# Rambling on--A continuation of "What is Ohm's Law?"

Discussion in 'Off-Topic' started by Ratch, Dec 30, 2008.

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1. ### Ratch Thread Starter New Member

Mar 20, 2007
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Dave,

It appears to me to be a distinction without a difference. I agreed before that Ohm's law is a statement about proportionality, specifically linear proportionality. That does not make it a definition of resistance.

R=V/I will always be correct anywhere on the diode V-I curve, because that is the definition of resistance. Three or more points are needed to determine linearity, and compliance with Ohm's law.

Ratch

2. ### Dave Retired Moderator

Nov 17, 2003
6,960
163

Georg Ohm's Definition: The amount of steady current through a material is directly proportional to the potential difference, or voltage, across the material, for some fixed temperature.

Ratch's Definition: The resistance of metallic conductor is the same no matter what applied voltage is used to measure it.

Ohm's definition is that of proportionality resulting in steady current and therefore can be applied, both locally and where applicable globally, to any conducting materials. Your definition is merely a sub-set where there are the conditions of any voltage resulting the same resistance, i.e. the material must be linear both locally and globally.

Not the same thing. Now who do I believe? Georg Ohm who formulated Ohms Law, or Ratch who trolls AAC looking for an argument?

No. Ohms Law says nothing more than for a given material at a given temperature that the steady current is proportional to the applied voltage. The resistance is a function of several parameters, material properties (resistivity), length, cross-sectional area, junction temperature; in other words the definition of resistance:

$R = \frac{\rho L}{A}.\alpha\left(T - T_{0} + 1\right)$

However that is not what Ohms Law is. I have said this to you previously, and I note that you took no notice whatsoever, that there is no mention in Ohms Law of the requirement for the resistance to remain constant across an I-V characteristic, just that there is proportionality between I and V for steady I; electrically that can be defined as the reciprocal of a "variable", where this "variable" - the material variable - is defined by other constructs. And it is only by investigating various material properties did he come to a law for steady current; a law that is Ohms Law.

I suggest you get his paper and read exactly what Ohm said. Until you are prepared to do this, then this discussion is going nowhere. This thread is closed, and if you are not happy with this then I suggest you ply your trade elsewhere - we are not here to micro-moderate you.

Dave