Re: delta V = I R

[Bob Sciamanda <trebor@VELOCITY.NET>]

----- Original Message -----
From: Michael Edmiston <edmiston@BLUFFTON.EDU>
To: <PHYS-L@lists.nau.edu>
Sent: Monday, November 08, 1999 10:16 AM
Subject: Re: delta V = I R

> Bob Sciamanda asks what we call (delta-V)/I for a non-ohmic device.  I
> don't think we call it anything.  Although some people would still call
> it "resistance," I don't think a non-ohmic device has a resistance.
>  "Having" a resistance implies a definite value.  If (delta-V)/I yields
> a different result for each value of delta-V, then this ratio has no
> set value for that device/material.  What would be the point of giving
> a name to it?
> * * * *

Whence comes the dictum that we only define constant properties of matter?

Whether constant or not, the ratios V/I and dV/dI are so useful that
"everyone" gives them names: resistance and dynamic resistance.
.
> We do this type of thing all the time.  PV = nRT is the ideal gas law.
>  It applies to any gas which is behaving ideally.  If the gas is not
> behaving ideally, then PV = nRT does not apply.

Not at all a parallel.  We have independent definitions of the quantities
involved.

> Likewise, delta-V = IR applies to devices that are behaving in an ohmic
> manner.  If the device is not ohmic, then this equation does not apply.
>  Is that too complicated?  If so, is PV = nRT too complicated?
>
> Michael D. Edmiston, Ph.D.              Phone/voice-mail:
419-358-3270
Bob Sciamanda (W3NLV)
Physics, Edinboro Univ of PA (em)
trebor@velocity.net
http://www.velocity.net/~trebor