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Homework Help: Finding potential of two points in a constant electric field

  1. Jan 27, 2010 #1
    1. The problem statement, all variables and given/known data
    The figure below shows two points in an E-field: Point 1: (3,4) and Point 2: (12,9) both in m. The electric field is constant, with a magnitude of 80 V/m, and is directed parallel to the +X axis. The potential at point 1 is 1100 V. What is the potential at point 2?

    2. Relevant equations
    E = change in V / change in X
    A^2 + B^2 = C^2

    3. The attempt at a solution
    I got that the distance (tangent) between points 1 and 2 is 10.296 m. If I plug that into the equation to solve for V, I get 80=(V2 - 1100) / 10.296, or V2 = 823.68. Since Point 2 is further away from the origin of the E-field, the potential should be lower, so 1100 - 823.68 = 276.32. This is wrong. I tried adding them to see if that was right, (1923.68) but that was wrong too.

    My main question is does the E-field value change? It says it is constant, so I don't know why it would. Thanks for the help!

    The second part of the question is to calculate the work required to move a negative charge of Q = -608 microC from point 1 to point 2, which I would use the formula: the change in PE = the change in V times the charge (q). Is this right?
  2. jcsd
  3. Jan 27, 2010 #2


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    Welcome to PF!

    Hi skibum143! Welcome to PF! :smile:

    (try using the X2 tag just above the Reply box :wink:)

    Potential = potential energy per charge …

    potential energy = work done …

    what is the work done if you go a distance 9 parallel to the x-axis?

    what is the work done if you go a distance 5 parallel to the y-axis? :wink:
  4. Jan 27, 2010 #3
    Hi tiny-tim. Thanks so much for the response!
    The work going a distance 5 parallel to the y-axis is zero, because it's on the same equipotential line.
    So should I only be using 9 as my "r" instead of the tangent of 10.296?
  5. Jan 27, 2010 #4
    sorry, i meant use 9 as my change in x...
  6. Jan 27, 2010 #5
    I guess my other main question is, if they say the electric field is constant, at 80 V/m, is that the value at every point in the electric field? If not, how do you calculate the change in the electric field?
  7. Jan 27, 2010 #6
    AH!!!! I got it!!!!!!!! Thank you so much!!!! :)
  8. Jan 27, 2010 #7


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    :biggrin: Woohoo! :biggrin:

    But, slow down in future! :smile:

    you don't usually get a quick response here, so you may as well take your time! :wink:
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