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Electric Potential homework

  1. Apr 14, 2016 #1
    1. The problem statement, all variables and given/known data
    The electric potential a distance r from a point charge q is 195 V, and the magnitude of the electric field is 2870 N/C. Find the values of q and r.


    2. Relevant equations
    2780q=F
    Fr=W
    W/q=195

    3. The attempt at a solution
    Using substitution I got r=.00679 meters, which is correct. I can't substitute to find q... I have no idea what to do.
     
  2. jcsd
  3. Apr 14, 2016 #2

    gneill

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    Staff: Mentor

    What are the equations for the electric field and electric potential at a distance r from a point charge q? Your text or class notes must have these two equations as they are quite fundamental. Hint: They both involve the constant from Coulomb's law.
     
  4. Apr 15, 2016 #3
    Ik the equation F=(kQ1Q2)/(r^2) that's Coulomb's law. We never really did anything with fields or potentials... I found that electric field=F/q which I already have up there.
    I already have the electric field equation up there and the electric potential is U=(kQq)/r
     
  5. Apr 15, 2016 #4

    gneill

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    Staff: Mentor

    I suspect that your method for finding the distance r was actually flawed, and your correct result was a coincidence. I say this because one of your relevant equations, Fr = W, is not correct for this situation. If F is meant to be force and W the work done, then it doesn't hold if the force varies with the distance (F is not constant so W = F⋅d doesn't hold).

    Your new equation, U=(kQq)/r, gives the electric potential energy (in Joules) for a system of two charges. That's the energy required to bring them from infinity to a separation distance of r. What you need is the electric potential (in Volts) for a point charge at distance r.

    The equations that you're seeking are:

    ##E = k \frac{q}{r^2}~~~~~~~~~~## Electric field strength (N/C)

    ##V = k \frac{q}{r}~~~~~~~~~~~## Electric potential (Volts)

    You should verify that these equations are given in your text book.
     
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