Re: delta V = I R

[Bob Sciamanda <trebor@VELOCITY.NET>]

It is common for one to use the characteristic curve (I vs. V) of a
non-linear resistive device in circuit design. Eg., vacuum tubes (I was 16
yrs old when the transistor was born), diodes, transistors, thermistors,
heaters, light bulbs, etc.  One can use the resistance value at the
desired operating point for quiescent (bias) state evaluations.  (eg., "DC
plate resistance" is a common specification for a vacuum tube, values are
listed for commonly used conditions.)

The main logical point is that one should have a definition of a quantity
before one asserts that its value is a constant for a certain material.

The main practical point is that, constant or not, R=V/I is a useful
quantity.  Perhaps the most common, obvious example is the home light
bulb, with  a much different on-resistance than cold resistance; it is
commonly used as an economical lab resistance with a high dissipative
capacity for loading various power devices (even RF transmitters) - its
(variable) resistance is here an important quantity.

I just thought of the Power Mosfet , which has various "on resistances"
specified for various gate voltages; a common motor controller.  In short,
V/I is an important quantity, perhaps even more so when it is not
constant.

Bob

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

----- Original Message -----
From: Michael Edmiston <edmiston@BLUFFTON.EDU>
To: <PHYS-L@lists.nau.edu>
Sent: Monday, November 08, 1999 11:00 AM
Subject: Re: delta V = I R


> Bob Sciamanda points out that we can define non-constant parameters.
>
> We can define non-constant values if there is utility in doing so.
>  What would be the utility of calculating and define a "resistance" for
> a non-ohmic device?  It cannot be used in I = (delta-V)/R to predict
> the current at any value of delta-V... it only works for one value.  A
> non-ohmic device needs a different (non-linear) equation to relate
> current to potential difference.  Or, if you wanted to tabulate values
> of R for various delta-V's, I guess you could do a "look-up table."
>  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.
>
> Michael D. Edmiston, Ph.D.              Phone/voice-mail:
419-358-3270
> Professor of Chemistry & Physics        FAX:
419-358-3323
> Chairman, Science Department            E-Mail
edmiston@bluffton.edu
> Bluffton College
> 280 West College Avenue
> Bluffton, OH 45817