Author Topic: Does Kirchhoff's Law Hold? Disagreeing with a Master  (Read 50732 times)

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Offline bsfeechannel

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #1000 on: April 21, 2019, 02:00:51 am »
I did your experiment. At the begin of the experiment the EMF must have been in the air gap between both secondary terminals.

Yep.

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However, as soon as I connected the voltmeter, I got the feeling the EMF jumped into my voltmeter

Yep.
 
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(as it is 10MOhm resistive.  As you say EMF is path dependent).

Under a varying magnetic field it is. Where the magnetic field doesn't vary, i.e. outside the transformer, it is not.

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With the 10k resistor I was expecting it to sit now in the Voltmeter AND in the resistor at the same time - according to your thesis.

I do not have a thesis. Maxwell does. You need to get along with him more often.

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And I always thought, the EMF is defined as the tangential force per unit charge in the wire integrated over length, once around the complete circuit/loop! But not at all, it was hiding in the resistor and my voltmeter!

You clearly thought wrong, I regret. EMFs will never "hide" inside (static) wires (considered as ideal, that is).

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But you confused me a bit later, when talking about the [elp]. The [elp] being the EMF (I guess the Israeli Prof meant [Epsilon loop] with this abbreviation). You tell me it’s invisible and can’t be measured? Although you told me before it’s sitting in the resistor and how to measure it?

It's understandable that you are confused. Electromag is unintuitive. It requires diligent study. You didn't pay attention when I said that the EMF can't be measured as a SEPARATE ENTITY in this circuit.

Once you measure the voltage on the 10Mohm of your meter or the 10kohm load, you won't find this voltage anywhere else in the circuit. All that you'll find is a wire with (ideally) zero volts. This violates Kirchhoff's law which demands that a second voltage be found around the path of the wire to balance the one you found across the resistor.

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On the other hand you say today’s convention is to put voltage drops AND EMFs on the left side. But insist at the same time to keep the -dFlux/dt on the right side? -dFlux/dt being the EMF.
Looks like you don’t consider -dFlux/dt as EMF, but rather the EMF as part of the line integral E ds?

Again you left out the phrase FOUND IN THE CIRCUIT. To the right are the EMFs FOUND IN THE CIRCUIT, i.e. directly measured. -dΦ/dt is an EMF that cannot be found in the circuit as a separate entity, so it must be on the right side of the equation.

But you are on the right track. You are at least questioning, instead of trying to disprove Faraday's law with pseudo-scientific assumptions, which is a good thing.

You'll understand electromagnetism sooner than those who thanked you.
 

Online ogden

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #1001 on: April 21, 2019, 02:15:01 am »
Once you measure the voltage on the 10Mohm of your meter or the 10kohm load, you won't find this voltage anywhere else in the circuit. All that you'll find is a wire with (ideally) zero volts.

Wow. How transformers with multi-tap secondary windings work then?

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You'll understand electromagnetism sooner than those who thanked you.

 :-DD

Right. Be prepared to become enlightened. Those who thanked you are lost souls - nonbelievers. They do not believe that Dr.Lewin is always right even when he is not.
« Last Edit: April 21, 2019, 02:35:42 am by ogden »
 

Offline bsfeechannel

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #1002 on: April 21, 2019, 02:35:33 am »
All that said, let me add that I have no hard feelings toward the site. As a matter of fact, apart from this 'incident', I believe it's one of the most relaxing places where to talk about electronics.

Yay! Sredni is back!

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But the moderator screwed up, and you lost a contributor.

Sredni, man, can't we leave this moderation imbroglio behind? After all, your access has been reinstated, you were allowed to rant on what happened, the thread is still open for us to discuss the subject and no one is bothering us.

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I am still leeching, tho.

You're not punishing Dave, you are punishing us. Dave and Simon have more important things to worry about than this "electromagnetism rubbish". So leave them be.

If you have something to contribute to the discussion, please do. We are in desperate need of people who can help us destroy bullshit thinking in engineering.
 

