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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
    we have 1/6 * n *L=(n*V_th)/6
    a bout this
    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
    the left side represent the total energy
    so i used this formula
    but in the solution they use
    why they added the n?
  6. Mar 24, 2010 #5


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    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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