# What exactly causes resistance?

According to Ohm's Law, $$V=IR$$.

Here $$R$$ denotes the resistance. It's the way I was introduced resistance. The definition is not convincing me. First of all I don't understand what causes resistance?

https://youtu.be/YMNZ2oYu-qI

In this video by Caltech, they show that resistance is caused due to collision of electrons with dust and impurities inside a metal as it cannot be perfect. Here I have a question that aren't those impurities themselves made of atoms and also have electrons? If the reason in this video is not true then what is exact reason behind electric resistance?

• Note that an impurity in materials science is an atom (sometimes a molecule, but in the case of conductors we are talking about individual atoms or ions). Commented Jun 7, 2021 at 12:29

Ohm's law in the form $$I \propto V$$ assumes that resistance is independent of current. This is a reasonably good approximation for constant or slowly varying applied voltages across metallic wires at roughly constant temperature. It does not apply if the current heats the wire significantly; if the applied voltage is varying rapidly; or in more complex non-linear circuit components such as capacitors or diodes.