coulter said:
mivey -
I read the hypephysics links - like most of the posters here, the articles talk about ohms law doesn't work for for some particular substance/device because the current is not porportional to the resistance - huh? I thought resistance was defined as v/i.
But the resistance formula is not, in itself, Ohm's Law.
Also, it is not really a law in the strictest sense, but a limited observation of the voltage current relationship in certain materials.
Ohm's Law states the ratio of voltage to current through a conductor is constant if influences, like temperature, are held constant (we know this is true only in a limited range for a lot of materials) and that this ratio is independent of the voltage and current.
This formula describes the resistance of such a conductor, and while commonly called Ohm's Law, it is NOT Ohm's Law in the strictest sense: R = V/I. This formula becomes the general definition for resistance and has been expanded to cover non-ohmic material.
These formulas, while based on the concept, do not describe an ohmic material, and these materials do not necessarily follow Ohm's Law:
Z = v/i
Z = v(i)/i
Z = v/i(v)
R = v(t)/i(t)
R(t) = v(t)/i(t)
Z = Ldi/dt / Cdv/dt
coulter said:
So where am I going? I think that when the term "ohms law doesn't apply" is used, what is meant is , "dv/di is not a constant"
carl
The term might be "doesn't follow Ohm's Law" instead of "doesn't apply". So when we say a material "doesn't follow Ohm's Law", the resistivity is not a constant, independent of the voltage and current.
Again, the formulas are not Ohm's Law, but are resistance definitions or voltage/current relationships. They are Ohm's Law equations that can be said to describe Ohm's Law.
My physics book does call the formula E = pJ "Ohms law for a substance", where E is the electric field, J is the current density, and p is the resistivity for ohmic substances. It states that nonlinear substances are materials where E is not proportional to J.
I have several texts that refer to the V=IR formula as Ohm's Law but that does not make the use totally correct.
FWIW, I also have the BJT thermal version of the Ohm's Law equation: Tj-Ta = @ja * PD where Tj is the junction temp, Ta is the ambient temp, @ja is the thermal resistance, and PD is the power dissipation.
How about Ohm's Law from a material's standpoint?: s = q*n*u where s = specific conductivity, q = charge, n = density of charge carriers, and u = charge carrier mobility.