Offline bsfeechannel

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #1003 on: April 22, 2019, 03:27:43 pm »
This was exactly my point. It doesn't make sense to talk about a voltage source as purely being an impedance.

A voltage source is NOT an impedance, much less "purely". Read Feynman lectures volume II chapter 22.

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When analyzing the circuit it makes more sense to think of it as an equivalent circuit. This separates the voltage source from the impedance part, allowing both to remain the same no matter the load.

Or is the use of equivalent models somehow forbidden in circuit analysis?

What determining the equivalent circuit of a battery has to do with measuring an impedance in an AC circuit under a varying magnetic field?

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What you describe is called mutual inductance in circuit analysis. As soon as inductors start sharing magnetic fields they also start sharing there inductance too.

So what causes the change in their behavior is the _ _ _ _ _.

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Here is a quick summary of how this works: https://physics.stackexchange.com/questions/119638/choosing-sign-for-kvl-mutual-inductance

So now how does the transformer secondary not have 90 degree phase shift if its an inductor? Because the voltage is all coming from mutual part of inductance and this inductance only cares about the current in the primary.

As you confirm, not only the secondary doesn't behave like an inductor (there's no 90 degree phase shift), but also it is not even an impedance: because the voltage on the secondary is a function of something external to itself, which is in fact the _ _ _ _ _ generated by the current in the primary.

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The current trough the load resistor actually affects the voltage of the primary side.

How can this be, once the primary is connected to a voltage source?

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Once all of these currents and voltages are summed up neither the primary or secondary have 90 degree phase shifts anymore. In an ideal 1:1 transformer with no leakage the currents and voltages are actually all perfectly in phase (If the load is a  resistor like in your example) or 180 out of phase depending on what way around you connect the coil.

So now not even the primary can be identified as an inductor anymore.

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All my hair is still in its place as there is no need to pull it out over a concept that works just fine, or is the use of mutual inductance somehow forbidden?

Forbidden? Why? It perfectly demonstrates what I said before: if you don't understand the underlying physics of electromagnetism you'll be limited in your ability to design and analyze circuits.

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So then back to the original question. Under what circumstances is a wire not an inductor and when it is?

If you read your own words again you'll see that you've already answered your question.
 

Offline bsfeechannel

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #1004 on: April 22, 2019, 04:16:20 pm »
:-DD

Right. Be prepared to become enlightened.
Since you rejected my baby-stepped method for guiding you to enlightenment, I'll be forced to reveal the shocking truth to you. The conservation of energy holds for when Kirchhoff fails because the energy the resistor is dissipating comes from the fuh...., the fuh-fieh..., the fie... No, I can't. I can't put you through this emotional sacrifice. I have scruples.

« Last Edit: April 22, 2019, 04:21:52 pm by bsfeechannel »
 

Online ogden

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #1005 on: April 22, 2019, 04:29:06 pm »
Right. Be prepared to become enlightened.
Since you rejected my baby-stepped method for guiding you to enlightenment, I'll be forced to reveal the shocking truth to you. The conservation of energy holds for when Kirchhoff fails because the energy the resistor is dissipating comes from the fuh...., the fuh-fieh..., the fie... No, I can't. I can't put you through this emotional sacrifice. I have scruples.

Again you show that you can't even read  :palm:

I did not ask to tell where energy comes from. I ask you to show law of conservation of electrical energy using Maxwell's equations. Apparently you are afraid of what comes next after you do it.
 

Offline bsfeechannel

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #1006 on: April 22, 2019, 05:42:17 pm »

Again you show that you can't even read  :palm:

I did not ask to tell where energy comes from. I ask you to show law of conservation of electrical energy using Maxwell's equations. Apparently you are afraid of what comes next after you do it.

Yes. I'm afraid you won't understand. Since you don't want to know where the energy comes from, it'll be impossible for you to understand it using Maxwell's equations, or whatever.
 

Online ogden

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #1007 on: April 22, 2019, 07:04:24 pm »
Since you don't want to know where the energy comes from, it'll be impossible for you to understand it using Maxwell's equations, or whatever.

