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Homework Help: Beginning physics, please help in finding the electric field for a uniform line charg

  1. Jan 29, 2010 #1
    Q:
    A uniform line charge that has a linear charge density λ = 4.4 nC/m is on the x axis between x = 0 to x = 5.0 m.
    (a) What is its total charge?

    (b) Find the electric field on the x axis at x = 6 m.

    (c) Find the electric field on the x axis at x = 11.0 m.

    (d) Find the electric field on the x axis at x = 240 m.


    To solve for a, I used the equation Q=lambda*length. My answer is 22nC, which is correct.

    To solve for the remaining portion of the question, I used E=kQ/[x^2], where I took x to be (L+a-x). L is defined as the length of the rod (5m), a is the difference between the given position and the rod (for b, 1m), and x is the distance from the origin to the midpoint of the rod (2.5m).
     
  2. jcsd
  3. Jan 29, 2010 #2

    ideasrule

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    Re: Beginning physics, please help in finding the electric field for a uniform line c

    The fact that you didn't get the right answer proves that E=kQ/[x^2] can only be used for point masses, not for extended objects. (The sphere is an exception, but that's a special case.) To solve this problem, you have to use calculus. What's the charge contributed by a length "dx" of rod a distance "x" from the given position?
     
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