Announcements 9/10/10
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Be sure to register your clicker on the class
website! (About 5 of you are not registered yet.)
a. After everyone is registered, I’ll tell the computer to
go back and re-grade all of the earlier quizzes to give
you credit.
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Results of the TA office hour survey, effective
next week:
a. Mon 3-4 pm
b. Wed 5-6 pm
c. Fri 5-6 pm
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My own office hours are still right after class, 23 pm.
Video
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Video: Barrel Crush
Worked problems:
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How much mass does the air in this room
have? (MM  0.029 kg/mol)
According to the ideal gas law, what is the
density of air at 300K? At arbitrary T?
A hot air balloon is 520 kg (including
passengers). It’s spherical, with radius = 8 m.
The temperature is 300K outside (80.3F).
How hot does the pilot have to get the air
inside the balloon for it to lift off?
Thought quiz (ungraded):
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In air, the molecular mass of oxygen
molecules is 32 g/mol; the molecular
mass of nitrogen molecules is 28 g/mol.
Which molecules are traveling faster on
average?
a. Oxygen
b. Nitrogen
c. Same speed
Demo: heavy vs light molecules
Equipartition Theorem
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“The total kinetic energy of a system is
shared equally among all of its
independent parts, on the average, once
the system has reached thermal
equilibrium.”
“independent”: e.g. x, y, z (for
translational KE)
“parts”: translational, rotational,
vibrational
Specifically, each “degree of freedom”, of
each molecule, has “thermal energy” of …
½kBT
Thought quiz
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Compare a monatomic molecule such as
Ne to a diatomic molecule such as O2. If
they are at the same temperature(*),
which has more kinetic energy?
a. Ne
b. O2
c. Same
d. Not enough information to tell
(*) let’s assume the temperature is “high”.
Relative to what, we’ll discuss in a minute.
Disclaimer
Thermal energy (measured by kBT) must be
comparable to the quantum energy levels, or some
degrees of freedom get “frozen out”
From section 21.4: diatomic hydrogen
Y-axis: heat
added, divided
by temperature
change (per
mole)
Units: J/molK
Translational KE and vrms
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Worked problem: what is average
speed (vrms) of oxygen molecules at
300K?
Molecular View of Pressure
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Related problem: pressure by baseballs
(m = 145 g) on a wall (A = 9 m2).
Speed = 85 mph (38 m/s). Elastic
collisions, each lasting for 0.05
seconds. (This is the time the ball is in
contact with the wall.) A baseball hits
the wall every 0.5 seconds.
Actual problem: a cube filled with gas
a. Pressure on right wall from one molecule
b. Pressure on right wall from all molecules
Molecular View of Pressure, cont.
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Result: P = N m ⅓ v2 / V
What does PV equal?
Compare to: PV = N kB T
What does T equal?
Quick writing: What does this tell you
about what temperature really is?
Demo
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Demo: kinetic theory machine