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Cant understand those lines in a text about solid state physics

  1. Mar 24, 2010 #1
    "we want to calculate the heat enrgy of a gas molecule
    n - is the number of molecules per volume
    v_th - is the average speed of gas molecule
    L - is the distance which travels one molecule in a certain time

    we have six directions for which they travel
    +-x +-y +-z

    so in each time n/6 of the molecules travel in every one of those 6 dircetion."


    i cant see how?
    we have n molecules why they divided evenly
    it could be the in the +z direction move 5/6n of the molecules

    ??
     
  2. jcsd
  3. Mar 24, 2010 #2
    Gas molecules are energetic and bounces off of walls and gazillions of other gas molecules. As a result they constantly change direction. That's why it's impractical to assign one specific direction because in the next moment a gas molecule will bounce off something and fly in the opposite direction. That's why the average of 6 possible directions is taken.
     
  4. Mar 24, 2010 #3
    thanks :)
    next question about missundertood text:(question 2)
    "the total amount of molecules which hit an area unit in a certain time
    equals 1/6 of the number of molecules which present in the rectangular box
    the area of its base is L=V_th*1
    so
    we have 1/6 * n *L=(n*V_th)/6
    "
    a bout this
    L=V_th*1
    V_th is the speed .sped times 1 cannot be area?

    how they got
    1/6 * n *L*1=(n*V_th)/6
    ?
    n is the number of molecules per volume ,L is the distance which travels in some time
    so i cant understand what is the result of multiplying dencity by distance
    its undefined.

    n*V_th is speed time density another undefined unit
    ??
     
    Last edited: Mar 24, 2010
  5. Mar 24, 2010 #4
    (question 3)
    formula question:"
    calculate the density n and its the material energy of a gas which is contained in presure 5 and termperature 325k"

    i am not asking you to solve this homw work question
    my question is about the formula
    i know that
    [tex]
    \frac{mv_{th}^2}{2}=\frac{3}{2}kT[/tex]
    the left side represent the total energy
    or
    [tex]
    E=\frac{3}{2}kT[/tex]
    so i used this formula
    but in the solution they use
    [tex]
    E=n\frac{3}{2}kT[/tex]
    why they added the n?
     
  6. Mar 24, 2010 #5

    dlgoff

    User Avatar
    Science Advisor
    Gold Member

    I'm not sure this is helpful, but if you are wanting to determine the density of an ideal gas, n, wouldn't you want to use p=nkT where p is the pressure, T is the temperature, and k is Boltzmann's constant?
     
  7. Mar 25, 2010 #6
    i need to calculate the energy too
    my dilema is about the formula they used its not like the formal formulaa
     
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