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Homework Help: Pressure due to molecular velocity problem

  1. Feb 19, 2006 #1
    Hey guys,

    I am having a little problem with a problem I am doing in Physics (thermo chapter).

    Well, here is the problem:

    "The mass of the H2 molecule is 3.3e24 g. If 10e23 H2 molecules per second strike 2.0 cm^2 of wall at an angle of 55° with the normal when moving with a speed of 1.7 10e5 cm/s, what pressure in Pascals do they exert on the wall?"

    I have attempted this problem numerous times for atleast 2 hours and 45 minutes and have not been able to get the correct solution.

    I keep on getting 2.51e5 Pa but it is not correct.

    If anyone could shed some light on this problem, I would greatly appreciate it!! I am more interested as to how you arrive to the solution rather that just an answer.

  2. jcsd
  3. Feb 19, 2006 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Show how you arrived at your answer. (Hint: Consider the change in momentum of the molecules as they bounce off the wall.)
  4. Feb 19, 2006 #3
    The value of the velocity that is used during momentum calculation is the velocity in the x direction which would be :

    Vx = 170 m/s * cos (55 degrees)
    I converted the velocity from centimeters to meters
    (velocity in centimeters = 170000 cm/s)

    and the pressure exerted is:
    P = [ n*m*(Na) / L^3 ] * Vx^2

    n is the number of moles
    m is the mass of the molecules
    Na is Avogadro's number
    Vx is the calculation above

    However, I just realized that the L is the length and I was using the area that is given to me in the problem. But how can I find the pressure if I do not have the length of one side?

  5. Feb 19, 2006 #4
    [tex]N\Delta p=F\Delta T=F[/tex]

    where [tex]p[/tex] is the momentum

    The pressure is [tex]P=\frac{F}{1m^2}=F[/tex]

    You just need to calculate the [tex]\Delta p[/tex]
    Last edited: Feb 20, 2006
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