[Phys-L] Kirchhoff''s "Laws" + legs + mesh - wikipedia

[John Denker <jsd@av8n.com>]

Hi --

Last March there was some discussion on the subject of:
  The status of Kirchhoff's laws

Here's my take on it, as of now:  Executive summary:

  Kirchhoff’s Circuit «Laws» provide a simple /approximate/ 
  description of how voltage and current behave in electric 
  circuits. People really like this simple way of looking at 
  things, and they often take pains to design their circuits 
  so as to minimize the importance of any Kirchhoff-law 
  violations. On the other hand, there are plenty of situations 
  where violations cannot be avoided. You cannot apply these 
  «laws» safely unless you know when, how, and why they break 
  down.

I decided to write a couple of web pages on this, and on
the related topic of "voltage" itself.

 *) Basic Notions of Voltage
    http://www.av8n.com/physics/voltage-intro.htm

 *) Kirchhoff’s Circuit «Laws»
    http://www.av8n.com/physics/kirchhoff-circuit-laws.htm



Also:  I explain the leg-by-leg approach (in contrast to the 
widely taught but less convenient loop-by-loop approach).
  http://www.av8n.com/physics/kirchhoff-circuit-laws.htm#sec-legs

Also:  I came up with an example circuit that (a) grossly
violates KVL, and (b) cannot be fixed.  There is AFAICT no
way you can add little "parasitic" components to this schematic
in such a way as to model the voltages and currents in the
real circuit.
  http://www.av8n.com/physics/kirchhoff-circuit-laws.htm#sec-mesh

This is not the sort of thing that you show to introductory-
level students, but I find that pondering it has deepened my 
understanding.  Also it comes in useful when arguing with people 
who think that KVL is some God-given 11th commandment.


This stuff is still new and drafty.  Questions, comments, and 
suggestions are welcome.

I am quite aware that my definition of voltage is far too 
mathematical to be suitable for the introductory course.
In the long run, this is fixable.  In the short run, I did 
it this way, because this is how I think about it.  This is 
how I explain it to myself.

I reckon there are more pictorial, more operational ways of 
presenting it;  I just haven't figured it out yet.



===============================

Here's a much less interesting "process" story.

You may be wondering why I decided to write my own pages,
rather than (say) maintaining the wikipedia pages on the
same subjects.

Well, I tried the wikipedia route.  It didn't work.

Back in late April 2013, I edited the wikipedia article on
Kirchhoff's Circuit «Laws» so as to make it not quite so
grossly wrong.  It was part of an experiment, to see how
long it would stay fixed.

After a few months it got worse, and lately it has gotten
dramatically much worse.  My favorite was the anonymous
contributor who informed us that
> >   "voltage" is a misconception people use to describe difference

As the wikipedia instructions say:

> If you don't want your writing to be edited mercilessly,
>  then don't submit it here.

To which I say OK, I won't.

In particular:  
 ++ If I submit something and other people make it better, 
  that's great, and it encourages me to submit more.
 -- If I submit something and other people incessantly
  make it worse, then I'm gonna bail.  I've got better
  things to do with my time.