Hi Ratch,

No one has said all materials are ohmic.

What I understand L&M to be saying is that if j is proportional to E,
the material is ohmic. Proportionality requires j and E to be
related by a CONSTANT (constant relative to j and E, that is).

If rho is a constant, the material is ohmic. If rho is not constant,
the material is not ohmic.

In quoting L&M, I left off the first paragraph where they discuss rho
being constant, to wit:


[4. OHM's LAW

If there is no electric field in a conductor, there is also no
electric current; the mean velocity (v) of the charge carriers
(electrons) vanishes. In many, although by no means all, materials the
current density is proportional to the electric field:

j = (1/rho) * E (9-16)

The quantity rho is called the resistivity of the material; its inverse
1/rho is usually called the conductivity. It is a property of the
material; in addition, it will vary with the temperature of the
conductor.

Eq. (9-16) describes the current density in terms of the electric
field at a point in a conductor (Fig. 9-11). It is called Ohm's law.
materials that obey Ohm's law are usually called ohmic conductors. This
relation enables us to calculate the current flowing through a wire of
length L which is connected to two terminals - points between which
there is a potential difference V....]



L&M could have said a bit more about what they meant about a material
not following Ohm's law; how they meant that a material that has a non
constant rho is non ohmic. However, I caught the meaning the first time
I read it, so it cannot have been too badly worded.

The trip from (9-16) to: V = RI is just a straight forward
rearrangement, and substitution. It still states the same thing as
(9-16). A material is non ohmic if R is not a constant.

-Chuck

Ratch wrote:
Yes, rho and R are proportional to each other, but that does not answer
the question I asked before (see the first paragraph above). How does L&M
define something as nonohmic when according to what they say, everything is
ohmic because it follows V=IR (which they say is Ohm's law).


So, as a result, if R is some function of I, the material is non ohmic.


I agree with that, but according to what you said about what L&M
writes, that never happens because all materials follow V=IR. Does L&M
mention
nonohmic materials? Ohm's law cannot be both V=IR and constant resistance
as current varies. Which one does L&M say it is? Ratch


There is no inconsistency.


Yes, according to what L&M says there is. Ratch