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Re: [Phys-l] Kirchhoff's laws, or not



On 07/07/2010 01:34 PM, Stefan Jeglinski wrote:
Both of Kirchhoff's laws
are flatly contradicted by the Maxwell equations, and
both are routinely violated in practice.

As a catch-all term, I assume you are referring to the "radiation"
aspect? Or am I completely missing the point? (a common happenstance)

I don't like to argue about terminology, but I don't
think "radiation" is a helpful description.

Could we agree on _AC circuit_ effects (as opposed to
"radiation" effects)? Most if not all violations of
Kirchhoff's law occur in AC circuits.

There's a lot you can do in terms of AC circuits at
the introductory level and even the intermediate level
without bothering with radiation effects. Capacitors,
inductors, transformers, etc. don't radiate.

2) Ground loops are a familiar example of a gross
violation of K's voltage law.

I have thought of this also as a radiation issue.

I prefer to think of it as a parasitic mutual capacitance,
i.e. a parasitic transformer. This is an AC circuit but
not really a radiation effect.

We once inherited a competitor's hardware from a customer that
replaced theirs with ours. We noticed a funny bulge in a cable and
took it apart, noticing that the shield had been broken and then
spliced with a 100-ohm resistor.

Components spliced into cables? That's something they
teach at clown college, not in engineering school.
1a) Sometimes the shield should be interrupted.
In this case, it should be interrupted properly,
with overlapping shielding, and it should be
interrupted at one end or the other, not in the
middle. Example:
http://www.av8n.com/physics/grounding-shielding.htm#sec-measure-phi-dot
1b) If you need an impedance in series with the
shield, it should almost certainly be the impedance
of a balun, not the impedance of a resistor.
2) If you sometimes need continuity and sometimes
need interruption, you should do it with a switch,
and not try to split the difference. And again,
the switch should be in the apparatus at one end
or the other, not in the middle of the cable.
3) If you want the shield to be interrupted with
respect to small signals, but still want backup
continuity to defend against electrostatic discharge
threats, consider crossed diodes. Again this should
be inside the chassis, not in the middle of the cable.

There's a whole lot more that could be said about grounding
and shielding, and about low-noise circuit design in
general. Just last week I was helping a grad student
knock down the noise in the custom A/D/A converter board
he designed. I typed up about 1% of my notes and put them
at
http://www.av8n.com/physics/grounding-shielding.htm