"Is Kirchoff really for the birds?"

[Muu9]

This video revisits the classic debate with some good physics and simulations in SPICE



You can skip to 42:34 to skip the physics review.

 

[Dale]

His whole "Kirchoff is for the birds" schtick is just a strawman argument. He starts by violating one of the basic assumptions of circuit theory and then tries to make a big deal about the fact that one of the laws of circuit theory doesn't work. Nobody claims that you can use circuit theory when the assumptions of circuit theory are violated, that doesn't mean that circuit theory is "for the birds". Every single theory ever developed is subject to that same silly criticism.

 

[DaveE]

Always keep in mind this first sentence of the Wikipedia entry:

"Kirchhoff's circuit laws are two equalities that deal with the current and potential difference (commonly known as voltage) in the lumped element model of electrical circuits."

The key phrase here is "lumped element model". This is the part that these videos always seem to ignore. It simply doesn't work for induced voltages unless you model that part as an inductor.

 

[kuruman]

It simply doesn't work for induced voltages ...

And the reason is that KVL is $\oint \mathbf E\cdot d\mathbf l = 0$ in disguise. Electric fields due to induced voltages are notoriously non-conservative.

Disclaimer: I have not watched the video because my laptop's speaker in inoperative at the moment.

 

[DaveE]

I think there's a broader issue on display here which is often neglected and not well taught in introductory physics.

CIRCUITS AREN'T REAL. These are always simplified approximate models that we make of the real world which allows us to use simple, powerful, analysis tools. As such, we must always keep in mind the limitations of our models. There should be no prizes for people that can point out the discrepancies between our models and the real world, we know already, we put them there.

The only mystery here is why people think it's a mystery. It's just a bad model in these cases. That's why we also have EM.