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Finding electric field 3 cm along a thin rod.

  1. May 8, 2009 #1
    1. The problem statement, all variables and given/known data
    The figure shows a thin rod of length L = 5.0 cm with total charge Q = 8.4 nC. What is the magnitude of the electric field E at x = 3.0 cm?

    Figure in the attachment
    2. Relevant equations
    E = Kq/r^2
    K = 8.99 * 10^9 N m^2/C^2;
    r is the distance between the point and the charge.
    Field for a line of charge:
    E = KQ/(d*(d^2 + (L/2)^2)^(1/2))

    3. The attempt at a solution
    We have Q, L and "d". But I am still unable to get the answer.
    When I plug in the values as they are (as what I think they are), I get 64458.88
    Answer is: 2.7*10^5 N/C.

    But, when I use linear charge density lambda = Q/L.
    E = lambda/4*pi*epsilon_0 * distance, I get 2.2634 N/C. Do you think this is the right way to get it?
    I am not sure why that equation does not work.
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     

    Attached Files:

  2. jcsd
  3. May 8, 2009 #2

    rl.bhat

    User Avatar
    Homework Helper

    Field due to a small charge element at the point is given by
    dE = k*dQ/r^2.
    dQ = Q/L*dr
    Hence E = Integration(dE) = k*Q/L*Int(dr/r^2).
    Find the lower and upper limit of r,Substitute it in the integration to get the field.
     
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