Matter & Energy


Matter is composed of atoms or groups of atoms called molecules. The arrangement of particles in a material depends on the physical state of the substance. In a solid, particles form a compact structure that resists flow. Particles in a liquid have more energy than those in a solid. They can flow past one another, but they remain close. Particles in a gas have the most energy. They move rapidly and are separated from one another by relatively large distances.

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 the positive end.

Insulators cannot conduct electric currents because all their electrons are tightly bound to their atoms. A perfect insulator would allow no charge to be forced through it, but no such substance is known at room temperature. The best insulators offer high but not infinite resistance at room temperature.

Some substances that ordinarily have no free electrons, such as silicon and germanium, can conduct electric currents when small amounts of certain impurities are added to them. Such substances are called semiconductors. Semiconductors generally have a higher resistance to the flow of current than does a conductor, such as copper, but a lower resistance than an insulator, such as glass.