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 1b. path 2c. 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 1b. path 2c. 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 heatb. …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