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Announcements 9/12/11

Announcements 9/12/11. Prayer I’ve got extra copies of the class directory On Wednesday I will go back and regrade all of the old clicker quizzes. As long as your clicker is registered by then, all of your quizzes will count. Unregistered clickers:. 1151FBBB 11A3D26 13C413C4 1416FEFC

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Announcements 9/12/11

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  1. Announcements 9/12/11 • Prayer • I’ve got extra copies of the class directory • On Wednesday I will go back and regrade all of the old clicker quizzes. As long as your clicker is registered by then, all of your quizzes will count. • Unregistered clickers: • 1151FBBB • 11A3D26 • 13C413C4 • 1416FEFC • 14710762 • 1473197E • 160B0B16 • 16B89D33 • 190E8D9A • 1A3B1E3F • 1AEE0CF8 • 1B495507 • 1DAE9D2E • 26395C43 • D68FB9E Make sure your clicker number is not on this list! (If it is, register it on the class website ASAP!)

  2. Characterizing velocities • Recall bouncing balls in jar. Focus on one type of molecule. Lots of questions, such as : • What’s the average velocity? • What’s the most popular velocity? • What’s the velocity that corresponds to the average kinetic energy? • How many molecules have velocities within a given range? How to answer: use statistical distributions, aka histograms

  3. Height Histogram (made up data): • Total students = 49 • Round heights to closest integer, plot histogram • What is the combined area of all bars? • If I pick a student at random, what are chances he/she will be 68 inches tall? • What is the area of the bar at 68 inches divided by the total area? • How many students will be exactly 68.000000 inches tall? • If I pick a student at random, what are chances he/she will be 61.5-64.5 inches tall? • What is average height of all students? (At least, how would you figure that out?)

  4. “Normalized” Histogram: • Total students = 49 • Y-axis now divided by total # of students. • What is combined area of all bars? • If I pick a student at random, what are chances he/she will be 61.5-64.5 inches tall? (At least, how would you figure that out?) • How many students have heights between 61.5 and 64.5 inches? • What is average height of all students?

  5. Probability Distribution Function • Imagine total # =10 billion. Tiny “bins”. Connect peaks of curve with line… becomes a function • What is combined area of all bars? • If I pick a person at random, what are chances he/she will be 63.6-67.2 inches tall? • How many people have heights between 63.6 and 67.2 inches? • What is average height of all people? (If a non-symmetric curve, this is not just the peak of the curve.)

  6. Velocity Distribution: “Maxwell-Boltzmann” with some constants out in front to normalize it • At 600K, how many molecules with speeds between 400 and 600 m/s? • What is “vmost probable”? • What is “vaverage”? • What is “vrms”? • How many molecules are at exactly the “most probable” velocity? Where does this eqn come from? Wait a few weeks.

  7. Calvin and Hobbes, Bill Watterson

  8. Calvin and Hobbes, Bill Watterson

  9. Calvin and Hobbes, Bill Watterson

  10. Heat = not a fluid! • Sir Benjamin Thompson, Count Rumford, 1753-1814 • Boiling water with a cannon Image credit: Wikipedia

  11. James Joule, 1818-1889 Image credit: Wikipedia

  12. Demo/Video • Demo: Boiling water with a vacuum • Video: Boiling water in a paper cup

  13. Reading Quiz • What name do we give to the heat capacity per unit mass? • entropy • internal energy • mass-pacity • normalized heat capacity • specific heat

  14. Specific Heat • Q=mcDT

  15. Thought Question • If you add 500 J of heat to a mass of water, and 500 J of heat to the same mass of copper, which one increases the most in temperature? • Water • Copper • Same

  16. Reading Quiz • Thermal energy that is used to melt or freeze something is called: • latent heat • mass heat • melty heat • molar heat • specific heat

  17. Water boiling Ice melting 100o C Water boils Ice warming Steam warming Water warming T 0o C Ice melts Heat energy added (Q) Phase Changes

  18. Latent Heats

  19. Thought Question • If you want to melt a cube of ice that’s initially at -40C, you must first raise its temperature to 0C, and then you must melt it. Which part takes the most energy? • Raising the temperature • Melting • Same

  20. Calorimetry • Worked problem (class designed): ____ grams of hot iron at _____ C is added to ____ g of water at _____ C in a styrofoam insulated container. What is the final temperature of the mixture? (Neglect the container.) • ciron = 448 J/kgC • cwater = 4186 J/kgC • Lwater-steam = 2.26  106 J/kg

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