Re: [Phys-l] Current as Vector

[Brian Whatcott <betwys1@sbcglobal.net>]

At 01:27 PM 3/22/2006,  John M., you wrote:
> Brian wrote:
>
> >I certainly can find counter examples to the assertion that resistance
> >is a scalar. Plastic films show notable non isotropism.
>
> You don't need exotic materials to make resistance "dependent" (in
> some sense) on "direction."  For instance, what is "the resistance"
> of a 1 cm x 2 cm x 3 cm block of graphite?
>
> Nevertheless, in my book, resistance is a SCALAR property of a) a
> given object and b) a well-specified electrical connection scheme*.
> If the object is anisotropic, the anisotropy is expressed through a
> conductivity TENSOR relating the local current density (a VECTOR) to
> the local electric field (another VECTOR).  That anisotropy is taken
> into account along with the connection details when determining the
> SCALAR resistance.
>
> * I recognize that resistance may also be a function of applied
> voltage, temperature, and perhaps other things, but that doesn't
> detract in any meaningful way from the point I am trying to make.
>
> --
> John "Slo" Mallinckrodt


Since you raised the idea of resistance properties in bulk, perhaps
I can return the favor and raise the properties of current in a
bulk material.
 Let us suppose I have a cube of graphite a kilometer on a side.
And suppose I consider a situation near the middle of this mass.
Suppose I have a driving voltage field in an x direction,
another driving field in the y direction, and a third  field
in the z direction.

Should I say that the current in the middle of this mass is a
scalar, now I don't have the invisible directional constraints
of conductive wires in air that allow me  ..er.. you to talk as though
directionality is a given?



Brian Whatcott    Altus OK    Eureka!