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Homework Help: Parallel plates - direction of electric field

  1. Apr 12, 2005 #1
    Indicate the direction of the electric field between the plates of the parallel plate capacitor shown in the drawing if the magnetic field is decreasing in time. Give your reasoning.

    Please help me.. how can i do this?

    Attached Files:

  2. jcsd
  3. Apr 13, 2005 #2
    Try understanding the physics of the situation before asking the question so abruptly---

    You would do well to recall Lenz Law... and with it, tell yourself what the direction the induced current in the wire will be (CW or CCW) ? Then decide how that would effect the E field between the capacitor plates.

    Oh and might I add... your right hand would most certainly be helpful!
    Last edited: Apr 13, 2005
  4. Apr 13, 2005 #3
    not left hand :/?
  5. Apr 14, 2005 #4
    Use left hand when dealing with electrons fired thru H fields :D
  6. Apr 14, 2005 #5
    would the E field be up? (the magnetic field would be decreasing over time, so the induced magnetic field would have to oppsoe the change, and therefore point out of page. the current goes up when magnetic field points out, so the current will create a + charge on lower end of capacitor, - on upper, so E field points up)
  7. Apr 14, 2005 #6
    current will go CCW right?? (god i hate conventional current)
  8. Apr 14, 2005 #7
    Hehe, yep you're absolutely right... imagine some unknown entity wishing to fire extra H field upward, since the H field is decreasing... so in using your right hand, you want to point your thumb in the direction of that entity is wishing to fire the H field, and your fingers will naturally curl around in the direction the induced current is traveling... in this case it is CCW like you stated!

    So yes, positive charge will accumulate on the bottom plate, and negative on the top... thus creating an E field going from the bottom plate to the top plate--- as for finding the strength of the E field... I doubt you'd be required to ever do that, however you'd analyze using Gauss and apply sigma/2epsilonnaught or whatnot.
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