1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook