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Transient response

  1. Nov 14, 2014 #1
    1. The problem statement, all variables and given/known data
    In given network (voltage source V & switch K) , a steady state is reached with the switch k open .At time t=0 , switch k is closed. Find voltage across C before switch is closed? and i1 &i2 at t=0+.Also values of di1/dt and di2/dt at t=0.


    2. Relevant equations


    3. The attempt at a solution
    since , before closing the switch steady state has reached . so , capacitor will be open & inductor open. so current i1(0-) will be V/R1+R2 =i1(0+). And current i2(0+) =V/R3.
    Now for voltage across capacitor before switch is closed , it'll be i1R2-Ldi1/dt but di1/dt is coming out be zero. so voltage should be i1R2. Also di2/dt is coming out to be zero.at t=0- .. Please check if it is correct or not??
     

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    Last edited by a moderator: Nov 17, 2014
  2. jcsd
  3. Nov 15, 2014 #2

    NascentOxygen

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    I would give only half marks for that, because it's lacking an essential pair of parentheses.

    Not correct.
     
  4. Nov 15, 2014 #3
    which parantheses you are talking about?
    Is the mistake occuring because I did'nt consider that a current is flowing through inductor when the switch was open while finding i2??
     
  5. Nov 15, 2014 #4

    NascentOxygen

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    Determine the voltage at t=0+ on each end of R3, take their difference and divide by R3. That will be i2 at that time.
     
  6. Nov 15, 2014 #5
    at t=0+ , the capacitor will be shorted..!! so potential difference across R3 will be potential difference across the inductor's branch..Is'nt it?
    i1(0+)R2- Ldi2(0+)/dt.. how we will find di2(0+)/dt..??
     
  7. Nov 15, 2014 #6

    NascentOxygen

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    What is the capacitor voltage at t = 0- ?
     
  8. Nov 16, 2014 #7

    rude man

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    First, the problem does not define when di1/dt and di2/dt are to be computed. Is it at t0- or t0+?
    So for the moment that leaves only i1(0+) and i2(0+), plus VC(0-) to be computed.
     
  9. Nov 16, 2014 #8

    rude man

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    I will limit my next reply to the verification of those computations only, if they are forthcoming..
     
    Last edited: Nov 16, 2014
  10. Nov 17, 2014 #9
    it'll be i1(0-)R2 where i2(0-) = V/R1+R2
     
  11. Nov 17, 2014 #10

    NascentOxygen

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    With the subscript fixed and an essential pair of parentheses added, that would be correct. :confused:

    So what would be the capacitor voltage at t=0+ ?

    Meaning the current i2(0+) would be ...?
     
  12. Nov 17, 2014 #11
    sorry , actually I did a mistake in subscript its i1(0-) = V/R1+R2 , i2(0-) will be zero..
     
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