how to find voltage in this resistor circuit (capacitive)

Discussion in 'Homework Help' started by safi chn, Feb 20, 2016.

  1. safi chn

    Thread Starter New Member

    Jan 30, 2016
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    ok , so i know the first step is to find t (tau)=RC but should i multiply C with the 6 ohms or with the 3 ohms ? and after that what is the next step ? the voltage signs are on the same place as the capacitor 2mf . plus up negative down 1450983374.jpg
     
  2. etpineda

    New Member

    Feb 20, 2016
    1
    0
    There are two time constants, one is before switch is closed that is 6 ohms x 2 mF, after switch is closed, the 3 ohms will be in parallel with 6 ohms... new time constant will be computed...
     
  3. dannyf

    Well-Known Member

    Sep 13, 2015
    1,782
    360
    Easy: before the switch is closed, the capacitor is charged to 6ohm*9a=54v.

    Right after the switch is closed, that voltage doesn't change. so the instantaneous wattage on the 3ohm resistor is 54^2/3=...
     
  4. WBahn

    Moderator

    Mar 31, 2012
    17,737
    4,789
    If you are unable to determine the time constant by inspection, then it would be good practice to fall back and analyze the circuit by finding the differential equation and seeing what the time constant works out to be. Then see if you can see how that could have been found by inspection.
     
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