Today I learnt about Ohm's Law(I had some basic knowledge earlier).When I came home I searched in the internet and found that at Ohm's Law, resistance should not change because then the circuit will not obey Ohm's Law.I googled about this "problem" but I found nothing. My physic professor says that resistance can change at Ohm's Law. I don't know which one to trust. Can anyone help me ?
Answer
Well, the thing with Ohm's law is that the ordinary expression $I=\frac{V}{R}$ (sorry, I'm on mobile and writing "nice math" here is a pain), works only for what is usually referred to as "static resistance", that is $R=\frac{V}{I}$. However, this implies a material whose resistance does not vary. There are some materials whose resistance varies inversely to its cross-sectional area and proportionally its length ($R=k\frac{l}{A}$, where $k$ is the coefficient of resistivity of a material, a constant really). For this materials you need a "dynamic resistance" which is given by $\frac{dV}{dI}$, simply the rate of change of $V$ with respect to $I$ (or the slope of said line).
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