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B Permittivity of free space

  1. Dec 7, 2015 #1
  2. jcsd
  3. Dec 7, 2015 #2

    Dale

    Staff: Mentor

    It is simply a SI unit conversion factor between Newtons, meters, and Coulombs. It does not even exist in other unit systems, like Gaussian units.
     
  4. Dec 7, 2015 #3
    what it does?
     
  5. Dec 7, 2015 #4

    Dale

    Staff: Mentor

    It converts units of Coulombs to units of Newton's and meters.
     
  6. Dec 7, 2015 #5
    how?
     
  7. Dec 7, 2015 #6

    Dale

    Staff: Mentor

    ##1\;C^2=4\pi \epsilon_0 \;N m^2##
     
  8. Dec 7, 2015 #7
    But I thought it is something abut allowing electric field to enter in a region.
     
  9. Dec 7, 2015 #8

    Dale

    Staff: Mentor

    It is just a unit conversion factor, and only in SI units. In Gaussian units charge is defined in terms of the dyne and cm such that there is no similar conversion factor.

    You may be thinking about the permittivity of a material, but free space is not a material.
     
  10. Dec 7, 2015 #9
    Ok what is permittivity of material?
     
  11. Dec 7, 2015 #10

    Dale

    Staff: Mentor

    I think you already have a separate thread on that. It is not a good idea to have two open threads discussing the same topic. Let's keep this thread for vacuum questions and the other thread for material questions.
     
  12. Dec 7, 2015 #11
    But that thread was about susceptibility not permittivity.
     
  13. Dec 7, 2015 #12

    Dale

    Staff: Mentor

    ##\epsilon = (1+X_e) \epsilon_0##
     
  14. Dec 7, 2015 #13
    Ok, susceptibility and permittivity are related but not the same thing.
     
  15. Dec 7, 2015 #14

    Dale

    Staff: Mentor

    More importantly, vacuum and material are not the same thing.
     
  16. Dec 7, 2015 #15
    yes.
     
  17. Dec 7, 2015 #16
    What each term indicates in your equation?
    ##ε##= permittivity of a material
    εo=permittivity of free space/vaccum

    χe =susceptibility of ?
     
  18. Dec 7, 2015 #17

    cnh1995

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    Homework Helper

    According to my understanding, permittivity is a measure of charge required to create an electric field in a dielectric. It is not the physical definition but certainly helped me understand its applications.
     
  19. Dec 7, 2015 #18

    cnh1995

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    Homework Helper

    The material..
     
  20. Dec 7, 2015 #19

    cnh1995

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    Homework Helper

    In this video example, a slab of conducting material is placed in vacuum and subjected to an electric field E. Due to this field, there will be rearrangement of the charges on the slab surfaces to make the field inside the slab 0. Due to rearrangement, there will be charge densities created on opposite faces. Now, this charge density will be of the lowest possible value in case of vacuum(εo is the lowest). If this slab were placed in water(higher ε), the charge density required(or allowed) to create the same electric field E would be higher (εr times).
    This is the principle used in the capacitors.
     
    Last edited: Dec 7, 2015
  21. Dec 7, 2015 #20
    Susceptibility is a measure of the ease with which a material may be polarized defined by the equation

    P = ε0χE

    with χ = K -1 K being the dielectric constant of the material and the permittivity ε = (1 + χ)ε0

    Thus permittivity is a measure of how a material affects an electric field by its presence. since

    D = εE since D is the electric field due to free charges and E is the net field in the presence of a dielectric.

     
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