"Chuck Harris" wrote in message
...
Hi Ratch,

I pulled out my old college physics text just to see what they had
to say, and Lobkowicz and Melissinos, "Physics for Scientists and
Engineers", Vol 2. starts out stating that Ohm's Law is:

j = E/rho, and therefore, E = j * rho

Where j is the current density, E is the electric field, and rho is
the resisitivity.

Then they set a few conditions to make the math easier: a homogeneous
material of constant crossection, and some unspecified length,
and derive the more familiar form:

V = [rho L I]/A = RI, where L is length, I is current, A is
cross sectional area, and R is resistance.

V = RI

From that point on, L&M use the two forms of the equation
interchangably... picking which ever one makes the solution
easier.

So, it would appear that both forms are actually equivalent,
one is more favorable for use in studing materials, and the
other is better for studying circuitry.

Every solid had a property called resistivity. Resistance is determined by
the resistivity, the shape of the solid, and the
placement of the two measurement leads into the solid.

Prof Serway says the same thing about resistance/resistivity as L&M, except
Serway does not call the formula E=rho*J Ohm's law. Instead Serway says
that if the resistivity (rho) is constant at any electric field (E), then
the material is ohmic. That means that E and J are linearly proportionate to
each other. How does L&M define an ohmic material? Ratch