Announcements 9/17/10 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 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 m 2. The gas company charges you $0.89 per “therm” (1.055  10 8 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. a.It equals minus the integral under the curve. b. b.It depends on the path taken. c. c.It cannot be calculated without knowing n and T. d. d.It is equal to ΔE int – Q. e. e.It has units of Joules.

Work done by an expanding gas 1 m 3 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 m 3. Draw the change on a P-V diagram How much work did the gas do as it expanded? a. 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. a.path 1 b. b.path 2 c. 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, E int (aka U) U = E int = Sum of all of the microscopic kinetic energies. Return to Equipartition Theorem: a. 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. b.Each “degree of freedom” of a molecule has energy k B T/2 c. 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  E int is the greatest (magnitude) is: a. a.path 1 b. b.path 2 c. 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”  E int = Q added + W on system What does that mean? You can add internal energy, by… a. a.…adding heat b. b.…compressing the gas Helpful rewrite: Q added =  E int + W by system

Three Specific Cases Constant pressure, “isobaric” Constant volume, “isovolumetric” Constant temperature, “isothermal”

Demo Demo: A constant volume process