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Homework Help: How is the Force related to Energy

  1. Oct 21, 2011 #1
    1. The problem statement, all variables and given/known data

    I'm confused as to how he makes the conclusion for the last portion of the question.

    2. Relevant equations

    3. The attempt at a solution

    I would have had the following thought process,

    [tex]\vec{E_{2}} = 0 = q\vec{F_{2}}[/tex]

    [tex]q \neq 0[/tex]

    [tex]\Rightarrow \vec{F_{2}} = 0[/tex]

    Is this incorrect?

    If so, where's the math behind his answer? I'm just not seeing it.

    Attached Files:

    Last edited: Oct 21, 2011
  2. jcsd
  3. Oct 23, 2011 #2


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    Staff Emeritus
    Science Advisor
    Homework Helper

    Well, the fixed voltage and charges (on the upper & lower plates) imply that the potential energy stored in the capacitor is a constant.

    Your professor is using the relation [itex]F = - \nabla \cdot U[/itex] to show that the constant potential implies zero force.
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