[Phys-l] Orientation of voltage drop

[James McLean <mclean@geneseo.edu>]

Concerning <http://www.av8n.com/physics/resistance.htm#sec-diag>,

John Denker wrote:

> James McLean wrote:
>
> > BTW, at the above web link, it says "To label the voltage of a node in 
> > the diagram, it suffices to put a symbol next to the node."  While this 
> > is true, I think it might be a tad misleading.  To obtain proper voltage 
> > *differences*, reference must be made to the same orientation choices 
> > that are made for description of the currents.  
>
>
> That's news to me.  What means "proper"?  What's the penalty for
> making an "improper" choice?
My apologies: now that I read the text near your figure 1, I see that 
you describe how to take the proper differences.  "DeltaV is the voltage 
drop across the device _in_some_chosen_direction_." (my emphasis)  The 
penalty is of course get the wrong answer, specifically to get the wrong 
sign on the current from Ohm's Law.

Notice a rather curious fact:  In figure 1, you say that the "voltage 
drop in the chosen direction" is V1-V2.  And I certainly know exactly 
what you mean.  But consider a student who has spent a year trying to 
become accustomed to the rule DeltaX=X_final-X_initial (which is 
apparently very unnatural to them).  They might likely interpret "the 
voltage drop in the chosen direction" to be V2-V1.

> > See 
> > <http://www.geneseo.edu/~mclean/AnalytII/demo/CircuitAnalysis.pdf>.
>
> The soi-disant "sure-fire never-fail technique" described there may
> be good enough for the most elementary applications, but is, to say
> the least, far from complete.
Absolutely.  I'm sure you picked up on the fact that the quoted phrase 
is hyperbole.  I presume that my students pick up on that too, but maybe 
I should check.

>   -- The near-universal convention for positive output voltage and positive
>    output current for a _battery_ violates rule 1.
Yes, annoying, isn't it.  For a chosen direction of negative to positive 
terminal, it "feels wrong" to say that DeltaV<0.  But, AFAICT this a 
"choose your poison" situation.  Assuming the natural choice of DeltaV 
for a resistor made above, you must either:
(1) For a battery use DeltaV<0 when the chosen direction is neg to pos, or
(2) Have a different rule for batteries and resistors concerning the 
sign applied while executing Kirchoffs loop rule.

I'm guessing that my students will have less trouble with (1) than with 
(2).  I tell them that the silver lining is that positive DeltaV will 
always mean that the electric field points in the chosen direction.  If 
you have another option, I'm all ears!

Cheers,
--James

--
Dr. James McLean                     phone: (585) 245-5897
Dept. of Physics and Astronomy       FAX:   (585) 245-5288
SUNY Geneseo                         email: mclean@geneseo.edu
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