Why is heat produced in a conductor due to the flow of electric current?

Answer 1

Metals are usually good conductors of heat and non-metals are poor conductors. This is put down to metals having many free electrons which move easily and transfer heat from one part of the metal sample to another, whereas non-metals do not have free electrons and can only conduct heat from atom to atom.

Thus silver and copper are good thermal conductors whilst air and cork and wood are poor ones. Silver and copper are also good electrical conductors and the others mentioned poor electrical conductors, and there is some correlation here between the two types of conduction.

Answer 2

With the exception of the superconductors, which have no resistance to electron current flow, conductors all exhibit some amount of resistance. In an aside, we don't really know how superconductors work. Want a Nobel Prize? There an observable phenomenon awaiting your explanation, and you'll definitely have the inside track on bagging it. Superconductors are excluded in this explanation, but we'll need to back up a bit to catch the basics, and then peg the answer. Follow along, if you please.

All materials are composed of matter, and matter is composed of atoms. (A plasma is matter in a high energy system where electrons are stripped off the atoms.) All matter we consider forms some kind of "matrix" across the space it occupies. If we wish to flow electron current across this matrix of atoms, we need the cooperation of the matrix to do so. The electrons in the matrix can help to pass electron current through the space, and they do this by some kind of "bucket brigade" or "musical electron" game where an electron enters the matrix at one side, its presence is "known" by the matrix, and an electron at the other side exits the matrix, all at the speed of the propagation of electricity through that matrix. That's how electricity works. One electron goes in one end of a piece of wire, and another at the other end of the wire exits. (The electron entering does not pass through the whole length of the wire to exit the other end producing the current. That's not how it works.)

In any matrix of atoms, all the electrons occupy various Fermi energy levels. In conductors, some of these levels are high enough that the electrons are not very "firmly" held by parent atoms and are available to support the conduction of electron flow as explained above. Some electrons have enough energy that they can be "moved" with little effort, and that is the mark of a conductor (as opposed to an insulator). But no electrons have infinitely high Fermi energy levels, so it takes some amount of energy to get that electron to help support the electron current flow. All electrons in a given matrix have some resistance to being forced to support current flow. Another way to say this might be that all electrons in the matrix have at least a bit of a hold on them from atomic nuclei in the matrix. The result is that some of the energy put into the matrix to move the electrons is "lost" in the matrix and appears as thermal energy, or heat.

In closing, one measure of a material's ability to conduct electricity is its electrical conductivity (which is opposite its electrical resistance). The higher the conductivity, the less resistance the material offers to current flow, and the less heat will be generated (compared to another material) when a given current flows through it. There is always a "cost" associated with forcing electron current flow through any material, and the only question is how difficult it is going to be enlisting the "cooperation" of the material in allow (supporting) that current flow through it. It is in the nature of all materials (superconductors excepted) to offer some resistance (or not to effortlessly cooperate) with the movement of electron current flow through it. And the "cost" appears in the form of the increase in the thermal energy of the matrix supporting the current flow. You want to make current flow through a material? It will warm up to some degree in doing so.