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Homework Help: Calculate the total energy stored in the capacitor as a function of x

  1. Jul 16, 2011 #1
    1. The problem statement, all variables and given/known data


    calculate the total energy stored in the capacitor, as a function of x [4 marks]

    2. Relevant equations


    3. The attempt at a solution

    I don't know what I am supposed to do here, if I just substitute C=... into the above formulae, it probably won't give me the answer they are looking for. Is there another equation I should use that is not the equation I mentioned? I do not know what Q is
  2. jcsd
  3. Jul 16, 2011 #2
    what do they mean by x, L and d here? they don't seem to be constants. what is the statement of the problem???
  4. Jul 16, 2011 #3
    here are some earlier parts of the question:
    consider a parallel plate capacitor with square plates of side L and distance d<<L apart. The bottom plate lies on the x-y plane, and the distance d is parallel to z. A block of dielectric material with dimensions (L*L*d) can completely fill the space between the plates.

    Let us consider instead the dielectric to be composed of two materials glued together, material 1 with dielectric constant epsilon1 and dimensions 0.6L*L*d(in x, y and z directions respectively) and material 2 with constant epsilon2 and dimensions 0.4L*L*d. The dielectric is free to move as a single block without friction along the x axis, parallel to the plates inside the capacitor, and it can also move outside the capacitor. Let us define as x the distance between the dielectric and the edge of the plate, along the x axis. A potential difference V is applied between the plates, and we can neglect the electric field outside the plates.
  5. Jul 18, 2011 #4
    sorry, i was offline a bit long.
    the key to this problem is to calculate the capacitance of the parallel plate when the combined slab is pulled a distance x toward +X axis. if u draw the figure u will see that the combined capacitance can be calculated from the combination of a series of capacitances containing dielectric e1 and e2 and with that combination in parallel a capacitance containing dielectric of e0 . now calculate c1, c2 and c0 by using the formula c = e*area/distance of seperation. be careful while putting the expressions of area of each capacitor. find the combined capacitance and put the expression in U=05c*v2. a bit of algebra is needed. u can find U.
  6. Jul 18, 2011 #5
    is this the eqn you have written:

    [itex]C \ = \ \epsilon_0 L \frac{(x + 0.6L)\epsilon_1 \ + \ \ (0.4 - x)\epsilon_2}{d} [/itex]
  7. Jul 18, 2011 #6
    no, the resultant c is of the form (c1*c2/c1+c2) + c0. just figure it from the series and parallel laws of capacitance.
    Last edited: Jul 19, 2011
  8. Aug 20, 2011 #7

    yes, that is the equation
  9. Aug 20, 2011 #8
    thank you by the way - I'm sure that made what I meant a whole lot clearer
  10. Aug 20, 2011 #9


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    Staff: Mentor

    Looks to me that all three capacitors should be in parallel.
  11. Aug 20, 2011 #10
    yes, the three capacitors are in parallel
  12. Sep 3, 2011 #11
    what am I supposed to substitute in for V?
  13. Sep 3, 2011 #12


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    Staff: Mentor

    :confused: V is just V. The problem statement says that a potential V is applied. Your result should be an expression that involves the symbol V along with the other parameters of the problem.
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