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Homework Help: Voltage across capacitor

  1. Jun 3, 2008 #1
    1. The problem statement, all variables and given/known data
    Voltage across 0.2-F cap is given by

    v(t)=4(1-Є^(-10t)) for t>0

    determine expression for the current i(t)

    2. Relevant equations

    i(t)=c (dv(t))/dt

    3. The attempt at a solution

    I'm really not sure were to begin with this problem what is the Є symbol and how do you take the derivative of it?
  2. jcsd
  3. Jun 3, 2008 #2


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    pmontone, you're in luck. That's not a Є symbol, but rather the constant e (= 2.718), the basis for the exponential function.
  4. Jun 10, 2008 #3
    First, look at your given equation v(t) and try to understand its meaning. What happens to v(t) as t increases? Is this equation representative of a "charging" or "discharging" capacitor? Now, based on this answer, how do you expect the current across the capacitor to behave over time? Use your equation and then analyze the result for physical accuracy. Remember that a fully-charged capacitor acts as an open circuit (no current).
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