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Homework Help: Electric Charge Questions, Please help

  1. Feb 14, 2012 #1
    Electric Charge Questions, Please help!!!

    1. The problem statement, all variables and given/known data
    I have three problems. I am absolutely desperate for answers. I cannot figure out what to do from the examples my book gives. It doesn't give examples in the book that relate to these problems, and I simply do not know what to do. It's very important that these get done fast. I'm basically screwed and this forum is my last resort, so please, please help.

    A small charge of -2 x 10^ -8 C is at the point x = 2 m, y = 0 on the x axis/ A second small charge of -3 x 10^ -6 C is at the point x=0, y= -3.0 m on the y axis. What is the electric force that the first charge exerts on the second? What is the force that the second charge exerts on the first? Express your answers as vectors.

    I know the answer is (-2.3 x 10^-5 N)i + (-3.5 x 10^-5 N)j, (2.3 * 10^-5 N)i + (3.5 * 10^-5 N)j, but I don't know how to get that.

    Point charges +Q and -2Q are separated by a distance d. A point charge q is equidistant from these charges, at a distance x from their midpoint. What is the electric force on q?

    A thin rod of length L is placed near a point charge q, with the nearest end a distance d from the charge and oriented radially as shown. The rod carries a uniform distribution of charge C coulombs per meter. Find the electric force that acts on the rod. (Hint: sum the force contribution dF = kqdq'r^2) due to each small charge dq' = Cdx on the rod to obtain the total force F = the antiderivative of dF.

    If you could help with any or all of these, I would much appreciate it.

    2. Relevant equations

    F = (8.99 x 10^9 N*m^2/C^2)(qQ/r^2)

    3. The attempt at a solution
    See above
  2. jcsd
  3. Feb 14, 2012 #2


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    Homework Helper

    hi calculusisrad! :smile:

    let's start with …
    using the formula, i don't understand why you're not getting that result :confused:

    how far have you got?

    did you get the magnitude ok?
  4. Feb 14, 2012 #3
    Re: Electric Charge Questions, Please help!!!

    Hi...The formula that u had provided in your post will give you only the magnitude of the FORCE vector...
    To get the direction of the resultant force, you must find the unit vector that represent the line joining the two charges... In this case , it is (3i - 2j)/√(9 + 4) = (3/√13)i - (2/√13)j.

    Multiply this vector with result that u get using that formula , you ll get the answer.
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