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Net electric field by two curved rods

  1. Dec 14, 2008 #1
    1. The problem statement, all variables and given/known data

    http://online.physics.uiuc.edu/cgi/courses/shell/common/showme.pl?courses/phys212/oldexams/exam1/sp08/fig4.gif [Broken]

    Two curved rods each have charge +Q uniformly distributed over their length. Which statement best describes the electric field due to these two rods at the midpoint between the two rods marked by an X ?

    (a) E points up.
    (b) E points down.
    (c) E = 0.

    2. Relevant equations

    It's more like geometry I think.

    3. The attempt at a solution

    I definitely thought E would point up since the rod at the bottom has extra length that has positive charge, which would affect electric field to the radially outward direction. But the answer is b, E points down. Could anyone explain the reason? Thanks in advance.
    Last edited by a moderator: May 3, 2017
  2. jcsd
  3. Dec 14, 2008 #2
    E should point up. If your problem statement and diagram are accurate, then the answer is wrong. The top section should cancel out exactly with the segment of the bottom rod that is directly opposite. This leaves two portions of the bottom rod to contribute to the direction of the field.
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