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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
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!)


Characterizing velocities
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


Height histogram made up data
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?)


Normalized histogram
“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?


Probability distribution function
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.)


Velocity distribution maxwell boltzmann
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.





Heat not a fluid
Heat = not a fluid!

  • Sir Benjamin Thompson, Count Rumford, 1753-1814

    • Boiling water with a cannon

Image credit: Wikipedia


James joule 1818 1889
James Joule, 1818-1889

Image credit: Wikipedia


Demo video
Demo/Video

  • Demo: Boiling water with a vacuum

  • Video: Boiling water in a paper cup


Reading quiz
Reading Quiz

  • What name do we give to the heat capacity per unit mass?

    • entropy

    • internal energy

    • mass-pacity

    • normalized heat capacity

    • specific heat



Thought question
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


Reading quiz1
Reading Quiz

  • Thermal energy that is used to melt or freeze something is called:

    • latent heat

    • mass heat

    • melty heat

    • molar heat

    • specific heat


Phase changes

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



Thought question1
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


Calorimetry
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