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Ohm's Law validity

  • Thread starter dlp248
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  • #1
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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.
 

Answers and Replies

  • #2
BvU
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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## :)
 

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