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I Why cant thelectric field be in circumferential direction?

  1. Nov 12, 2016 #1
    Sorry

    in this question i have several things to ask
    1.why cant the E field in the circumferential direction?
    2.How can i find the direction of B field

    3.according to the formula of curl E=-dB/dt and the curlB=u J
    did they share the same property so that the direction of E is circumferential (just like B dl=u i which is ampere law so that the direction B is circumferential) and dB/dt is going straight(just the the current is going straight which is enclosed inside the boundary)?

    thank
     

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  2. jcsd
  3. Nov 12, 2016 #2

    Simon Bridge

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    1. I don't know what you mean by "circumferential".
    Clearly it is possible to set up an electric field that goes in a loop: it's called an electric circuit.
    But you cannot have a circular field line because the line has to start and end at different potentials.

    2. You use a small magnetised needle balanced on a pivot.

    3. The similarity in the equations shows that there is a similarity in the equations ...
    Note, they should be:
    ##\nabla\times\vec E = -\partial_t\vec B## and
    ##\nabla\times\vec B = \mu_0\left( \vec J +\epsilon_0\partial_t \vec E \right)##
    ... if ##\vec J = 0## the two equations look even more alike ;)

    Check what "curl" means:
    https://en.wikipedia.org/wiki/Curl_(mathematics)
     
  4. Nov 12, 2016 #3

    you can see this picture clearly
    the solution states:The magnetic field is “circumferential” in the quasistatic approximation."


    .....but i dont know how did the solution concluded "Thus the direction of the electric field is longitudinal" from the simiarlty between the relation between the electric and magnetic fields?
     

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  5. Nov 13, 2016 #4
    What breaks the symmetry between the magnetic and the electric is the lack of magnetic monopoles: north and south magnetic charges that emanate magnetic field in the way positive and negative electric charges emanate electric field. If you had a magnetic wire carrying magnetic monopoles in a straight line they would generate a circumferential electric field in the same way an electric current generates a circumferential magnetic field.
     
  6. Nov 13, 2016 #5
    But Why did the Electric field outside vanished in this case?
     
  7. Nov 13, 2016 #6
    But Why did the Electric field outside vanished in this case?
     
  8. Nov 13, 2016 #7

    Simon Bridge

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    In what case? Please be specific.
     
  9. Nov 13, 2016 #8
    See the statement inside the red circle.
     

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  10. Nov 13, 2016 #9

    Simon Bridge

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    Oh so the question had nothing to do with the quoted text? Fine.
    To see why the electric field vanishes, try doing the problem.
     
  11. Nov 13, 2016 #10
    because the total current inside the object is 0?
     
  12. Nov 13, 2016 #11

    mfb

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    That contributes to it, yes, but the main point is the absence of magnetic charges and the fact that the problem assumes a static situation.
     
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