Please with circuit ASAP, thanks first

Discussion in 'Homework Help' started by tinhnho, Oct 12, 2004.

1. tinhnho Thread Starter Active Member

Aug 31, 2004
34
0
hi everyone

can someone helps me out this proplem? just tell me which to start, i'm totally dont undestand this proplem. Thanks alot

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2. haditya Senior Member

Jan 19, 2004
220
0
writing all diff eqns via keyboard will be a pain... if u need immediate answers refer fundamentals of physics-by resnick, halliday, walker under the chapters capacitor and inductors fo all diff equations

remember- caps offer infinite impedance to steady(dc) current...inductors offer no impedance to dc currents.......
so at t=infinity, and the source is dc, caps can be disconnected from ckt(as the offer infinite impedence) and at t=0 caps behave like short circuits
inductors on the other hand behave like open circuits at t=0 and like short ckts when there is no change in current flowing thru it(ie at t=infinity for dc sources)

3. tinhnho Thread Starter Active Member

Aug 31, 2004
34
0
First thanks for respond. Herer what i did :
Case 1
i(t)= (v0/R)*e^(t/R*C)
i(0)= vo/R
i(infinity)= 0

Case 2
v(t)=v0*e^(-t/RC)
v(0+)= v(0)
v(infinity)= 0

Case 3
i(t)= -C*(dv/dt)= -C* d(v0*e^(t/R*C))/dt
i(0)= -C*d(v0)/dt
i(infinity)= 0

Case 4
v(t)=v0*e^(-t/RC)
v(0+)= v(0)
v(infinity)= 0

Case 5
i(t)= (i0 - v0/R)*e^(-t/RC)
i(0) = i0 - v0/R
i(infinity) = 0

Case 6
v(t)= i0*R + (v0 - i0*R)*e^(-t/RC)
v(0) = v0
v(infinity)= i0 * R

Case 7
i(t) = 0
i(0) = 0
i(infinity) = 0
Because in figure 2, from b to a is open circuit

Case 8
v(t)= 0
v(0) =0
v(infinity)= 0
Because in figure 2, from b to a is open circuit

are those right ?.i 've been working on it last couple hours,since it will be dued tomorrow.so tired and frustrating, can anyone help me out ? .thanks alot and also anyway thanks for reading.

4. haditya Senior Member

Jan 19, 2004
220
0
case 3 i(0)=Vo/R....cap behaves like a short
case 4 v(0)=0 , v(infinity)=Vo
for chargin v(t)=Vo(1-e^(t/RC))

with this data jus check up ur values

again if possible go thru circuits and inductance in resnick haliday and walker...it shud be easily available in your college library

5. tinhnho Thread Starter Active Member

Aug 31, 2004
34
0
thanks for telling that, but my library was out of that book. i got case 1 to case 6, but dont know case 7,8, Do you have any idea? thanks

6. haditya Senior Member

Jan 19, 2004
220
0
well ya the thing is relatively simple
case 7
at t=0, the cap begins to charge, and doesnt offer any resistance to current...is its a short and current is infinite
at t=infi, the cap is fully charged its open, current is zero
the diff eqn is dt=-CRdi/i , where i is the current thru cap
its easily obtainable using some simple manipulations and kirchoffs laws

c if this helps u

7. tinhnho Thread Starter Active Member

Aug 31, 2004
34
0
thanks for the answer. Last question, do i have to explain after switch from a -->b or from b --> a ? i guess i have too right ?

8. haditya Senior Member

Jan 19, 2004
220
0
i dun know...buti guess there is no harm in explainin