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Homework Help: Energy stored in a Capacitator

  1. Aug 4, 2008 #1
    1. The problem statement, all variables and given/known data
    A parallel-plate capacitor has a charge Q and plates of area A. What force acts on one plate to attract it toward the other plate? Because the electric field between the plates is E = Q/A*epsilon, you might think the force is QE = Q^2/A*epsilon. This conclusion is wrong because the field E includes contributions from both plates and the field created by the positive plate cannot exert any force on the positive plate. Show that the force exerted on each plate is actually F = Q^/2*A*epsilon. Suggestion: Let C = epsilon*A/x for an arbitrary plate seperation x and note that the work done in seperating the two charged plates is W = integral of F times dx.

    2. Relevant equations
    U = V * dq

    3. The attempt at a solution
    I get really confused whenever work comes into it.

    I don't see why the answer cannot just be the electric field due to one plate times Q, i.e. E = Q/2A*epsilon, times charge Q which yields the required answer.

    I really don't know where to start with the work method.
  2. jcsd
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