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Homework Help: Conceptual: two positive point charges value of E

  1. Dec 7, 2013 #1
    1. The problem statement, all variables and given/known data

    Both Q1 = Q2 = +5E-6 and are harnessed to screen and in a vacuum.

    ----Q1---A----Q2------B--------P this is x axis, A and B are distances

    2. Relevant equations


    3. The attempt at a solution

    So the E field at point P is KQ1/(A+B)^2 + KQ2/(B)^2

    My question is Why isnt Q1 E field affected by the repulsion of the two charges. IE if photons carry the force of the charged particle, why arent the photons affected by the presence of Q2 emitting photons 360 degrees. So to the left of Q2, Q2 sends its signed wave to the left while a same signed wave is moving to the right from Q1. Shouldnt they interact, that is how they would repel if not anchored to the screen.
  2. jcsd
  3. Dec 7, 2013 #2
    In classical electrodynamics, electric fields are governed by the principle of superposition, i.e., the total field at any point in space is the sum of the fields due to individual sources. In quantum electrodynamics, the principle of superposition holds until fields become very strong. This is known as the Schwinger limit.
  4. Dec 7, 2013 #3
    Ok i see it now. The two charges exert destructive interference, but the waves dont disappear. Once Q1 wave passes Q2 it combines and it diminishes as 1/r^2.
    Last edited by a moderator: Sep 25, 2014
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