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Homework Help: Electricity and Magnetism

  1. Sep 24, 2005 #1
    Electricity and Magnetism urgent!!!pls!!!

    An infinite long line of charge of constant charge density a is located near the line AB which carries uniform charges with charge density b. Suppose both two lines are in the same plane, calculate the electrostatic force exerted on the line AB.

    i dunno how to start......
    Last edited: Sep 24, 2005
  2. jcsd
  3. Sep 24, 2005 #2


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    Homework Helper

    Always start with a diagram, with labels
    and a coordinate system. Label the items
    described in the problems' situation,
    and the quantity being asked about.

    F=qE becomes F_vec = integral (dq E_vec).
    Gauss says the infinitely-long charge density
    carries a radial E-field that drops off as 1/r,
    so unless AB is parallel to it, E=E(r).

    If this sounds like gobbledy-gook, drop.
    If this sounds like simple stuff you already knew,
    the don't say "I dunno how to start".
  4. Sep 24, 2005 #3
    the diagram

    what is the limit of the integal then???
    a to b or 0 to b???

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  5. Sep 24, 2005 #4


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    you only need to integrate where the charges are ...
    Your Diagram should include a dx (label it!)
    and an E-field (presuming lamda_1 >0) at dx

    F_on_dq = (dq)(E_at_dq)

    If you integrate from 0 to A, use dq =0 there!
    between A and B , dq = "b"dx .
  6. Sep 25, 2005 #5
    what is the direction of dx & e-field then?
  7. Sep 25, 2005 #6


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    Come on! E-field due to the infinite line of charge
    points _away_ from the positive charges there.

    dx or dr is the coordinate (NOT vector) along line AB,
    which is a geometric way of keeping track of the charges.
    You're just adding F_vectors for each charge on AB ...
    the total amount of charge on line AB is :
    integral(dq) = integral(lamda_2 * dx) = lamda_2 * Length.

    But don't expect F_total to be E(a+b/2) * lamda_2 * L !
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