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Physics 6B
Heat, Work and the Ideal
Gas Law
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Housekeeping
•
If you gave me a DRC form, please confirm via e-mail what you’ll need
for midterm.
•
Homework 2 solutions posted
•
Practice Exam (from Spring 2009) posted
•
Review session for midterm: Sunday 2/6, 7-9pm, Classroom Unit 2
•
Supplemental Problems
!
extra-credit to make up lost homework points.
.
!
find them at the bottom of the assignments list on MP
3
Thermodynamics: using heat to do work
•
Doing work on an object requires a source of energy…
!
the energy source can be heat
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Heat
"
Pressure
"
Force
"
Work
•
Work is force acting over a distance
!
W = F
•
d
•
Heat doesn’t create a force directly
!
just increases thermal motion
of atoms and molecules
•
But heat can be used to
increase pressure
•
Pressure acting over
a distance does work

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Ideal Gas Law -- Molar and Particle forms
•
“Particle” form:
PV = N k T
!
N
is number of particles
!
k
= 1.38
!
10
-23
J/K is
Boltzmann’s constant
!
P,V in SI units
•
“Molar” form 1:
PV = n R T
!
n
is number of moles
!
R
= 8.314 J/(K·mol) is the “universal gas constant”
!
P,V
in SI units
•
“Molar” form 2:
PV = n R T
!
n
is number of moles
!
R
= 0.08206 (L·atm)/(K·mol) is the “universal gas constant”
!
P
in atmospheres
!
V
in Liters
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Temperature, particle speed, particle energy
•
Simple ideal gas model gave us…
•
This relationship between particle speed and temperatures holds for
complicated molecules as well as monatomic gases.
•
We will see (soon) that relationship between T and internal energy is
!
1/2 kT per “degree of freedom”
!
3 degrees of freedom are associated with translational motion
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Degrees of freedom
•
The more independen
t ways a molecule can move
!
the more
degrees of freedom
it has.
•
Single atom (“monatomic”) particles (e.g., helium, neon, argon…)
!
3 degrees of freedom, corresponding to x, y, z motion
•
Two atom (“diatomic”) particles (e.g. H
2
, N
2
, O
2
…)
!
add 2 rotational degrees of freedom for a total of 5
8
More degrees of freedom
•
More complicated particles (e.g., H
2
O, CO
2
, NH
3
…) have an additional
rotational degree of freedom for a total of 6

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