1. Announcements 9/10/10 Be sure to register your clicker on the class website! (About 5 of you are not registered yet.) a. 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. Results of the TA office hour survey, effective next week: a. a.Mon 3-4 pm b. b.Wed 5-6 pm c. c.Fri 5-6 pm My own office hours are still right after class, 2- 3 pm.

2. Video Video: Barrel Crush

3. Worked problems: 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?

4. Thought quiz (ungraded): 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. a.Oxygen b. b.Nitrogen c. c.Same speed Demo: heavy vs light molecules

5. Equipartition Theorem “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 … ½k B T

6. Thought quiz Compare a monatomic molecule such as Ne to a diatomic molecule such as O 2. If they are at the same temperature(*), which has more kinetic energy? a. a.Ne b. b.O 2 c. c.Same d. d.Not enough information to tell (*) let’s assume the temperature is “high”. Relative to what, we’ll discuss in a minute.

7. Disclaimer Thermal energy (measured by k B T) 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

8. Translational KE and v rms Worked problem: what is average speed (v rms ) of oxygen molecules at 300K?

9. Molecular View of Pressure Related problem: pressure by baseballs (m = 145 g) on a wall (A = 9 m 2 ). 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. a.Pressure on right wall from one molecule b. b.Pressure on right wall from all molecules

10. Molecular View of Pressure, cont. Result: P = N m ⅓ v 2 / V What does PV equal? Compare to: PV = N k B T What does T equal? Quick writing: What does this tell you about what temperature really is?

11. Demo Demo: kinetic theory machine