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Homework Help: Charge flowing through/across a Capacitor?

  1. Mar 6, 2015 #1
    1. The problem statement, all variables and given/known data
    Dry air has a resistivity of about 3*10^13 Ωm. A capacitor has square plates 10cm on a side separated by 1.2mm of dry air. The capacitor is charged to 250V. Assuming the potential difference does not change as the charge flows, what fraction of the charge will flow across the gap in 1 minute?

    2. Relevant equations
    C = Q/V, ρ = E/J, J = I/A = n*q*v, V = (ρ*L*I)/A

    3. The attempt at a solution
    I found a capacitance using ε, area and plate separation, multiplied this capacitance by voltage and then divided this found charge by the time period - but I strong feeling that I am on the incorrect path altogether.
  2. jcsd
  3. Mar 6, 2015 #2


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    Science Advisor
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    Gold Member

    What does that give you?
    Have you omitted some of the information you were given?
  4. Mar 6, 2015 #3
    No, I wrote out the exact question with all given information.
  5. Mar 6, 2015 #4


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    Staff: Mentor

    The air between the plates has a resistivity. That means it forms a resistor between the plates. They're asking about the resulting leakage current, and how much charge flows over the given time period.
  6. Mar 6, 2015 #5
    So, I calculated a resistance and then current from this and the voltage contained in the capacitor to get the Energy per second. But I need charge per second. What can I do?

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  7. Mar 6, 2015 #6


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    Staff: Mentor

    Your resistance value is not correct as your area value is not right. (10 cm)2 is not a square meter.

    The above error means that your current value is off by a few orders of magnitude.

    Current divided by time is not energy. What does the Ampere unit represent?
  8. Mar 6, 2015 #7
    Ah, I should have converted units first. My bad. That fixes the resistance. And then Current is Charge/Second. Thank you, I should have the correct answer now!
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