Do not confuse public forum with private chat, tell for those who you think will understand.

To cut the crap: KVL is based on conservation of the charge which is based on law of conservation of energy. As soon as you write energy_generated_inthe_loop = energy_dissipated_inthe_resistor equation, KVL can be derived from it. Seems, you know it well and seems I know what abbreviation "BSFEE" means.
« Last Edit: April 23, 2019, 07:26:56 pm by ogden »
 

Offline Berni

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #1008 on: April 22, 2019, 09:44:46 pm »
This was exactly my point. It doesn't make sense to talk about a voltage source as purely being an impedance.

A voltage source is NOT an impedance, much less "purely". Read Feynman lectures volume II chapter 22.
...
What determining the equivalent circuit of a battery has to do with measuring an impedance in an AC circuit under a varying magnetic field?
...
So what causes the change in their behavior is the _ _ _ _ _.
Yes exactly my point that a voltage source is not an impedance.

In that chapter 22 that you mention it clearly shows that induced voltage acts like a voltage source. So including the induced voltage when calculating impedance of a component is just as nonsense as directly calculating the impedance of a voltage source using the voltage across its terminals.

So putting it all together:
2 terminal components can't have multiple impedance at the same moment in time.
2 terminal components can't have multiple different currents running trough it at the same moment in time.
Since impedance relates current to voltage this also means the before mentioned impedance can only have one well defined voltage across it. But this also defines the path of the voltage as being trough this impedance, hence why it doesn't go against Maxwell. The induced voltage didn't disappear, but the component it self doesn't care about it. Its only the circuit around it that sees the voltage (Since the loop needs to be closed) as this voltage adds on to whatever voltage is presented on the terminals. Much like the equivalent model of a voltage source looks on its terminals.

is there any reason why you replaced the world field with underscores? Its not exactly a secret that magnetic fields are what makes inductors work.

...
As you confirm, not only the secondary doesn't behave like an inductor (there's no 90 degree phase shift), but also it is not even an impedance: because the voltage on the secondary is a function of something external to itself, which is in fact the _ _ _ _ _ generated by the current in the primary.
...
How can this be, once the primary is connected to a voltage source?
...
So now not even the primary can be identified as an inductor anymore.
...
Forbidden? Why? It perfectly demonstrates what I said before: if you don't understand the underlying physics of electromagnetism you'll be limited in your ability to design and analyze circuits.
...
If you read your own words again you'll see that you've already answered your question.

Exactly it doesn't behave like an inductor anymore, but instead behaves like a coupled inductor.

But it doesn't mean its inductance simply disappeared. The same magnetizing current is still needed to hold up the field, but because the two inductors are sharing the same flux means they can also share the magnetizing current. Since the resistor on the secondary can't provide a source of reactive current means that all the magnetizing current comes from the voltage source on the primary. The resistive load does however cause a in phase current that makes a opposing field in the secondary that affects the field in the primary, where it induces voltage according to Faradays law. Since there is a voltage source forcing a well defined voltage across the primary, this instead draws extra current to correct this field. This extra  in phase current is what drags the total current back away from lagging 90 degrees. As for the secondary, it has no need for out of phase current, because all of its magnetizing current is provided by the primary.

So the inductance didn't go anywhere. The effect of inductance is simply buried under the other currents.

So yeah, sorry i still don't see the exact circumstance when a piece of wire stops being an inductor. Or does becoming a coupled inductor not count as being inductive? If so please explain why.

At what point is my inductance explanation inconsistent with physics?
 

Offline bsfeechannel

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #1009 on: April 23, 2019, 07:01:23 pm »
Today at 08:42:17 am » Insert Quote
You are ignoring this user.

It seems that you can't ignore me. Thank you. No wonder. I am managing to destroy mercilessly and systematically the myths you always held dear. But cheer up, dude. You'll be empowered by your new disbelief.

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Do not confuse public forum with private chat, tell for those who you think will understand.

