# Impedance Matching - value of R needed for max Vc+Vr?

#### RogerMc

Joined Feb 19, 2014
12
We've been discussing impedance a bit in AC class lately as related to RC circuits.

We've been given an extra credit problem seemingly associated with this derived variation of the Z equation.
$$R=sqrt {(Z)^2 - {(X_{C})}^2}$$

The question is as follows:
What value of R will give maximum value of Vc & Vr?

It's extra credit so I'm having trouble getting things setup to solve -- a stretch. I can't seem to understand how I would determine the maximum of Vc & Vr.

Would the max be? $$sqrt {(V_{C})^2 + ({V_{R}})^2$$

Can anyone offer a suggestion to get me pointed in the right direction, please?

Thanks!

#### t_n_k

Joined Mar 6, 2009
5,447
The question is curious. Presumably this is in relation to a series RC circuit. Can both be maximum (simultaneously)? Surely one will be maximum when the other is minimum.

#### RogerMc

Joined Feb 19, 2014
12
The question is curious. Presumably this is in relation to a series RC circuit. Can both be maximum (simultaneously)? Surely one will be maximum when the other is minimum.
Yes, correct this is a series RC circuit. The Vc and Vr are 90 degrees out of phase so the max won't occur at the same point. What is important is the max of Vc+Vr.

#### WBahn

Joined Mar 31, 2012
25,099
You need to be clear on what the problem is. In your original post you said the max of "Vc & Vr" and in the last post you said the max of "Vc + Vr". Assuming the latter, are you talking about the max of the rms voltages across the two? That has little physical meaning. But if you are talking about the max instantaneous voltage that makes even less sense because you say it is a series circuit and therefore the sub of the two voltages is always equal to the supply voltage.

It would be helpful if you both provided a circuit diagram and clearly stated exactly what the problem is. Then you need to show YOUR attempt to solve YOUR extra credit assignment.

#### RogerMc

Joined Feb 19, 2014
12
You need to be clear on what the problem is. In your original post you said the max of "Vc & Vr" and in the last post you said the max of "Vc + Vr". Assuming the latter, are you talking about the max of the rms voltages across the two? That has little physical meaning. But if you are talking about the max instantaneous voltage that makes even less sense because you say it is a series circuit and therefore the sub of the two voltages is always equal to the supply voltage.

It would be helpful if you both provided a circuit diagram and clearly stated exactly what the problem is. Then you need to show YOUR attempt to solve YOUR extra credit assignment.

#### WBahn

Joined Mar 31, 2012
25,099
We don't delete threads unless they violate the ToS in such a way that not deleting them is worse then letting them stand.

We are more than willing to help, but you need to show us YOUR efforts. That is what gives us something to go on -- we can see what you are doing right and what you are doing wrong, once we understand what it is that you are trying to do in the first place.

#### RogerMc

Joined Feb 19, 2014
12
We don't delete threads unless they violate the ToS in such a way that not deleting them is worse then letting them stand.

We are more than willing to help, but you need to show us YOUR efforts. That is what gives us something to go on -- we can see what you are doing right and what you are doing wrong, once we understand what it is that you are trying to do in the first place.
I am finding your tone unpleasant and regret asking for help. Very sorry I wasn't adequately clear in the first place, must be because I am just learning.

#### WBahn

Joined Mar 31, 2012
25,099
I am finding your tone unpleasant and regret asking for help. Very sorry I wasn't adequately clear in the first place, must be because I am just learning.
I'm sorry that you find it unpleasant to be asked to show some effort to solve your own problem, but that doesn't change the expectation. Not being clear is not a big issue, simply an issue that needs to be resolved. Having that it was unclear pointed out is how you learn. The next step is to try to apply what you've learned and make a more clear explanation of what it is you are trying to do. That, and to show YOUR efforts to solve YOUR homework. If you are expecting to just have someone give you the solution then you have, indeed, come to the wrong place. If you are willing to put in the effort to learn the material, with our guidance, then you have come to the right place. The difference between those two is all up to you.

#### RogerMc

Joined Feb 19, 2014
12
Last time when I asked for help on this forum, I supplied my work in progress and was admonished for providing a solution. This time I am confronted with a condescending attitude.

I have indeed come to the wrong place, but not for any of the reasons you cite.

If you have the capability, please remove my account and all history.

#### WBahn

Joined Mar 31, 2012
25,099
Again, we don't delete posts or accounts without compelling cause -- there are a number of reasons for this not least of which is that it messes with post counts and internal references. You are certainly free to change your notification preferences so that you won't be notified of future replies. You are also certainly free to never return, if that is your desire.

#### RogerMc

Joined Feb 19, 2014
12
Again, we don't delete posts or accounts without compelling cause -- there are a number of reasons for this not least of which is that it messes with post counts and internal references. You are certainly free to change your notification preferences so that you won't be notified of future replies. You are also certainly free to never return, if that is your desire.
I have asked nicely but you have failed to honor my request.

I am notifying you that my threads are a copyrighted work and you are not authorized to publish this work online.

To rectify this infringement of my rights, I require that you remove my copyrighted material from this website.

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#### t_n_k

Joined Mar 6, 2009
5,447
And while you are at it ...... Any chance of the mods removing any or all of my posts that I now regard as unduly embarrassing?

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#### WBahn

Joined Mar 31, 2012
25,099
I have asked nicely but you have failed to honor my request.

I am notifying you that my threads are a copyrighted work and you are not authorized to publish this work online.

To rectify this infringement of my rights, I require that you remove my copyrighted material from this website.
This would be like going to a book publisher and asking them to publish and distribute your work and then demanding that they go out and collect and destroy all of the copies of it. Perhaps you should have read the Terms of Service you agreed to (which you indicated that you HAD done as part of registering).

In particular, "You are granting us with a non-exclusive, permanent, irrevocable, unlimited license to use, publish, or re-publish your Content in connection with the Service. You retain copyright over the Content."

#### MrAl

Joined Jun 17, 2014
6,968
I am finding your tone unpleasant and regret asking for help. Very sorry I wasn't adequately clear in the first place, must be because I am just learning.
Hi there,

No need to get upset over a simple issue here. It's just that no one here seems to be able tu understand just what you are asking for exactly. My guess is that you have paraphrased the original question and thus left out some details, but again dont get upset over this because this happens a lot. A good idea would be to post the original question verbatim so we can all read it, and if you know how to start the problem solution then you should show that too so we all know how your instructor wants you to proceed, or at least how you believe we should proceed.
Sometimes it just takes a few back and forth relies to get all the required info out there, and then people start to suggest ways to solve the problem. It sounds like it is an easier type of problem so im sure you'll find help if you keep at it.

Look at it this way...if you take the time to post a few more times and eventually get the help you need, you will have managed to solve two problems: the first is the original problem, and the second is the problem of good personal internet communications.

I for one would like to hear more about the original problem and if you can, how you would start to find a solution. If you dont know how to start then i am sure people here would be happy to help. Keep in mind though that you do have to be at least a little tolerant of other peoples web personalities, just as they have to be at least a little tolerant of yours