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Homework Help: Electric potential problem with two plates and different points (i'll elaborate)

  1. Feb 1, 2013 #1
    1. The problem statement, all variables and given/known data
    There are two parallel plates. The plate on the left has an electric potential of +60 Volts. The right plate has an electric potential of 0 Volts. The plates are 8.0mm apart. Between the two plates lies a point P 2.0mm from the left plate with a charge -3.5pC. Point R lies a mass right in the middle. Point S lies 2.0mm from the right plate.

    i) What is the electric potential at point R?
    ii) What is the electric potential at point S?

    Point R lies a mass of 4*10^-19 kg with a charge -2pC.
    B) What is the speed of the mass at R when it reaches point S if it has an initial velocity of 3.0*10^4 m/s.

    2. Relevant equations
    V= kq/r

    3. The attempt at a solution

    I solved this problem for Ai) using kq/r where r is the distance from R and P and q is charge at P. I then added the voltage of 60 volts to that number to get the total potential. I believe this is where I went wrong but I don't see how the 60 voltage plate could not affect the potential at the point. Any help would be great. Thanks
  2. jcsd
  3. Feb 1, 2013 #2

    rude man

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    Homework Helper
    Gold Member

    If the potential of the RH plate is zero then the potential at a point x away from the RH plate towards the LH plate is 60V*x/d, x = distance from RH plate to R or S, ignoring the extra charge at P. Now the potential due to the extra charge at P is just the extra work you have to apply to a unit positive test charge to bring it from the RH plate to R or S.

    The usual definition of potential as being the work on a unit test charge brought from infinity to the observation point does not apply here. That's because the point of zero potential is the RH plate and not infinity.
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