I've already done it. But you rejected it unceremoniously.

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To cut the crap: KVL is based on conservation of the charge which is based on law of conservation of energy.

Charge conservation and energy conservation are two separate principles. Both can be derived for electromagnetism from Maxwell's equations.

Let's start with charge conservation, which is easier for a circuit-head guy like you to understand. You can find it demonstrated here.

In the picture below you can see how KCL fails, but charge conservation holds.



The sum of currents going in and coming out of the volume represented by the closed dashed line to the left is different from zero, violating KCL. But conservation of charge holds because the excess charge is being stored in the body inside the volume. On the volume to the right KCL holds, because no charge is being stored in the volume, so all the currents must add up to zero.

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As soon as you write energy_generated_inthe_loop = energy_dissipated_inthe_resistor equation, KVL can be derived from it.

Energy conservation in electromagnetism is a little bit more complicated, and can be derived, as in the case of conservation of charge, from the modified (by Maxwell) Ampère's law, however you need to understand that the energy that's powering the resistor is not coming from a component in the circuit. While you don't accept this you'll be looking in vain around the wire for an EMF that will balance the voltage drop, and smashing your head against the wall.

And as in the case of charge conservation, we can also have KVL failing while energy conservation holds.

The difficulty Mehdi expressed in his first video comes exactly from not understanding this. For him, KVL can never fail, because that would represent a violation of energy conservation.

Nothing could be further than the truth.

EDIT: corrected the charge conservation equation in the attached picture (had forgot the minus sign).
« Last Edit: April 24, 2019, 10:57:35 am by bsfeechannel »
 

Online ogden

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #1010 on: April 23, 2019, 08:15:50 pm »
Energy conservation in electromagnetism is a little bit more complicated, and can be derived, as in the case of conservation of charge, from the modified (by Maxwell) Ampère's law, however you need to understand that the energy that's powering the resistor is not coming from a component in the circuit.

Energy is delivered by the component of the circuit - wire loop. Most sources of electrical energy are energy conversion devices - so what? Instead of talking about law of electrical energy conservation, you play games around meaning of the word "generator". I can rewrite equation that needs to be filled-in with your wizdom if you are so picky about words I use: energy_delivered_bythe_wire_loop = energy_dissipated_inthe_resistor.
 

Online ogden

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #1011 on: April 23, 2019, 11:13:02 pm »
Let's start with charge conservation, which is easier for a circuit-head guy like you to understand. You can find it demonstrated here.

In the picture below you can see how KCL fails, but charge conservation holds.



The sum of currents going in and coming out of the volume represented by the closed dashed line to the left is different from zero, violating KCL. But conservation of charge holds because the excess charge is being stored in the body inside the volume.

It's just next fallacy of yours. Those two round objects on the left together form charged capacitor and wire between them - load where energy that is stored in the capacitor, dissipates. KCL holds.
« Last Edit: April 24, 2019, 01:37:41 am by ogden »
 

Offline Berni

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #1012 on: April 24, 2019, 01:24:52 am »
I dont see how charge is conserved inside the dotted circle. Curent is charge divided by time anyway.

But yeah i can see what the point is, an example of KVL not holding. We KNOW that KVL is not a law of physics. It holds in most cases but not all, it only always holds in lumped circuit meshes. Hence why it cant be directly slapped onto any physical circuit, much like the rest of circuit analysis math. Go ahed and try to find a quote of me claiming it always holds in physical circuits in any of this thread
 

Online ogden

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #1013 on: April 24, 2019, 01:46:50 am »
I dont see how charge is conserved inside the dotted circle. Curent is charge divided by time anyway.

He most likely did mean that charge is distributed evenly between two round objects. Yes, it is so - if there is separate point of reference (ground?) to measure voltages/charges of both round objects. In such case we have two capacitor paradox which again agrees with KVL.
« Last Edit: April 24, 2019, 01:48:23 am by ogden »
 

Offline bsfeechannel

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #1014 on: April 24, 2019, 02:05:21 pm »
I dont see how charge is conserved inside the dotted circle.

