Re: delta V = I R

[Bob Sciamanda <trebor@VELOCITY.NET>]

R can depend on many things, often usefully so, eg: photoresistive devices
(like CdS) have light-sensitive resistance.  Bulk materials even have
non-isotropic resistance (best described by a matrix).  An Ne2 neon bulb
has almost infinite resistance below a firing voltage (c. 75 Volts), above
which it is almost a short circuit.  A gas discharge can even have a
negative DYNAMIC resistance (as can a tunnel diode).

Some gas sniffing detectors rely on resistance changes in the presence of
certain gases (often as the result of a chemical reaction.

Bob

Bob Sciamanda (W3NLV)
Physics, Edinboro Univ of PA (em)
trebor@velocity.net
http://www.velocity.net/~trebor

----- Original Message -----
From: Ludwik Kowalski <KowalskiL@MAIL.MONTCLAIR.EDU>
To: <PHYS-L@lists.nau.edu>
Sent: Monday, November 08, 1999 1:19 PM
Subject: Re: delta V = I R


> Definitely yes, try to measure the leakage resistance of an
> electrolytic capacitor. It will change by at least one order
> of magnitude before R is suddenly reduced to nearly zero.
> Do not forget to say good bye to the device before this
> happens at some high voltage. I once nearly became blind
> when a capacitor exploded. (See the old thread "A Myth
> About Capacitors in Series".)
>
> "Paul O. Johnson" wrote:
>
> > Michael Edmiston wrote:
> >
> > > Please tell me what utility one would gain from knowing the
"resistance" of a non-ohmic
> > > device given that the value is only valid at one potential
difference.
> > I have always believed and taught that the resistance value depends on
the device's
> > temperature, not on the potential difference across it. Is there
really a functional
> > relationship between R and delta V at constant T?
> >
> > poj