Re: Ohm's 'law' & minus sign

[Paolo Cavallo <ton0621@IPERBOLE.BOLOGNA.IT>]

----- Original Message -----
From: "John S. Denker" <jsd@MONMOUTH.COM>
To: <PHYS-L@lists.nau.edu>
Sent: Tuesday, February 11, 2003 4:28 PM
Subject: Re: Ohm's 'law' & minus sign


> The usual form of Ohm's law is delta V = R I, with
> everything a scalar.
>
>
> Apparently it is conventional to define
>         delta V := -L Del V
> which can be thought of as an instruction as to
> where you put the red lead and where you put the
> black lead of the voltmeter.

This is correct, no doubt.
But everytime I discuss the topic of Ohm's law, some
student gets uneasy with the "usual form". I am very
strict with every Delta-something we happen to write
in our five years course, so they feel we are making
some trick here. And I think they're right, after all.
If we follow the circuit along with the current, we
meet the higher potential before the lower one. If
Delta V is to mean V(x+dx)-V(x), it seems to me that
if we see a positive current, we see also a negative
Delta V. This way, Kirkhhoff rule is also easier.

Please, forgive my poor English.

Paolo Cavallo