Charge conservation is just a matter of bookkeeping (as a matter of fact all conservation principles are). The dollar you spend has to be discounted from your account balance. That sort of thing.

∇·J = - ∂ρ/∂t means that the charges you see going away for good have to be discounted from the charges you hoard inside your closed surface (represented by the dashed line).

Or a little more rigorously, the total amount of charge you lose per unit area, per unit time, to the exterior of a close volume (∇·J) is equal to the rate of change over time of a decrease (hence the minus sign) on the amount of charge per unit volume (- ∂ρ/∂t).

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Curent is charge divided by time anyway.

Precisely. Current is the rate of charges crossing an area per unit time.

So choose a closed region of space. Count the amount of charge you have in that volume. Say 10 coulombs. If you measure a current of 1 A, i.e. 1 coulomb per second, going out of that volume (and you see no other current), in one second you'll have 9 coulombs, because 1 coulomb will be gone.

If you have this current, plus another with a 2 amp intensity going in the volume, you will end up with 11 coulombs after one second.

You can have a situation in which the amount coulombs stored in the volume must be kept constant. In that case all that comes in per unit time has to go out. In other words, all currents must add up to zero. And that's KCL.

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But yeah i can see what the point is, an example of KVL not holding. We KNOW that KVL is not a law of physics.

Kirchhoff studied carefully the behavior of currents and voltages in circuits and published the results of his findings in the "Annals of Physics". Then he derived his theorems, as he called them, from those empirical data. His discovery was a major breakthrough.

So KVL and KCL ARE laws of physics. But a law of physics has not to work under whatever condition. As important as it is to understand KVL and KCL it is to know when they hold and when they fail.

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It holds in most cases but not all,

I would say that KVL and KCL do NOT hold most of the times. Where can you find a place on earth where you don't have varying electromagnetic fields? The thing is that we PRETEND that KVL and KCL hold by approximation. We stash fields inside capacitors and inductors, we create ground planes, employ shielded conductors, we decouple lines, inductors, capacitances, all to avoid having to deal with fields. And when we have to deal with them, we employ rules of thumb and equivalent approximate models.

And what we cannot tame and make to conform to KVL/KCL we call "parasitics".

This creates the illusion that KVL and KCL hold "in most cases". But it's only an illusion. Not that we will abandon this illusion all of a sudden. We need to know what it means, and not to try to linger to it if it clearly shows that we will be limited in our ability to interpret the phenomena around us.
 

Offline Berni

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #1015 on: April 24, 2019, 03:53:49 pm »
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But yeah i can see what the point is, an example of KVL not holding. We KNOW that KVL is not a law of physics.

Kirchhoff studied carefully the behavior of currents and voltages in circuits and published the results of his findings in the "Annals of Physics". Then he derived his theorems, as he called them, from those empirical data. His discovery was a major breakthrough.

So KVL and KCL ARE laws of physics. But a law of physics has not to work under whatever condition. As important as it is to understand KVL and KCL it is to know when they hold and when they fail.

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It holds in most cases but not all,

I would say that KVL and KCL do NOT hold most of the times. Where can you find a place on earth where you don't have varying electromagnetic fields? The thing is that we PRETEND that KVL and KCL hold by approximation. We stash fields inside capacitors and inductors, we create ground planes, employ shielded conductors, we decouple lines, inductors, capacitances, all to avoid having to deal with fields. And when we have to deal with them, we employ rules of thumb and equivalent approximate models.

And what we cannot tame and make to conform to KVL/KCL we call "parasitics".

This creates the illusion that KVL and KCL hold "in most cases". But it's only an illusion. Not that we will abandon this illusion all of a sudden. We need to know what it means, and not to try to linger to it if it clearly shows that we will be limited in our ability to interpret the phenomena around us.

