th
Edition)
AJM:2/17/01
1
Section 1
The ideal gas law is the natural result of a very simple model of
a gas (in which molecules behave like particles that undergo
elastic contact collisions with walls and other molecules) and an
interpretation of temperature as a measure of the average
energy per molecule.
In this model the pressure arises from the average force of the
collisions between the molecules and the walls and is directly
associated with the density and the average translational kinetic
energy of the molecules.
The ideal gas law then reveals
temperature as a pure function of the average translational kinetic
energy.
With
far
more generality than is implied by the derivation of
this simple model, the “equipartition theorem” says that, for a
system in thermal equilibrium,
every possible way
in which a
molecule can store energy (each socalled “degree of freedom”)
will on average store an energy determined solely by the
temperature.
P
simple model
=
2
3
N
V
1
2
mv
2
( 29
T
ideal gas
=
2
3
k
B
1
2
mv
2
( 29
1
2
mv
2
=
3
2
k
B
T
Energy per
degreee of
freedom
=
1
2
k
B
T
Section 2
The equipartition theorem allows us easily to determine the
internal energy of
an ideal gas since the value depends only on
how many molecules there are and on how many ways each
molecules can translate, rotate and/or vibrate.
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This note was uploaded on 03/16/2008 for the course PHYSICS 132 taught by Professor Idontknow during the Spring '08 term at Cal Poly Pomona.
 Spring '08
 IdontKnow
 Physics

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