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Homework Help: Year 12: Cambridge Physics Problems (Joule's classification on molecule factor)

  1. Jun 8, 2012 #1
    Derive an expression for the number of impact of gas molecules on unit area in unit time in terms of the number of molecules per unit volume, n, and their mean speed, <c>. Explain the assumptions you make.

    A disc of radius "a" rotates with constant angular velocity "omega" in a gas at low pressure so that the molecules of the gas strike the disc from random directions. If the molecules are momentarily attached to the disc and leave in random directions relative to the disc, derive an expression for the torque exerted on the disc by the gas in terms of <c>, the mean speed of the gas molecules, rho, the density of gas, "a" and omega.

    The guide says such:
    Whoa. Whadahell just happened? Is this something beyond a CIE A Level Physics student's reach?
  2. jcsd
  3. Jun 8, 2012 #2
    This is pretty similar to one of the questions I got two years ago. Since the guidelines say I can't just tell you the answer I'll give you the advice that helped me most.

    Brush up on Brownian motion(although not exactly what you're doing here, it's similar) and make sure you aren't just picking bits from the question to put in an equation. Some of the most important stuff here is given by the way the question is written.
  4. Jun 8, 2012 #3
    I get n<c>/6.

    I don't know about Joule classification of velocities, but how I solved the problem is assumed that the unit volume where there are 'n' molecules is in form of a cube(this is the standard kinetic theory assumption while calculating velocities), and so the number of molecules dashing out of every face of the cube is n<c>. But only one face of the cube with unit area is facing the disc....:wink:

    I can't make sense of the other hollow cone method.
  5. Jun 14, 2012 #4

    This is AWESOME! I would have never thought of something like this. But what I don't get is the part
    . And why do we have to integrate [tex]\displaystyle\int kx^3 dx[/tex]?
    Last edited: Jun 14, 2012
  6. Jun 19, 2012 #5
  7. Jun 29, 2012 #6
    Come on guys let's get pumping!! XD
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