Announcements 9/17/10
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HW 7-5: the temperature of the air above the
water should be -12C, not +12C.
Thought for the day (from Dr. Van Huele’s
colloquium talk on Wed afternoon):
“Physics  Nature”
What I think he meant: “Physics just gives you
approximations (models) of nature.”
Worked Problem
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You foolishly decide to build the walls of your new
house out of solid aluminum, 5 cm thick. As a
result, in the wintertime heat leaks out like a
sieve. How much money will this cost you each
day? The inside temp is 70 F (21.1 C), the
average outside temperature is 25 F (-3.9 C).
The surface area is 280 m2. The gas company
charges you $0.89 per “therm” (1.055  108 J).
Only count heat loss through conduction.
Reading quiz
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Which of the following is NOT true of the
work done on a gas as it goes from one point
on a PV diagram to another?
a. It equals minus the integral under the
curve.
b. It depends on the path taken.
c. It cannot be calculated without knowing n
and T.
d. It is equal to ΔEint – Q.
e. It has units of Joules.
Work done by an expanding gas
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1 m3 of an ideal gas at 300 K
supports a weight in a piston
such that the pressure in the
gas is 200,000 Pa (about 2
atm). The gas is heated up. It
expands to 3 m3.
Draw the change on a P-V
diagram
How much work did the gas do
as it expanded?
a. How do you know it did
work?
More on Work…
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PV diagrams
What if pressure doesn’t
stay constant?
Work done on gas vs
work done by gas
Thought question
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A gas in a piston
expands from point A
to point B on the P-V
plot, via either path 1
or path 2. Path 2 is a
“combo path,” going
down first, then over.
The gas does the most
work in:
a. path 1
b. path 2
c. neither; it’s the same
Quick Writing
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Describe with words how
you could actually make
a gas (in some sort of
container) change as in
path 2.
Internal Energy, Eint (aka U)
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U = Eint = Sum of all of the microscopic kinetic energies.
Return to Equipartition Theorem:
a. “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.”
b. Each “degree of freedom” of a molecule has energy
kBT/2
c. independent parts: larger for molecules that can
– rotate
– vibrate
(requires more than one atom)
→ such molecules have more “internal energy”
Internal Energy
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Result for monatomic ideal gas
Result for diatomic gas (around room
temperature)
Thought question
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The process in which
Eint is the greatest
(magnitude) is:
a. path 1
b. path 2
c. neither; it’s the same
Isothermal Contours
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A gas changes its volume and pressure
simultaneously to keep the temperature
constant the whole time as it expands to twice
the initial volume. What does this look like on a
PV diagram?
“First Law”
Eint = Qadded + Won system
 What does that mean? You can add internal
energy, by…
a. …adding heat
b. …compressing the gas
 Helpful rewrite: Qadded = Eint + Wby system
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Three Specific Cases
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Constant pressure, “isobaric”
Constant volume, “isovolumetric”
Constant temperature, “isothermal”
Demo
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Demo: A constant volume process