Yes for a time Kirchhoffs circuit laws ware the best explanation of electricity we had, but this was back in the mid 1800s. At that time we had no idea what an atom is made out of, we didn't even know electrons existed, radio waves have yet to be used to transmit information, mathematicians ware still discovering methods for working with complex numbers and some parts of calculus ware still being figured out.

Just like our understanding of what matter is made up of went from alchemy to Mendeleevs periodic table to atoms, to explaining atoms are made of some stuff in the middle, to figuring out its actually protons and neutrons in there, saying that are fundamental particles, but then later on managing to split them apart and finding quarks inside of them.

In the same way Maxwell did the next big step by explaining how electricity works in better more accurate detail that Kirchhoff could. At that point Kirchhoffs laws ware no longer considered laws of physics, its even in the name, they are called "Kirchoffs Cirucit Laws" since its circuits they apply to, not the theory of electricity itself, Maxwell has that now. But it didn't stop there, science kept on going and upon closer inspection found out that actually Maxwells explanation is still not quite the whole story, so the theory of Quantum electrodynamics was put together and it now carries the crown for the best theory of electricity (To this date that is, who knows for how long, there might be a even more fundamental one we haven't figured out yet).

So why do we still use Maxwells equations if they are wrong according to Quantum electrodynamics?  Its much like the reason why we still use Kirchhoffs circuit laws despite being wrong according to Maxwell. On a macroscopic level and under certain conditions these laws still work just fine, while being much more convenient to work with when you want to actually calculate them with actual numbers. The 3 sets of laws are basically just different abstraction layers for electricity. Just chose the desired abstraction level and be aware of its known limitations.

Tho to be honest there is very very little reason to go any deeper than Maxwells level of abstraction for engineering use.
 

Online ogden

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #1016 on: April 24, 2019, 03:59:34 pm »
As important as it is to understand KVL and KCL it is to know when they hold and when they fail.

Sure. Even more important is to understand when you do not apply KVL and KCL (Kirchoff's Circuit Law). You can't draw construct which is not even circuit - that has no return path for current, try to convince that current is flowing into nowhere, then happily conclude that KCL does not hold. It's not even laughable.

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I would say that KVL and KCL do NOT hold most of the times. Where can you find a place on earth where you don't have varying electromagnetic fields? The thing is that we PRETEND that KVL and KCL hold by approximation. We stash fields inside capacitors and inductors, we create ground planes, employ shielded conductors, we decouple lines, inductors, capacitances, all to avoid having to deal with fields. And when we have to deal with them, we employ rules of thumb and equivalent approximate models.

And what we cannot tame and make to conform to KVL/KCL we call "parasitics".

Wow. With such rhetoric one can invalidate nearly every law of physics.

P.S. Nice video regarding electromagnetics
« Last Edit: April 25, 2019, 03:55:15 am by ogden »
 

Offline SparkyFX

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #1017 on: April 24, 2019, 07:56:01 pm »
As I said before, scientific populism is going to be a big problem a few yeas ahead.
Opinion based decisions are surely a problem (and i am guilty of making them myself), but anyway, science teaches to check your results, so "scientific populism" is an oxymoron - "pseudoscience" would be the word. Science also teaches that the observation of a process might interfere with the result (and therefore needs taken care of).

One usually does not need to understand astrophysics or biology (or have an opinion about them) to pick a fruit from a tree and eat it -> results count, the fruit exists, the process therefore works. It starts to matter once you want to know why and how the fruit grew, manipulate the process or want to make predictions about it.

I think it is a bit on the troll side to find two different perspectives on the same subject and try to push a wedge between the two, although both are able to produce the same result -> describe the same process. I reckon it is a good conversation starter and helped students to understand the differences between abstractions, simplifications (wiring diagrams do usually not contain dimensions, equivalent circuits) and physical reality, but if you start thinking in "camps" or "sides", insult others, the cause is somehow lost.

Coming from a practical approach you measure voltages, not fields - and field sensors will usually output an equivalent voltage. So the proof seems complicated without KVL. Trying to measure fields using the force exerted on an object takes the object (and eddy currents in it) into the equations, making it as complicated to calculate as a wiring diagram with all equivalent components (parasitic inductance, capacitance) in place. So both suffer from the same problem, even if one is proven with "all fields are considered" and the other is disproven using "observation influenced the result".
 

