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.


-Chuck



Ratch wrote:

It is certainly true that what a professor writes is going to be what
he believes to be factual. And impossible to discern how he came upon his
knowledge. But you have to ask yourself, why did Professors Resnick and
Serway go out of their way to make a point that Ohm's law is a property of a
material and not V=IR? Does that not indicate that they looked into the
matter more closely that their contemporaries? The next time I get to a
good college library, I will look at other college physics textbooks. Ratch