# RC / LC / switch questions

Discussion in 'Homework Help' started by KungFutZe, Oct 18, 2010.

1. ### KungFutZe Thread Starter New Member

Sep 27, 2010
2
0
Hi guys,
I'm new here so HI I'm Jean from PR.
I'm bit at a loss with my review exercises, my test is due tomorrow and I cant figure this out. I need help with 1) 2) 3) 5) 4) is the only one I have managed to figure out and I'm not sure 100% if it's been done correctly.

4)
What I did with that one is I did a source transformation on the 6mA I source to a 150V in series with the 25kohm resistor and found the voltage drop across the Capacitor
so I managed to squeeze a formula:
vc(t)= Vf+ (Vo-Vf)e^t/RC
vc(t)= 150 + (80 - 150) e^-1000t V
Vc(t) 150-70e^-1000t

Vf= 150 V
Vo=80 V
tau=RC= 1^-3
i/tau=1000

so
i(t)= C dv/dt
i(t)= (40nF) ((-1000)(-70e^-kt)
i(t)= 2.8 e^-kt A

So please correct me if this one has any errors, All I did was follow a similar example in my book with few modifications.

Excersises 1, 2,3,5 I'm out of ideas on how to begin with

I'll be posting my progress so you can correct or guide me thru this.

Hope you can help me in any way.

by the way I have tried to simplify 1,2,3 with the help of source transformations or going to a Thevenin / norton equivalent but I'm not so sure which route to take.

JM

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2. ### KungFutZe Thread Starter New Member

Sep 27, 2010
2
0
for exercise 1:
What I have so far is :
I did a source transformation and I got a 4000 V in series with a 400 Resistor and the 15mH L
for t<0
i(t) = Vg/R (1-e^-5tau/tau Vg (voltage of the source)
i(t) = 10-10e^-5
i(t) = 9.93

but v(t) ? I'm stuck there