CHARGE

 It was a major scientific accomplishment to integrate an understanding of electricity with fundamental concepts about the microscopic nature of matter. Observations of static electricity like those mentioned earlier were elegantly explained by Benjamin Franklin in the late 1700s as follows: There exist in nature two types of a property called charge, arbitrarily labeled “positive” and “negative.” Opposite charges attract each other, while like charges repel. When certain materials rub together, one type of charge can be transferred by friction and “charge up” objects that subsequently repel objects of the same kind (hair), or attract objects of a different kind (polyester and cotton, for instance).

Through a host of ingenious experiments, scientists arrived at a model of the atom as being composed of smaller individual particles with opposite charges, held together by their electrical attraction. Specifically, the nucleus of an atom, which constitutes the vast majority of its mass, contains protons with a positive charge, and is enshrouded by electrons with a negative charge. The nucleus also contains

neutrons, which resemble protons, except they have no charge. The electric attraction between protons and electrons just balances the electrons’ natural tendency to escape, which results from both their rapid movement, or kinetic energy, and their mutual electric repulsion. (The repulsion among protons in the nucleus is overcome by another type of force called the strong nuclear interaction, which only acts over very short distances.)

This model explains both why most materials exhibit no obvious electrical properties, and how they can become “charged” under certain circumstances: The opposite charges carried by electrons and protons are equivalent in magnitude, and when electrons and protons are present in equal numbers (as they are in a normal atom), these charges “cancel” each other in terms of their effect on their environment. Thus, from the outside, the entire atom appears as if it had no charge whatsoever; it is electrically neutral.

Yet individual electrons can sometimes escape from their atoms and travel elsewhere. Friction, for instance, can cause electrons to be transferred from one material into another. As a result, the material with excess electrons becomes negatively charged, and the material with a deficit of electrons becomes positively charged (since the positive charge of its protons is no longer compensated). The ability of electrons to travel also explains the phenomenon of electric current, as we will see shortly.

Some atoms or groups of atoms (molecules) naturally occur with a net charge because they contain an imbalanced number of protons and electrons; they are called ions. The propensity of an atom or molecule to become an ion—namely, to release electrons or accept additional ones—results from peculiarities in the geometric pattern by which electrons occupy the space around the nuclei. Even electrically neutral molecules can have a local appearance of charge that results from imbalances in the spatial distribution of electrons—that is, electrons favoring one side over the other side of the molecule. These electrical phenomena within molecules determine most of the physical and chemical properties of all the substances we know.

While on the microscopic level, one deals with fundamental units of charge (that of a single electron or proton), the practical unit of charge in the context of electric power is the coulomb (C). One coulomb corresponds to the charge of 6.25 x 10 to power protons. Stated the other way around, one proton has a charge

of 1.6 x 10 to power -19 C. One electron has a negative charge of the same magnitude, 21.6 x 10 to power -19 C. In equations, charge is conventionally denoted by the symbol Q or q.