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Dealing with time constant

  1. Jun 9, 2012 #1
    1. The problem statement, all variables and given/known data
    A student is designing an alarm circuit. She wants a time interval between closing a switch and the circuit becoming active. She decides to charge a 2200 mu (x10 -6) F capacitor through a resistor from a 5V supply. The potential difference across the cap. must rise to 4.3 V for the alarm to become active. Calculate the size of the resistor that she should connect in the circuit to acheive a 1 minute delay


    2. Relevant equations
    V=V(initial)e^-t/RC


    3. The attempt at a solution
    t should be 60 seconds
    ln(4.3/5)=-60/Rx2200x10-6
    RC=719.5831403
    R should be .. Oh .. That's too big.. I NEED HELP!
     
  2. jcsd
  3. Jun 9, 2012 #2
    Notice that the problem says the voltage across the capacitor should rise to 4.3 volts, yet when we plug in t=0 into your formula, you get 5 volts. I would consult notes and make sure that this is the correct formula for the capacitor's voltage.
     
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