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A simple capacitor problem

  1. Dec 29, 2013 #1
    1. The problem statement, all variables and given/known data
    Thin parallel plate capacitor is connected to a U=600 V current source. Find the electric current flowing in the circuit if one of the electrodes is pulled at v=6 m/s velocity. The electrodes are square (l=10 cm), the distance between them is d=1 mm.

    2. Relevant equations
    C=ε0[itex]\frac{A}{d+vt}[/itex], C=[itex]\frac{q}{U}[/itex], I=[itex]\frac{dq}{dt}[/itex]

    3. The attempt at a solution
    I tried finding the current by first expressing q from those capacitor equations and plugging it into the dq/dt equation, but there is still time t left. I am not sure if the answer depends on time or is it possible to find some constant value of I?
  2. jcsd
  3. Dec 29, 2013 #2


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    No, the current is not constant. Find the time dependence of the current, and the current at t=0.

  4. Dec 29, 2013 #3
    OK thanks!
  5. Dec 30, 2013 #4


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    I assume you mean a voltage source.
  6. Dec 30, 2013 #5

    rude man

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    Hint: i = C dV/dt is not the complete story.

    The complete story is i = d/dt (CV).
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