Offline bsfeechannel

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #1018 on: April 25, 2019, 07:43:10 am »
It's just next fallacy of yours. Those two round objects on the left together form charged capacitor and wire between them - load where energy that is stored in the capacitor, dissipates. KCL holds.

Let's examine KCL in the words of Kirchhoff himself:

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Wird ein System von Dräten, die auf eine ganz beliebige Weise mit einander verbunden sind, von galvanischen Strömen durchflossen, so ist:

1) wenn die Drähte 1, 2, ...µ in einem Punkte zusammenstoßen,

I1 + I2 + ...+Iµ = 0,

wo, I1, I2, ... die Intensitäten der Ströme bezeichnen, die jene Drähte durchfließen, alle nach dem Berührungspunkte zu als positiv gerechnet;

Translation

Let a system of wires, which are connnected with each other in an entirely arbitrary way, be traversed by galvanic currents [i.e. DC], then:

1) if the wires 1, 2, ...µ meet at one point,

                                I1 + I2 + ...+Iµ = 0,

where, I1, I2, ... designate the intensities of the currents, which flow through each wire, all calculated as positive in the direction of the point of contact;

We have just one wire, with just one current. This problem is analogous to the KVL problem: we have just one voltage, and no other to balance it that can be directly measured in the circuit.
« Last Edit: April 25, 2019, 07:45:39 am by bsfeechannel »
 

Offline bsfeechannel

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #1019 on: April 25, 2019, 07:51:39 am »
So why do we still use Maxwells equations if they are wrong according to Quantum electrodynamics?  Its much like the reason why we still use Kirchhoffs circuit laws despite being wrong according to Maxwell.

Kirchhoff's laws are not wrong according to Maxwell. Maxwell's equations do not only confirm that KVL and KCL are right, but also state in WHAT CONDITION they are right.

The source of the errors is elsewhere.

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On a macroscopic level and under certain conditions these laws still work just fine, while being much more convenient to work with when you want to actually calculate them with actual numbers. The 3 sets of laws are basically just different abstraction layers for electricity. Just chose the desired abstraction level and be aware of its known limitations.

A cat is a special case of mammal, and the description of a mammal is an abstraction of a cat (and every other mammal, as a matter of fact).

QED is an abstraction of Maxwell's equations, as Maxwell's equations are abstractions of KVL/KCL.

KVL/KCL are a special case of Maxwell's equations, as Maxwell's equations are special cases of QED.

KVL/KCL are NOT abstractions of Maxwell's equations by the simple fact that they don't work for all the cases for which Maxwell's equations do.

This is another illusion. Because we ram KVL/KCL down the throats of our circuits and close our eyes to the errors due to this approximation (and we manage to get away with it in many cases) we have the impression that KVL/KCL are just simplified (hence abstracted) cases of Maxwell's equations. But this mindset will bite you in the butt sooner or later. You need to be always aware of the limitations of KCL/KVL and, if the error due to approximating Maxwell to Kirchhoff is gross enough, resort to what will give you the most accurate prediction.

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Tho to be honest there is very very little reason to go any deeper than Maxwells level of abstraction for engineering use.

I think the inventors of the transistor would not agree with you, but anyway...
« Last Edit: April 25, 2019, 07:57:11 am by bsfeechannel »
 

Offline bsfeechannel

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #1020 on: April 25, 2019, 08:02:51 am »
I dont see how charge is conserved inside the dotted circle. Curent is charge divided by time anyway.

He most likely did mean that charge is distributed evenly between two round objects. Yes, it is so - if there is separate point of reference (ground?) to measure voltages/charges of both round objects. In such case we have two capacitor paradox which again agrees with KVL.

