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?


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?

  • $\begingroup$ 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). $\endgroup$
    – Steeven
    Commented Jun 7, 2021 at 12:29

1 Answer 1


... resistance is caused due to collision of electrons with dust and impurities inside a metal ...

I am not sure where you get "dust" from (I don't think this is mentioned in the video ?), but the flow of electrons through a metal such as copper is certainly impeded by imperfections in the metallic lattice, by the presence of impurities (atoms of other elements present in small quantities), and by the thermal vibrations of the copper atoms themselves - which is why resistivity increases with temperature.

The "pinball" model of electron flow that the video refers to is more formally called the Drude model or the Drude-Lorentz model.

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.

  • $\begingroup$ gandalf61, do you mean that electrons collide with lattice ions, if not can you please elaborate this point. $\endgroup$
    – RAHUL
    Commented Jun 7, 2021 at 11:16
  • $\begingroup$ @RAHUL Yes. The Wikipedia article that I linked to says “the microscopic behaviour of electrons in a solid may be treated classically and looks much like a pinball machine, with a sea of constantly jittering electrons bouncing and re-bouncing off heavier, relatively immobile positive ions”. $\endgroup$
    – gandalf61
    Commented Jun 7, 2021 at 13:00
  • $\begingroup$ I agree that this was one of assumptions in drude theory. But isn't it wrong according to quantum mechanical view? $\endgroup$
    – RAHUL
    Commented Jun 7, 2021 at 16:11
  • $\begingroup$ @RAHUL The Drude model is a classical model, but it fits the observed facts for metallic conductors at room temperature, which is the scenario where Ohm's law is typically used. Yes, you need a more complex quantum mechanical model to explain the behaviour of superconductors and semiconductors, for example, but that is a different topic. $\endgroup$
    – gandalf61
    Commented Jun 7, 2021 at 20:13

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