# 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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Jan 19, 2004
220
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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.

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

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 ?