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Conducting Plates, electrical fields w/ conductor.

  1. Jul 23, 2007 #1
    1.Two plates placed a distance "D" apart. One charged +Q, the other -Q. An uncharged slab of metal is placed between the plates.



    2. Does this slab change the electric field outside or between the plates (but outside the slab)? Do forces exerted on particle sitting between the plates (but outside the slab) change in any way with the slab as compared to before it was placed between the plates?



    3. The slab acts as a conductor with zero net electrical force. I have no clue about the rest? Does the E-field increase between the plates because of the slab?

    BONUS...same situation as in (1), but the plates are discharged, and a battery is hooked up with voltage V. An uncharged slab is again placed between the plates. Describe the e-field inside the metal slab.... :eek:

    :mad::mad::mad::mad::mad::uhh::uhh::uhh::frown::frown:
     
  2. jcsd
  3. Jul 24, 2007 #2

    mgb_phys

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    What do we know about capacitors:
    Charge is conserved - so we can't change the Q on the plates.
    There is no field inside a conductor ( assume the metal slab is a conductor!)
    The voltage on each side of the slab is the same.
    A conducting slab doesn't act like a dielectric

    What is the field exactly half way between the plates? Now imaging a very thin slab placed here, what is the field on each side of the slab.
    If the slab is thicker and fills more of the gap, the voltage on each side of the slab is still the same and the voltage on the plates is the same, what happens to the field as the gap betwen the slab and the plates gets smaller?
     
  4. Jul 24, 2007 #3
    If voltage stays the same and the gaps get smaller, the field has to get larger.

    V=ED
     
  5. Jul 24, 2007 #4
    This is true ONLY if the particles inside the conductor are in static equilibrium.
     
  6. Jul 24, 2007 #5
    it is GIVEN in the problem that: the e-field inside the metal slab is zero.
     
  7. Jul 24, 2007 #6

    mgb_phys

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    Sounds right to me, an ungrounded uncharged conductor between the plates of a capcitor shouldn't have any other effect except to make the gap effectively smaller.
     
  8. Jul 24, 2007 #7
    I'm afraid it is NOT given in the problem statement, which is strangely worded as you indicated in the original post. The statement

    "The slab acts as a conductor with zero net electrical force."

    is utter nonsense. A conductor, as a single entity, cannot possess force.

    A more concrete thing to say would something about the net charge on the slab. The intent is probably for the metal slab to be neutral. Now, what does THAT imply will happen when the slab is introduced between the capacitor's plates?
     
  9. Jul 24, 2007 #8

    mgb_phys

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    I assumed this meant the slab had no potential. Voltage translates as electric force (as in EMF) in a lot of langauges.
     
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