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Work done by E field to move electron from negative plate to postive.

  1. Feb 17, 2013 #1
    The facing surfaces of two large parallel conducting plates separated
    by 10.0 cm have uniform surface charge densities that are equal in magnitude but
    opposite in sign. The difference in potential between the plates is 500 V. An electron is released from rest next to the negatively charged surface. Find the work done by the electric field on the electron as the electron moves from the release point to the positive plate.

    Ok so I am getting -8E-17 J

    by ΔU=qΔV, W=qΔV

    and I know ΔV is positive because we are going form low potential to high potential therefore Δv=500V

    so i am getting W=(-1.6E-19)(500)=-8E-17 however I am concerned because I am seeing a positive work somewhere else. and that just does not make sense to me. or can someone explain to me why this would be positive work?
     
  2. jcsd
  3. Feb 17, 2013 #2
    Re: work done by E field to move electron from negative plate to posti

    The electron is released next to the negative plate. The electron will be attracted to this plate, so to get it to the other plate you must give it energy, and therefore work is positive.
     
  4. Feb 17, 2013 #3
    Re: work done by E field to move electron from negative plate to posti

    I thought the electron would be repelled by the negative plate since they have the same charge!?!?
     
  5. Feb 17, 2013 #4
    Re: work done by E field to move electron from negative plate to posti

    Ok I believe I have everything sorted out now, I would be doing negative work If an EXTERNAL FORCE was applied to move the electron to the positive plate, the problem asks for the work done the the ELECTRIC FIELD, and since the electric field is going to the opposite direction the of the motion of the electron w=qvcos(180)= -+-=+work.
     
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