Ohm's Law Validity: Does it Change with Heat?

[dlp248]

Homework Statement

When would Ohm's law not be valid. Does Ohm's law only work with specific resistors, i.e. when R = 25 ohms? As the current flows and the resistor heats up does resistance change?

Homework Equations

V = IR

The Attempt at a Solution

I would say that no, Ohm's law is not going to be valid all the time. There are specific materials that are designed to be resistors and can handle a certain amount of current. I've been reading about diodes and I'm not really sure what they are, but I believe they do not hold true to Ohm's law. I also want to say that conductors and transistors do not apply to Ohm's law. From these conclusions, I think that as current flows and the resistor heats up, resistance does change because the resistor is meant to handle a certain work load. As those parameters change, the resistor would probably fail and resistance would lessen.

 

[BvU]

If one simply defines resistance as Voltage/Current, then V = I R holds. But, as you clearly point out, for a lot of situations R is not a constant, which takes away the predictive capability of Ohm's law for those situations.

But even for diodes, light dependent resistors, etc. one can call $\Delta V / \Delta I$ the resistance at a given V, and for (very) small $\Delta V$.

Common understanding of Ohm's law includes the silent assumption / implicit condition that R is constant. It is a linear model, a simplification of reality. In practice it holds over many orders of magnitude for the devices we call resistors, but not for all: too much current and they blow up. Then you have $R=\infty$ :)