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Constant question

  1. Sep 27, 2009 #1
    I cannot for the life of me figure out what value my professor used for E(0) in this equation (vacuum permittivity). Here's what he wrote down:

    http://img38.imageshack.us/img38/5663/78976562.jpg [Broken]

    It's talking about the first ionization energy of an electron, so I've tried using numbers I do know to find a value for E(0)/e^2 (because I'm not entirely sure on the numerical value of e either, though it doesn't matter too much because the only time I need to use this constant it's in the form E(0)/e^2).

    If anyone could help out, all I need is a number value to about 4 places.
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Sep 27, 2009 #2
    Hello FatCat-
    All of the constants you need can be found at:
    This includes the electron charge, the permittivity of free space, and the classical Bohr radius, all to about 6 places. The number, 13.60569172 eV, is the Rydberg energy, also on this page. pi is about 22/7.
    Bob S
  4. Sep 27, 2009 #3


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    Staff: Mentor

    Did you try a Google search on "vacuum permittivity"? The first hit is a Wikipedia page which gives the value in two different sets of units near the beginning.
  5. Sep 27, 2009 #4
    I can and have looked up the constants, but I still can't find any combination of them that makes sense, either in units or in outcome. Using 8.854*10^-12 for E(0) I get this:
    Which is very clearly nowhere near...well, anything that makes sense.
  6. Sep 27, 2009 #5
    Oh, that answer comes out to 3.13926*^-14 by the way. Copied and pasted badly there....
  7. Sep 28, 2009 #6
    Hi Fatcat-

    The answer you get is in units of Joules

    E = e2/8 pi ε0 a0 = 2.18 x 10-18 Joules

    using 1.6 x 10-19 Joules = 1 eV (electron volt)

    we get E = 13.6 eV

    Bob S
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