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Why do we multiply permittivity of vacuum with relative permittivity?

  1. Oct 28, 2015 #1
    Why do we multiply permittivity of vacuum with relative permittivity?
    What does this calculation mean?
  2. jcsd
  3. Oct 28, 2015 #2
    You do that in order to get the permitivity of the medium. That's all.
    Rather than working with absolute values of permitivities it is preferred to use the vacuum permitivity as reference and tabulate the ratio of the permitivity of the medium to that of vacuum.
  4. Oct 28, 2015 #3
    Why is the vacuum permitivity used as a reference?
  5. Oct 28, 2015 #4
    Good question. What would you choose as reference?
  6. Oct 28, 2015 #5
    If vacuum is empty with no materiaparticle then why can we not just use the relative permittivity as a reference? If there is only emptyness why can we not dismiss vaacum and only focus on relative permittivity?
    But then if there are no materia particle in vacuum how can pressure exist and be less than atmospheric pressure? It makes it seem as we can not dissmiss vacuum and from that it gives me second thoughts on what I just said about dissmissing vacuum and only use relative permittivity. I am not sure nasu..
  7. Oct 28, 2015 #6
    I am going out of my main question but if it is only "emptyness" then how can talk about as if it exists?
  8. Oct 28, 2015 #7
    There is an electric force between charged particles in vacuum. So you cannot just dismiss it.
    When you put the same particles in a medium, the force is reduced by a factor. That factor is the relative permitivity of the medium.
    This is one reason to choose vacuum as reference. The force is largest in vacuum.

    The rest is not physics but sort of philosophy. What means to "exist" and things like this. Really not interested in discussing these.
  9. Oct 28, 2015 #8
    When I looked up vacuum, the source told me that there is not materia particle, then how does an electric force exist if there is no materia particle?
  10. Oct 28, 2015 #9
    Why do you think you need "materia" particles to have electric forces? Of course you have two charged particles, at least. But there is nothing else between or around them. This is what is meant by electric force in vacuum.
  11. Oct 28, 2015 #10
    Because thats how I thought myself through it, that the existence of materia is by particle, therefore if particles exist thats somehow materia. Thank you, I have understood now.
  12. Oct 29, 2015 #11


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    Gold Member

    Actually, the real reason for the so called "vacuum permeability" is that the SI electrical units (based on an Ampere) are not consistent with the rest and we need a fudge factor to fix that inconsistency.
    Switch to CGS system and the "vacuum permeability" disappears !!!
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