Conduction in Solids
. The conduction of electric currents in solid substances is made possible by the presence of free electrons (electrons that are free to move about). Most of the electrons in a bar of copper, for example, are tightly bound to individual copper atoms. However, some are free to move from atom to atom, enabling current to flow. Ordinarily the motion of the free electrons is random; that is, as many of them are moving in one direction as in another. However, if a voltage is applied to the two ends of a copper bar by means of a battery, the free electrons tend to drift toward one end. This end is said to be at a higher potential and is called the positive end. The other end is said to be at a lower potential and is called the negative end. The function of a battery or other source of electric current is to maintain potential difference. A battery does this by supplying electrons to the negative end of the bar to replace those that drift to the positive end and also by absorbing electrons at