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Homework Help: Help filling in a formula correctly? Not sure about SI units etc!

  1. May 18, 2012 #1
    1. The problem statement, all variables and given/known data

    It is estimated that a particular diffuse hydrogen cloud has a particle density of 3e7 cm^-3, and the temperature is 150 K.
    Calculate Jeans mass.

    2. Relevant equations

    Jeans mass is given by: (5kT/GmewM(H))^(3/2) . (3/4∏p)^(1/2)

    where k = boltzmanns constant = 1.38e-23
    T = temperature in Kelvin = 150K
    G = Gravitational constant = 6.673e-11
    mew = mean molecular weight = ?
    m(H)= mass of gas particles = ?
    p = cloud mass density = ?

    3. The attempt at a solution

    I know I just have to fill in the formula, so it should be simple, but I'm really having trouble calculating the parts of the formula.

    I don't have a clue about mean molecular weight OR mass of gas particles, and I THINK the 'cloud mass density' is the number given in the question (3e7 cm^-3) but I don't understand how to convert that to metres (the unit cm^-3 is throwing me off.

    I really want to calculate this as accurately as I can, could anyone help please?

    Thank you.
  2. jcsd
  3. May 19, 2012 #2


    User Avatar

    You know that the gas is hydrogen, so look it up, keeping in mind that hydrogen gas is H2.

    Express the centimeter in meters, then cube it.
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