Physics 9 Monday, February 13, 2012

Transcription

1 Physics 9 Monday, February 13, 2012 learningcatalytics.com class session ID: This week and next week: heat, thermal physics, etc. It s an important topic for you, so we re reading two authors presentations of the same ideas. Equation sheet is up-to-date (see homework on web site): positron.hep.upenn.edu/physics9/#homework You can now see your own grade info when you click my info on the online response web page. Let me know if I have recorded anything incorrectly! HW5: problems 2, 3, 4, 6, 9, 10, 11 difficult?! Did anyone who came to class on Friday find #11 difficult? HW6 is more fluids, plus thermal expansion, ideal gas law

2 Roughly, 90% is A, >96% is A+. A couple of people signed up late and are still catching up.

3 A few selected comments from today/friday I thought this homework was a bit harder than the others just because the concepts of fluid dynamics are harder for me. Ultimately I think the problems helped me understand the concepts better. Why is it that when I look out over the philadelphia skyline one day that the lights twinkle a little bit, but then on other days they twinkle a lot more? I find it hard to understand the concept of heat death and energy is degraded in Chapter 14. It was interesting to read about how the heat engine works, the alternative energy resources and thermal pollution. I d like to know why the specific heat of water is so high or what determines an object s specific heat in general. Why is a wet surface slippery?

4 Edited from Argonne National Lab s ask a scientist site: Surprisingly, good lubricants are sometimes sticky. The stickiness of a liquid is due to surface tension trying to pull the liquid into contact with solid surfaces. Lubrication is something that happens when a space between two solid surfaces is filled with liquid, and then is difficult to expel when forces push the solids toward one another. Imagine a thin liquid layer on the surface of a solid. If the layer is broken up into millions of tiny spots, wetted spots on a solid surface tend to be stuck to one spot, so friction is high. Once you fill the entire space between surfaces, no boundaries remain, and friction is low. High viscosity in an extremely thin layer of liquid might still provide some drag, but it would allow continuous creeping slippage.

5 I estimated 0.04% of an atmosphere (about 40 N/m 2 ) gauge pressure inside this dome. The Wikipedia Air-Supported structure article estimates 250 N/m 2 (0.25% atm) as a commonly used value, with a typical range of N/m 2. So it looks as if I underestimated by about a factor of 5. Bill Berner went over there Friday afternoon with his altimeter wristwatch, and measured atm higher pressure (about 200 N/m 2 ) inside than outside.

6 By the way, what holds this thing up? What are the forces on a small square of fabric at the top of the dome? What balances the force of gravity to hold the fabric up? Penn Current says canvas weighs lbs (32 tonnes) and is 386 ft 242 ft (93400 sqft = 8700 m 2 ): mass area g = 36 N/m2 Note that 14.7 psi = N/m = %

7 Chapter G13: temperature & kinetic theory I found these topics to be very interesting, especially the expanding and contracting of matter. It would be helpful if we went through some exercises using gas laws and avogadro s number. The chapter was straightforward, except for kinetic theory. I thought that section 13-5 (Anomalous Behavior or Water Below 4 Degrees Celsius) was really interesting. I liked the connection with everyday life. What good are moles? I enjoyed thermal stresses important to consider in architecture. (Personally, I found the water (anomalous behavior) part and the discussion of vapor pressure / dew point / relative humidity to be most interesting.)

8 Chapter G13: temperature & kinetic theory I d like to talk more about diffusion. Let s spend some time discussing the ideal gas law. The reading on phase changes and evaporation was intriguing. How does a car engine work? 0-4 celsius, water acts crazy and expands, but I still don t understand why atomically H2O expands where other liquids may contract as they cool? Brownian movement was interesting the path of the pollen being jostled by water molecules. H.S. chemistry teacher favorite example: old grandmas driving Buicks around parking lot. More speed more collisions! I m still having trouble grasping what Young s modulus means.

9 Will grandfather clock run fast or slow on a hot day? It will run a little slow because on a hot day the brass rod will get longer. Period of oscillation of a pendulum depends on the length of a pendulum. It will run slow on a hot day, because the rod will expand (lengthen) a little bit due to the high temperature, thereby increasing the period of the pendulum swing, making each (reported) second a little longer. When the temperature increases, the rod of the pendulum becomes longer. Time period of a simple pendulum depends on square root of its length L- with increases in the temperature the length will increase, so the time period will increase. The addition of heat will add additional energy. The increase in energy creates thermal expansion. A longer rod means a longer period of oscillation so the clock will run slower on a hot day. Maybe this is why very hot days feel so long.

10

11 Why do we exhale white clouds in winter? Because there is moisture in your breath, when you breath out into cold air the moisture condenses to form a white cloud of vapor. The exhaled air condenses into water droplets when it hits the cold air. Because the water droplet sizes are varied and random, the light comes back to you looking white. Your breath contains water vapor. When you exhale, the warm water vapor mixes with the cold winter air, and the water condenses, changing from gas to liquid.

12 What good are moles, anyway? Atomic mass unit (u): 1 u = kg This is very close to the mass of a proton: m proton = kg What is (roughly) the mass of one water molecule? Is this easier to answer quickly in kg or in u? What s the mass (roughly) of a mole of protons? (One mole of some kind of object is N A = of those objects.)

13 Textbook s illustration of solid / liquid / gas

14 Ideal gas law Anybody remember this from high school chemistry? PV = nrt R = J mol K R = L atm mol K

15 Thermal expansion It turns out that a simple coefficient of linear (or volume) expansion works pretty well for most materials. But these things are usually tabulated as a function of temperature. L = αl 0 T dl dt = αl 0 V = βv 0 dv dt = βv 0 If V = L 3 and material is isotropic, then dv dt = so β 3α for most materials. d dt (L3 ) = 3L 2 0(αL 0 ) = 3αL 2 0 = (3α)V 0

16 Thermal stress Remember from the the fall term that (E = Young s modulus) L L 0 = 1 E If I heat something up so that it tries to expand thermally: F A L L 0 = α T but I don t let it expand (because it is held in place rigidly), then it is effectively being squished by a factor α T So holidng it at its original length when it heats up induces a stress (force per unit area): F A = Eα T which the material may or may not be able to tolerate (depending on its compressive strength).