A Kirchoff's Law Question

JoeJester

Joined Apr 26, 2005
4,390
If the votage drop over each risistor adds up to the supply voltage and the current through each parrallel resistor adds up to total current (IT) it simply proves that your calculations are correct. It does not proof the law !
The voltage drop across each resistor is the same AS the source voltage. Nothing is added.

from https://isaacphysics.org/concepts/cp_kirchhoffs_laws

Kirchhoff's laws are fundamental to circuit theory. They quantify how current flows through a circuit and how voltage varies around a loop in a circuit. Kirchhoff's current law (1st Law) states that current flowing into a node (or a junction) must be equal to current flowing out of it. This is a consequence of charge conservation. Kirchhoff's voltage law (2nd Law) states that the sum of all voltages around any closed loop in a circuit must equal zero. This is a consequence of charge conservation and also conservation of energy.
 

WBahn

Joined Mar 31, 2012
26,398
Simulation result for Post #1 Question.

View attachment 169736

Simulation result for Post #5 Question.

View attachment 169737
Ask yourself if your results make sense.

You have 10 V across 4 Ω. Do you not expect the current to be 2.5 A?

Your sim is showing 2.27 A. Your 10 V source is also showing 9.09 V.

Always, always, ALWAYS ask if your results make sense.

These only make sense if the 10 V source has an internal resistance of 0.1 Ω.
 

WBahn

Joined Mar 31, 2012
26,398
Thanks for that I see were I was going wrong. You need to find the total current in this case 20A, then if you look at the amps going into each of the resistors it should add up to the total current.
But you have to be able to find the total current independently of the current in each of the resistors.

If you find the total current of 20 A by adding up the three given currents, then it proves nothing if you then "prove" that this is what you get by adding up the current in each resistor. Once again, circular reasoning.
 

WBahn

Joined Mar 31, 2012
26,398
I = I1 + 12 + 13 +...

V/Rt = V/R1 + V/R2 + V/R3 +...

KCL = Sum of all cuurrents entering a node = Sum of all currents leaving a node.

20A entering and 20A leaving.

View attachment 169741
NO! That proves NOTHING!

How did you find Rt?

If you found it by taking the reciprocal of the sum of the reciprocals of the parallel resistors, then you found Rt by ASSUMING that KCL applies. That's were that formula comes from!

You can't assume A to be true and then claim that because B is true as a result of that assumption that this means that A is actually true.
 

WBahn

Joined Mar 31, 2012
26,398
So then how do you prove KCL?
KCL and KVL are each a special case of Maxwell's Equations under specific conditions. KCL results from the constraint that the volume involved does not store net charge. KVL results from the constraint that the electric field is conservative. If either of those constraints are not met (and there are plenty of real-world situations in which this is the case), then the corresponding "law" simply does not apply and can be violated quite easily.

So the best you can do is show that the circuit obeys KCL or KVL, but even then you can't use KCL, directly or indirectly, to obtain the results by which you then claim to show that it obeys KCL. That's circular reasoning.
 

DrewStupid

Joined Nov 28, 2018
64
But you have to be able to find the total current independently of the current in each of the resistors.

If you find the total current of 20 A by adding up the three given currents, then it proves nothing if you then "prove" that this is what you get by adding up the current in each resistor. Once again, circular reasoning.
Ok ..i get the hint ! PROVE (on the lighter side)
 

JoeJester

Joined Apr 26, 2005
4,390
Ok Joe ...sorry !
I only pointed it out for you to read the stuff you write. If that didn't make sense to you, it sure didn't make sense to me, other then it could be an English as a Second Language issue.

Exercise care (due diligence) in all your work. WBahn always give the advice on "does your answer make sense?"and "Track your units." Those tidbits will help you in your future analysis.
 

DrewStupid

Joined Nov 28, 2018
64
I only pointed it out for you to read the stuff you write. If that didn't make sense to you, it sure didn't make sense to me, other then it could be an English as a Second Language issue.

Exercise care (due diligence) in all your work. WBahn always give the advice on "does your answer make sense?"and "Track your units." Those tidbits will help you in your future analysis.
What are you trying to tell me ? Must i post in my mother tongue. It is Afrikaans.
 

WBahn

Joined Mar 31, 2012
26,398
What are you trying to tell me ? Must i post in my mother tongue. It is Afrikaans.
Try not to take anything too personally. I have NO doubt that your English is far, far better than Joe's, or my, ability to communicate in your native tongue (though Joe might surprise us both). I know I for one respect anyone's ability to write in more than one language.

But also keep in mind that this IS an English-language forum. So don't be surprised when there are glitches in communication due to this being a second language for you (heck, there are plenty of glitches amongst us native-English speakers!). When someone points something out, look at it as both an opportunity to clear up that particular miscommunication, but also an opportunity to improve your overall ability to communicate in English. I assure you, many of us are continually doing the same thing.
 

DrewStupid

Joined Nov 28, 2018
64
Try not to take anything too personally. I have NO doubt that your English is far, far better than Joe's, or my, ability to communicate in your native tongue (though Joe might surprise us both). I know I for one respect anyone's ability to write in more than one language.

But also keep in mind that this IS an English-language forum. So don't be surprised when there are glitches in communication due to this being a second language for you (heck, there are plenty of glitches amongst us native-English speakers!). When someone points something out, look at it as both an opportunity to clear up that particular miscommunication, but also an opportunity to improve your overall ability to communicate in English. I assure you, many of us are continually doing the same thing.
Thank you Sir.. i could not phrase it better.
 

JoeJester

Joined Apr 26, 2005
4,390
@DrewStupid WBahn put is exactly correct. Yes, I know english is a very tough language to write and speak. That is why people will seek more information for clarity.

No harm was intended.

This is a help us help you type forum. Sometimes the discussions go off on a wild tangent from an interpretation of something that wasn't what the original poster meant.
 
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