Why some materials obey ohm's law in terms of properties?

Answer 1

"Compliance" with Ohm's Law is an interesting topic. (The specific answer to the question is at the bottom of this answer, but the introductory material is essential to understanding.

In point of fact, Ohm's Law applies in every single case, every time. It simply states that voltage equals current times resistance.

The problem that many people have is one of comprehension, and the fact that no device maintains constant resistance when voltage or current changes, so it appears that Ohm's Law does not apply, when it fact it does. It is simply a matter of perspective.

Take a diode, for instance, in a forward bias condition. As you increase the current, the voltage increases, but the curve does not appear to comply with Ohm's Law. This is even more apparent with a zener diode in reverse bias condition. We say that the diode has dynamic resistance. While this is true, the greater truth is that, for any single condition of voltage, current, and resistance, Ohm's Law applies. That is the fundamental definition of resistance.

The same example can be made of an ordinary light bulb. It has a cold resistance which is vastly different than its hot resistance, but you can still use Ohm's Law to evaluate its performance in any single situation.

All of that said, to answer the question, no material purely obeys Ohm's Law. In order to "obey", the material must have a constant resistance that does not change when current or voltage changes. That is a theoretical concept that does not exist in practice.

Answer

There seems to be a major misconception surrounding Ohm's Law. Ohm's Law simply states that 'the current flowing along a wire, at constant temperature, is directly proportional to the potential difference across its ends'.

Yet some people have 'rewritten' it, so that it reads: 'Ohm's Law states that the current flowing through a conductor is directly proportional to the potential difference and inversely proportional to resistance', or 'Ohm's Law states that current is voltage divided by resistance'. In fact, neither of these statements is true. Ohm's Law makes no direct reference to 'resistance', and does not give any formula for determining resistance!

Put simply, if the ratio of voltage to current remains constant for variations in voltage, then a conductor or device is obeying Ohm's Law. If this ratio changes for variations in voltage, then it is NOT obeying Ohm's Law. It's as simple as that!

Having said that, the ratio of voltage to current (resistance) will always tell us what the resistance of the circuit or device happens to be FOR THAT PARTICULAR RATIO.

The equation I = V/Ris not Ohm's Law formula, as many people believe. There is NO Ohm's Law 'formula'! The equation comes from the definition of an ohm, which is equivalent to a 'volt per ampere' -hence, I = V/R.