Transient response

1. Nov 14, 2014

ranju

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. Nov 15, 2014

Staff: Mentor

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

Not correct.

3. Nov 15, 2014

ranju

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??

4. Nov 15, 2014

Staff: Mentor

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.

5. Nov 15, 2014

ranju

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..??

6. Nov 15, 2014

Staff: Mentor

What is the capacitor voltage at t = 0- ?

7. Nov 16, 2014

rude man

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.

8. Nov 16, 2014

rude man

I will limit my next reply to the verification of those computations only, if they are forthcoming..

Last edited: Nov 16, 2014
9. Nov 17, 2014

ranju

it'll be i1(0-)R2 where i2(0-) = V/R1+R2

10. Nov 17, 2014

Staff: Mentor

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

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

Meaning the current i2(0+) would be ...?

11. Nov 17, 2014

ranju

sorry , actually I did a mistake in subscript its i1(0-) = V/R1+R2 , i2(0-) will be zero..