In this world we are into, one feels ashamed even to ask a question like that. Don't electrons leave a source of current, move along a wire driven by an electric field and enter the current source again, like water pumped into a pipe?
Yet we are not ashamed. Where does electric resistance come from? A conductor is not a pipe, the walls are not rough. Why does a metal filled with so many current carriers offer resistance to the flow of current?
This is one of those naive questions whose answer is far from simple.
Electric currents have been known for over 200 years, while the answer to our query came to light only about eighty years ago.
Here is how classical physics explained electrical resistance. The directed movement of electrons - what we call current - is all the time upset by the thermal vibrations of ions in the skeleton of the metal. These vibrations
impede the motion of electrons. Electrons begin to move like people in a building during an earthquake - walls and floors rising and falling and swinging and shaking.
Obviously, the smaller the vibrations of walls and floors, the easier it is to walk about the building. At the absolute zero of temperature, when the thermal vibrations of the ions cease altogether, electrical resistance should drop to
zero. Which is very close to the truth as regards very pure metals almost devoid of impurities. The whole trouble lies with these impurities. As the temperature falls, the resistance of such 'dirty' metals does not tend to zero, but rather to some nonzero value which depends on the content and type of impurities in the metal. The more impurities there are, the higher this residual resistance
What does classical physics have to say on that score?
Just nothing. It doesn't distinguish an atom of the metal from an atom of impurity: at the same temperature they vibrate in the same manner and impede the electron motion in exactly the same way.Now quantum mechanics proved a little more observant. These different atoms in the lattice are distinguished very clearly, almost as if they were of different colours.
Then how do we account for the electrical resistance?
First we think on electron diffraction on a crystal. There the electrons which impinged on the outer layers of the atoms of the crystal were partially reflected and formed diffraction on a photographic plate.
Couldn't we consider the electron current in a metal as a beam of electrons? Well, yes. Here the electrons stream along in one general direction, only the beam is wider occupying the whole cross-section of the piece of metal.
But then it is inevitably follows that the passage of electrons in a metal should be accompanied by an 'internal diffraction', as it were, of electrons on the ions of the lattice. If we could put a photographic plate inside the metal, we should be able to get a diffraction pattern.
Diffraction has an interesting property: if there is the slightest deviation in the regularity of the objects scattering the waves, the clear cut pattern vanishes and the plate is uniformly fogged. As physicists say, the scattering of the waves has become homogeneous.
It is just such disorder that is introduced into the regular structure of a metallic crystal by ionic vibrations and by the presence of impurity atoms. As a result, the waves of the electrons participating in the current are scattered in all directions.
As a rule, impurity atoms have quite different dimensions and electron shells than the atoms of the metal. The impurity atoms distort the lattice. Pushing the analogy further still we could say that the impurity atoms twist the corridors, bend the walls and deform floor of our building, It is clear that such defects remain even when the floor and walls cease to tremble. Sure enough, the distortions introduced into a metallic lattice by impurity atoms are independent of the temperature and remain even at absolute zero.
The scattering of electron waves on these lattice imperfections is the cause of the residual electrical resistance of metals that was so incomprehensible to classical physics.
Thus, it turns out that metals are far from perfect as conductors of current. True, not all of them and not at all times. Nature, feeling that it just had to produce something better, created superconductors.
A number of metals and alloys (as yet, just a few) begin to behave very strangely at extremely low temperatures. At just ten or so degrees above absolute zero, these substances suddenly lose practically all their electrical resistance.
This phenomenon, discovered half a century ago, became known as superconductivity.
Classical physics could not find an explanation for it. It is interesting to note that even the powerful quantum mechanics had to work hard for about teighty years before it came up with anything reasonable.
The enigma of superconductivity was resolved only some years ago. A big contribution to disentangling this mystery was made by the Soviet physicist N. N. Bogolyubov and his pupils.The superconductivity trick is due to the fact that at very low temperatures close to absolute zero the interaction of the electron cloud with the ionic skeleton in a number of metals changes drastically due to certain peculiarities of structure. Whereas before, each soldier of the electron army fought on his own, at the low temperature of superconductivity the electrons form into pairs.
The effect on the war between the electrons and ions is immediate Whereas before, each electron fought separately with the ions and could easily be put out of commission, now these electron teams warded off the blows of individual ions without batting an eyelash. The electrons ceased to notice the aggressive ionic encirclement, as it were. The difficulties of the electron army were reduced, and finally the electrical resistance of the metal fell off catastrophically.
In the language of physics, the new type of war consists in the fact that now the wavelengths corresponding to electron motion in the metal are thousands and tens of thousands of times greater than the distances between ions.
The wavelength of an electron pair is so much greater than the dimensions of the ionic obstacles in its path that
the scattering of individual electrons, which accompanies the passage of current through a metal under ordinary conditions, disappears-and with it, the resistance to current.
This ideal organization of the electron army is maintained only so long as the temperature is sufficiently low. As the temperature rises above a certain limit, the clashes with ions break up the pairs into separate soldiers. The balance of
forces has changed and the electrical resistance of the metal is restored.
So it was worth asking how current flows in metals.
Reference
Modern Classical Physics: Thorne, K. and Blandford, R
Analysis of electric current : Susan M. StocklmayerDavid F. Treagust
Internet.