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Homework Help: Electric Force Question

  1. May 14, 2009 #1
    1. The problem statement, all variables and given/known data
    Object I has a charge of + 3 x 10^6 coulomb and a mass of 0.0025 kilogram.

    a. What is the electric potential at point P, 0.30 meter from object I ?

    Object II, of the same mass as object I, but having a charge of + 1 x 10^6 coulomb, is brought from infinity to point P, as shown above.

    b. How much work must be done to bring the object II from infinity to point P ?

    c. What is the magnitude of the electric force between the two objects when they are 0.30 meter apart?

    d. What are the magnitude and direction of the electric field at the point midway between the two objects?

    The two objects are then released simultaneously and move apart due to the electric force between them. No other forces act on the objects.

    e. What is the speed of object I when the objects are very far apart?

    2. Relevant equations
    V = ke*(q/r)
    [tex]\Delta[/tex]PE = -WAB = -qEx[tex]\Delta[/tex]x

    3. The attempt at a solution
    a. V = ke*(q/r) = 9.0*10^4 V

    For b, would you use:
    -WAB = -qEx[tex]\Delta[/tex]x ?

    Your help is appreciated.
  2. jcsd
  3. May 14, 2009 #2
    You can, but usually you use W = qΔV. In this case this seems easier since you already know V at P from part a.
  4. May 14, 2009 #3
    My apologies... the answer was much easier than I had originally thought!

    W = Vq2

    I believe I have it from here.
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