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Homework Help: Electric potential and magnitude

  1. Feb 15, 2016 #1
    1. The problem statement, all variables and given/known data
    You start at a distance r from a point charge. The electric potential is 4500V at that point, and the electric field is 2.3x105N/C. a) What is r? b) What is the electric potential and magnitude of the electric field at a distance of r/4?

    2. Relevant equations
    v= kq/r E=kq/r^2 substitute everything and get q= v^2/kE?


    solve for q which will then allow you to solve for r?

    3. The attempt at a solution

    V = 4500, k = 8.9x10^9 and E = 2.3x10^5 that will give us a q

    This q will then be put into the equation r=kq/v and that will give you the answer for a?

    Part b would you take that distance solve for the magnitude and e-field?
  2. jcsd
  3. Feb 15, 2016 #2


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    Welcome to PF! Your outline of a solution should work. However, you don't actually need to find q first. Consider the ratio V/E.
  4. Feb 15, 2016 #3
    He will, however, eventually solve for q, but I would suggest to solve it after you find r. And yes, you would just plug in the values you get into the equations to do part b (but use r/4 instead of r).
  5. Feb 15, 2016 #4


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    If you think about how V and E depend on r, you can answer part (b) without knowing the charge. Of course there is nothing wrong with finding q if you want.
  6. Feb 15, 2016 #5
    Ah, I see what you are talking about.

    OP: using this strategy will take away most of the calculations you think you need to do.
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