Looking for some circuits help

[Born2bwire]

Originally posted by: Gibson486

Originally posted by: Born2bwire

Originally posted by: Gibson486

Originally posted by: Born2bwire

Originally posted by: Fiveohhh

Originally posted by: Born2bwire

Originally posted by: Fiveohhh
What I was doing was ((1/12.5k)+j(1000)(c))^-1

From here I plugged it out by hand and a calculator to get the imaginary part. I than set this imaginary part(which was negative since this portion is capacitive) equal to the reactance of j5000 from the inductor. doing this I get 2 real roots at 160nF and 40nF. Which I would think is not correct since there should only be one value as the circuit moves from inductive to capacitive it should only be purely resistive once.

s = j\omega, not j\nu.

Not following ya.

1/12.5k<---admittance of resistor
j(1000)(c)<--------admittance of cap

Adding them together and taking the inverse gets the impedance of the rc section. add to that the impedance of the inductor(1000)(5). Set to zero and solve for c.

I've been staring at this problem all day, so it's hard for me to get a fresh view on it, which is why I need some fresh eyes

You're taking the s-transform to find the impedances in phasor space. s is j*\omega, the angular velocity not frequency. \omega = 2*pi*frequency.

he may have not been introduced to laplace yet.....if he has been, it would make this so much easier

Can't imagine how he would know to replace C with 1/(j\omega C) and L with j\omega L without knowing Laplace transform.

welll....that is what sinosoidal capacitance looks like. You do not repalce C with 1/(jwC)....1/(jwc) is the defination of impedance for a cap in sinosoidal analysis.

Laplace tells you to replace anything related to the imaginary part with s.

The Laplace/s trasform is how you generate the complex impedances from the original differential forms. It seemed more natural to me that they would have introduced the general theory on how the phasor forms were derived first. But I guess they could do it either way.

Originally posted by: Fiveohhh

Originally posted by: Born2bwire

Originally posted by: Fiveohhh


I think I see what you're saying. It is supposed to be 1000 rads/sec. I just drew that up and didn't change the units.

Well in that case your answer is 0.16 \mu F and 0.04 \mu F.

Does it make sense that there are two solution? I'm still new to most of this AC, but I would think that as the circuit progresses from inductive to capacitive that it would only be in phase once.

In what sense is it progressing? You're changing the value of the capacitance and the general nature of the circuit, whether it is capacitive or inductive, is dependent upon the entire circuit as a whole. Depending upon the relationships, increasing the capacitance in one part of a circuit can end up making the entire circuit more inductive as is the case here.

 

[Gibson486]

Originally posted by: Born2bwire


The Laplace/s trasform is how you generate the complex impedances from the original differential forms. It seemed more natural to me that they would have introduced the general theory on how the phasor forms were derived first. But I guess they could do it either way.

Understood, but you do not learn laplace until your pass RC and RL circuits (first order circuits). You can easily solve those without laplace. When you move to RLC, things get messy without it, so they teach it to you then so you can "appreciate" it more.

 

[Gibson486]

Originally posted by: Fiveohhh

Originally posted by: Born2bwire

Originally posted by: Fiveohhh


I think I see what you're saying. It is supposed to be 1000 rads/sec. I just drew that up and didn't change the units.

Well in that case your answer is 0.16 \mu F and 0.04 \mu F.

Does it make sense that there are two solution? I'm still new to most of this AC, but I would think that as the circuit progresses from inductive to capacitive that it would only be in phase once.

I see what is going on here.

First off, when you get into linear systems, you will learn that since there is a 2nd order equation, there will be two solutions (remember the fundamental therom of algebra....???). In circuits, it is not different, but only one solution will make sense in you the cases you get introduced too.

2nd....you need to look at the circuit as a whole. It's asking how we get the phase to be the same when we get the last part of the circuit. Wether it is inductive or capacitive doe snot matter at this point. Just make the phase match.

 

[Fiveohhh]

Thanks for the help everyone, I was just under the assumption there was some linearity in the way the capacitance affected the circuit. That's what I get for assuming

Just fyi:
we started with in the time domain(very briefly), than moved into phasors, and now laplace. laplace was hinted to earlier on, but nothing in detail.

 

[Gibson486]

Originally posted by: Fiveohhh
Thanks for the help everyone, I was just under the assumption there was some linearity in the way the capacitance affected the circuit. That's what I get for assuming

Just fyi:
we started with in the time domain(very briefly), than moved into phasors, and now laplace. laplace was hinted to earlier on, but nothing in detail.

nah man...making those assumptions and getting them wrong is the first step to learning. Atleast you are doing your hw. When i was in college, I never did it. What happened? i ended up making those assumptions while on the test and I ended up with a very bad grade. I learned from my mistakes after the tests, but it was too late. I ended up getting a C- and when jobs saw this, they questioned my circuits ability. Do good in this class and the other classes will be cake (until you hit emag).

 

[Fiveohhh]

This is by far the hardest class I've taken. I have never worked this long on homework!

Again thanks for the help, I was not expecting to get so much on a seemingly obscure topic, late on a Sunday evening! Will ATOT ever cease to amaze me?

 

[Gibson486]

Originally posted by: Fiveohhh
This is by far the hardest class I've taken. I have never worked this long on homework!

Again thanks for the help, I was not expecting to get so much on a seemingly obscure topic, late on a Sunday evening! Will ATOT ever cease to amaze me?

I thought the same thing when i took it. The reason I did not do the hw was because it took too long Unfortuately, it only gets harder.

Here is a general rundown of how things will take place:

Circuits- hard
Electronics- easy if you understand kvl kcl
Linear Systems- depends on who teaches you...my experience was that it was easy
Emag- tough and hard
Engineering Statistics- Dear god....you will want to hang yourself
Communications theory- Easy...you only need 1% of what you learned in statistics.

 

[Fiveohhh]

Originally posted by: Gibson486

Originally posted by: Fiveohhh
This is by far the hardest class I've taken. I have never worked this long on homework!

Again thanks for the help, I was not expecting to get so much on a seemingly obscure topic, late on a Sunday evening! Will ATOT ever cease to amaze me?

I thought the same thing when i took it. The reason I did not do the hw was because it took too long Unfortuately, it only gets harder.

Here is a general rundown of how things will take place:

Circuits- hard
Electronics- easy if you understand kvl kcl
Linear Systems- depends on who teaches you...my experience was that it was easy
Emag- tough and hard
Engineering Statistics- Dear god....you will want to hang yourself
Communications theory- Easy...you only need 1% of what you learned in statistics.

Can't wait!!:roll: