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Physics concept questions (electric field & potential diff.)

  1. Nov 10, 2015 #1
    1. The problem statement, all variables and given/known data
    upload_2015-11-10_18-58-59.png

    2. Relevant equations


    3. The attempt at a solution
    1) charges should remain the same since there is no path for them to go anyway.
    2) Now, if the panels were of infinite length, I believe the electric field should be the same regardless of distance? But in this case, the panels aren't infinitely long so there are sharp edges at each end and electric fields are stronger at sharp edges so I believe the answer should be : electric field increases.
    3) using V=Ed , if d decreases and E increases... I have no idea then because it would depend on the magnitudes of d and E.
     
  2. jcsd
  3. Nov 10, 2015 #2
    #2)
    upload_2015-11-10_19-15-12.png

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    #2a) the battery maintains the electric potential in each plate so the potential difference should remain the same.

    #2b) using V = Ed, if V is constant, d is decreased, E should increase then.

    #2c) I guess charges would remain the same since positive charges on the left plate would be require work to reach the positive terminal of the battery? and same goes for the left side.
     

    Attached Files:

  4. Nov 10, 2015 #3
    upload_2015-11-10_19-37-24.png

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    Here is how I picture this:

    upload_2015-11-10_19-37-56.png

    using V=Ed, since V is constant, and the distance between the slab of metal and each plate is d/4, E should increase.
     
  5. Nov 10, 2015 #4

    haruspex

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    In respect of the first problem:
    I don't think you are supposed to worry about those details here. Treat the plates as infinite.
    Yes, the field should not change. What does that tell you about potential difference?

    Also, remember that the potential at any point is the sum of the potentials due to all the different charges. Consider the potential at one plate. As the other plate gets closer, will the potential there change?

    For your other problems, please post each in a separate thread. Gets too confused otherwise.
     
  6. Nov 10, 2015 #5
    If the field doesn't change, then V should decrease as d decreases.

    but why do we treat the plates as if they were infinitely long? say the plates are 1 meter long, each, would the field still remain the same?
     
  7. Nov 10, 2015 #6

    haruspex

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    Yes, The potential difference should decrease.
    The plates are not infinitely long, so the field is a bit more complicated, but I see no reason why it should change the general trend of the answer. You are not given the relationship between the plate lengths and separation, so it is not possible to be more precise.
     
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