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I Elementary question on Electric flux units

  1. Feb 23, 2017 #1
    The net flux over a closed surface is:

    Flux = The surface integral of the field E = charge / permitivity in space (Gauss law)

    The permitivity in space is:

    8.85 10-12 Coulombs2/N m 2

    So I understand the units of Flux are:

    Newton meter2/Coulombs

    Is this correct? I don't understand why in a book they say that the net flux is simply the charge, ignoring the permittivity units.

    ALternatively the permittivity could be given in Farad/meter so our flux would be:

    Coulombs meter/ Farad

    Is this correct?

    Thanks!
     
  2. jcsd
  3. Feb 24, 2017 #2
    Nobody?
     
  4. Feb 24, 2017 #3

    BvU

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    a book, eh ? Are they talking about ##\vec E## or about ##\vec D## ?
     
  5. Feb 24, 2017 #4
    Does your book use SI units or Gaussian? You should really identify the book.
    In SI, electric flux is volts meters. (Because electric field is volts/meter, and flux is an integral over area)
     
  6. Feb 24, 2017 #5

    jtbell

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    As a wild guess, perhaps it uses Gaussian units, in which the vacuum permittivity constant ##\varepsilon_0## does not exist.

    https://en.wikipedia.org/wiki/Gaussian_units

    [ah, now I see Khashishi beat me to it!]
     
  7. Feb 25, 2017 #6
    Ok, so if it is gaussian units it is just the charge, if it is SI units we have what I said at the start.
    I hope this is right, if not let me know.
     
  8. Feb 25, 2017 #7

    BvU

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    Did you miss
     
  9. Feb 25, 2017 #8
    It is Purcell's classic. It is a fantastic book, it says in gaussian units it is just the charge, but in SI doesn't explicitly say "these are the units", I wanted to check, since I consider that very basic.
     
  10. Feb 25, 2017 #9

    BvU

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    Purcell Ch1 (probably the rest too) is CGS. He touches upon SI which to me is potentially confusing. Check out appendix E where he says 'in our CGS system'.
     
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