Nope. The two capacitor paradox is about energy conservation not charge conservation. In the paradox, you have two wires. Here we have just one. In the paradox the wires are ideal. Here the wire can have resistance, inductance, and nonzero length.
 

Online ogden

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #1021 on: April 25, 2019, 08:31:16 am »
We have just one wire, with just one current.

In case you insist that there is just one wire and just one current meaning no path for return current - then I say that you don't even have circuit, thus Kirchoff's Circuit Law do not apply. There is huge difference between "do not apply" and "do not hold".

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This problem is analogous to the KVL problem: we have just one voltage, and no other to balance it that can be directly measured in the circuit.

You just mentioned voltage. Very nice. Charge was mentioned before - so those two round objects together indeed is capacitor, C=q/V. Here's your circuit - capacitor with load in form of wire. I already said so you just do not listen:

It's just next fallacy of yours. Those two round objects on the left together form charged capacitor and wire between them - load where energy that is stored in the capacitor, dissipates. KCL holds.
« Last Edit: April 25, 2019, 08:42:59 am by ogden »
 

Offline bsfeechannel

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #1022 on: April 25, 2019, 10:55:49 am »
In case you insist that there is just one wire and just one current meaning no path for return current - then I say that you don't even have circuit, thus Kirchoff's Circuit Law do not apply. There is huge difference between "do not apply" and "do not hold".

Nope. Kirchhoff says clearly that the wires can be connected in an entirely arbitrary way. So there is no requirement for them to form a circuit or to provide a path for a return current.

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so those two round objects together indeed is capacitor, C=q/V.

Not inside the dashed line. There is only one wire providing charges to (or removing from) that area.
« Last Edit: April 25, 2019, 12:36:35 pm by bsfeechannel »
 

Offline seagreh

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #1023 on: April 25, 2019, 12:01:25 pm »

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And I always thought, the EMF is defined as the tangential force per unit charge in the wire integrated over length, once around the complete circuit/loop! But not at all, it was hiding in the resistor and my voltmeter!

You clearly thought wrong, I regret. EMFs will never "hide" inside (static) wires (considered as ideal, that is).
Sorry, my wording was wrong, I should have said ‘I always believed, the EMF...’.!
Because the clearly wrong ‘thought’ is from Feynman Volume II chapter 16 Induced Currents.
As you say, electromag is so unintuitive, so much bullshit around.... you can’t even trust books anymore.
« Last Edit: April 25, 2019, 12:27:34 pm by seagreh »
 

Offline seagreh

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Re: Does Kirchhoff's Law Hold? Disagreeing with a Master
« Reply #1024 on: April 25, 2019, 12:23:58 pm »

Let's examine KCL in the words of Kirchhoff himself:

Quote
Wird ein System von Dräten, die auf eine ganz beliebige Weise mit einander verbunden sind, von galvanischen Strömen durchflossen, so ist:

1) wenn die Drähte 1, 2, ...µ in einem Punkte zusammenstoßen,

I1 + I2 + ...+Iµ = 0,

wo, I1, I2, ... die Intensitäten der Ströme bezeichnen, die jene Drähte durchfließen, alle nach dem Berührungspunkte zu als positiv gerechnet;

Translation

Let a system of wires, which are connnected with each other in an entirely arbitrary way, be traversed by galvanic currents [i.e. DC], then:

1) if the wires 1, 2, ...µ meet at one point,

                                I1 + I2 + ...+Iµ = 0,

where, I1, I2, ... designate the intensities of the currents, which flow through each wire, all calculated as positive in the direction of the point of contact;

We have just one wire, with just one current. This problem is analogous to the KVL problem: we have just one voltage, and no other to balance it that can be directly measured in the circuit.
This phenomena is well known in literature as the ‘One-Wire-Single Current-Sophism”.
For sure, can only be solved with Maxwell equations   :clap:
Maybe you missed the point, the ‘connection point’ (Berührungspunkt)?
Hence, a single wire hitting the connection point is a dead end, hence I1 = 0 .
« Last Edit: April 25, 2019, 12:31:42 pm by seagreh »
 


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