22.2 : Sources and Properties of Electric Charge

All objects we see around us consist of atoms, which combine to form molecules. The lightest element in the universe is hydrogen, and a hydrogen atom consists of a positively charged proton and a negatively charged electron. The magnitude of charge that a proton and an electron carry are the same, and it is the fundamental unit of charge. In SI units, it is 1.602 times 10^-19 coulomb.

Most atoms additionally constitute another fundamental particle, the neutron. It carries no electrical charge. A neutron and a proton have almost identical masses, which are about 2000 times greater than the mass of an electron.

Heavier atoms, for example, helium, consist of twice the number of protons and neutrons. In all atoms, protons and neutrons are bound together in a nucleus. Hence, most of the mass of an atom is concentrated in the nucleus. The binding force of mutually repelling positively charged protons and electrically neutral neutrons is not electrical. Instead, it is different from gravitational and electrical forces. This force is called the strong nuclear force.

The relative difference in mass of the nucleus and the electrons makes the electrons freer to move. If an electron is removed from an atom, it becomes positively charged, whereas it becomes negatively charged if an electron is added to it. Such atoms are called ions.

It is empirically observed that electric charge can neither be created nor destroyed; it can only be transferred from place to place, from one object to another. Frequently, we speak of two charges "canceling"; this is verbal shorthand. It means that if two objects with equal and opposite charges are physically close to each other, then the equal and opposite forces they apply on any other charged object nullify, resulting in zero net force on the latter. However, the charges on the objects by no means disappear. The net charge of the universe is constant.

The conservation of electric charge is not just a global phenomenon concerning the entire universe but also a local phenomenon. In principle, if a negative charge disappeared from a laboratory and reappeared on the Moon, charge conservation would still hold. However, this never happens. If the total charge on the lab bench is changing, there will be a measurable flow of charge into or out of the system. Again, charges can and do move around, and their effects can and do cancel, but the net charge in the closed, local environment is conserved. This idea is called the law of conservation of charge.