Announcements 9/17/10
HW 7-5: the temperature of the air above the
water should be -12C, not +12C.
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
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
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
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…
PV diagrams
What if pressure doesn’t
stay constant?
Work done on gas vs
work done by gas
Thought question
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
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)
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
Result for monatomic ideal gas
Result for diatomic gas (around room
temperature)
Thought question
The process in which
Eint is the greatest
(magnitude) is:
a. path 1
b. path 2
c. neither; it’s the same
Isothermal Contours
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
Three Specific Cases
Constant pressure, “isobaric”
Constant volume, “isovolumetric”
Constant temperature, “isothermal”
Demo
Demo: A constant volume process
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