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Finding the time constant given % of max value

  1. Apr 8, 2010 #1
    1. The problem statement, all variables and given/known data

    After how many time constants does the current in the figure below ( http://www.webassign.net/gianpse3/30-06.gif ) reach the following values?

    (a) within 67 percent of its maximum value


    (b) within 6.7 percent of its maximum value


    (c) within 0.67 percent of its maximum value


    2. Relevant equations
    tao (time constant) = L (inductor) /R (resistance)
    I = Vo/R (1 - e^-time/tao)
    Vc = V(1-e^-t/RC)


    3. The attempt at a solution

    Ok so I know that tao is the time required for the current (I) to reach the % of max value (1-1/e). However, given NO numerical values and just constants, I can't figure out how to find how many time constants. I tried taking the natural log of both sides and figuring out tao that way, but then I dont know the time t. I know that it is discharging because it isn't connected to the voltage anymore. please help!
     
  2. jcsd
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