Re: [Phys-l] Textbook petpeeves: circuits

[John Denker <jsd@av8n.com>]

Rauber, Joel wrote:
> While we are on circuits, one of my pet peeves is the use of "V" for
> potential difference in circuits.
>  
> At least at first I'd prefer the use of "Delta V"

Another way to solve the problem is to label all the nodes
and just write the voltage difference as (ahem) a difference
of voltages:

    Va - Vb = I r1
    Vb - Vc = I r2
    Vc - Vd = I r3


    I
 -->>>-|
       |__ Va
       |
       Z
       Z r1
       Z
       |__ Vb
       |
       Z
       Z r2
       Z
       |__ Vc
       |
       Z
       Z r3
       Z
       |__ Vd
       |


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Also you don't want to be toooo quick to mark down every instance
of "V", because the student is allowed to exercise his gauge
freedom and tie one point of the circuit to "ground".  That
typically turns some instances of ΔV into just V.  Sometimes
using gauge-invariant terminology is just not worth the bother.
Writing ΔV for every voltage in an ordinary digital circuit
would be pure pedantry.

OTOH I don't want to make any too-sweeping generalizations, because
there are some non-trivial downsides.  Sometimes students (and others)
become toooo enamored of the "ground" idea:
  a) Sometimes they think "ground" is required, i.e. they lose
   every vestige of the idea of gauge invariance, and
  b) Sometimes they think that labelling N different things
   "ground" automagically makes them all have the same voltage.
   It doesn't.  Instead it gives rise to the dreaded "ground loop".

I once ran into a TA -- a grad student in the EE department -- who
did not know why a voltmeter measured a voltage using two wires,
when you could measure it with just one wire using an oscilloscope.
(I didn't mind answering his question, but it raised other questions,
like whether he was qualified to be a TA, and whether he ought to
spend a couple years taking undergrad courses before starting his
graduate work.)