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Homework Help: Solving for Electric Potential Difference in Square diagram

  1. Sep 14, 2015 #1
    1. The problem statement, all variables and given/known data
    I've attached a url with a picture of the problem setup and diagram
    https://instagram.com/p/7nYNXTscre/ [Broken]

    2. Relevant equations
    I used the equation V = KΣiqi/ri to solve for potential at A and B
    3. The attempt at a solution
    For Va I used K((Q/d)+ (2Q/(d^2+d^2)^.5))
    I just plugged in
    d = .02
    Q = 5E-9
    2Q = 10E-9
    and arrived to Va = 5.43E3 Volts.

    For Va I used K((Q/(d^2+d^2)^.5) + (2Q/d))
    I just plugged in
    d = .02
    Q = 5E-9
    2Q = 10E-9
    and arrived to Vb =6.08E3 Volts.

    I am just stuck on part c.. Which asks "What is the electric potential difference between B and A" I'm not sure exactly which equation so use to solve for electric potential difference.. I think I am misunderstanding the concept of an "electric potential difference".. is it just the difference between the electric potentials at the two points a & b? If so, I'm not sure which point (a or b) would be considered the "initial potential" which I would use to take Vfinal - VInitial. If this approach is correct, picking the wrong potential as the initial or final would result in a sign error in the final answer....

    Thank you guys for your help :smile:
    Last edited by a moderator: May 7, 2017
  2. jcsd
  3. Sep 14, 2015 #2
    As soon as I figure out how to add a photo
  4. Sep 14, 2015 #3


    User Avatar
    Science Advisor
    Homework Helper
    2017 Award

    Ah, there's some more guidelines on the PF forum: the threads remain on PF forever (well...) whereas photos on instagram and sites like that disappear in a short while. So we urge folks to type their problem statement (good exercise: collecting only the necessary stuff and checking for completeness; listing variables, dimensions, knowns , etc... !) and upload pictures using the button.
    After all, the helpers do take the trouble to type and typeset their assistance, so it's fair to ask that the requesters do likewise...

    (I wish I had a shortcut button for the guidelines and this here little admonition -- you are definitely not the only one :smile: ).

    Asking for potential difference between B and A is indeed awkward. You could answer VB - VA = ... on paper, but in a computer program you indeed risk a sign error.

    Your chances are better than 50%, because often the problem writer doesn't care about the sign, so a positive number of Volts might be the safest bet.

    This guy (at t=5:07) says the potential difference between A and B is the work/charge to bring a charge from A to B, so he calculates VB - VA

    (How did we do that in mechanics with potential energy from gravity ?)
    Last edited by a moderator: May 7, 2017
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