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Homework Help: Capacitor charging time

  1. May 7, 2010 #1
    1. The problem statement, all variables and given/known data

    In a camera flash unit, a capacitor is charged up over a period of several seconds (from a battery). The capacitor is discharged very rapidly -- in about 1 millisecond -- into a gas tube producing a very brief, bright flash of light. How long will it take to charge a 195 µF capacitor to a potential difference of 125 volts with a current of 3.71 mA? Answer in seconds.

    I was given a hint to solve for Q first.

    2. Relevant equations

    Q= CV

    Q= 1-e^(t/rc)

    3. The attempt at a solution

    FIrst I calculated Q= CV to be 195 µF * 125v = 24.3 mC

    than I used ohms law to R= V/I = 33.693Kohms

    Than I put those values into Q= 1-e^(t/rc)
    and solved for t which gave me 162 msec

    which was incorrect so I'm note sure where I am going wrong

    any pointers?
  2. jcsd
  3. May 7, 2010 #2


    User Avatar

    In this problem the current is constant and not exponential. So you have simply
    Q = IT
  4. May 7, 2010 #3
    Thanks mate your a ledgend

    FIrst I calculated Q= CV to be 195 µF * 125v = 24.3 mC

    Q = IT solve for t = Q/I t = 6.55 seconds
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