The word “electricity” may evoke an image of complex modern technology: lights, motors, electronics, and computers. But the electric force plays an even deeper role in our lives.
The word electricity comes from the Greek word elektron, which means “amber.” Amber is petrified tree resin, and the ancients knew that if you rub a piece of amber with a cloth, the amber attracts small pieces of leaves or dust. A piece of hard rubber, a glass rod, or a plastic ruler rubbed with a cloth will also display this “amber effect,” or static electricity as we call it today.
You have probably experienced static electricity when combing your hair or when taking a synthetic blouse or shirt from a clothes dryer. In each case, an object becomes “charged” as a result of rubbing, and is said to possess a net electric charge.
There are two, and only two, types of electric charge. Each type of charge repels the same type but attracts the opposite type. That is: unlike charges attract; like charges repel. The two types of electric charge were referred to as positive and negative by the American statesman, philosopher, and scientist Benjamin Franklin.
The choice of which name went with which type of charge was arbitrary. Franklin’s choice set the charge on the rubbed glass rod to be positive charge, so the charge on a rubbed plastic ruler is called negative charge. We still follow this convention today.
Franklin argued that whenever a certain amount of charge is produced on one object, an equal amount of the opposite type of charge is produced on another object. The positive and negative are to be treated algebraically, so during any process, the net change in the amount of charge produced is zero.
When a plastic ruler is rubbed with a paper towel, the plastic acquires a negative charge and the towel acquires an equal amount of positive charge. The charges are separated, but the sum of the two is zero.
The law of conservation of electric charge states that: the net amount of electric charge produced in any process is zero or no net electric charge can be created or destroyed.
If one object (or a region of space) acquires a positive charge, then an equal amount of negative charge will be found in neighboring areas or objects.
A simplified model of an atom shows it as having a tiny but heavy, positively charged nucleus surrounded by one or more negatively charged electrons.
The nucleus contains protons, which are positively charged, and neutrons, which have no net electric charge.
All protons and all electrons have exactly the same magnitude of electric charge; but their signs are opposite.
Hence neutral atoms, having no net charge, contain equal numbers of protons and electrons. Sometimes an atom may lose one or more of its electrons, or may gain extra electrons, in which case it will have a net positive or negative charge and is called an ion.
In solid materials the nuclei tend to remain close to fixed positions, whereas some of the electrons may move quite freely. When an object is neutral, it contains equal amounts of positive and negative charge. The charging of a solid object by rubbing can be explained by the transfer of electrons from one object to the other.
In liquids and gases, nuclei or ions can move as well as electrons.
Suppose we have two metal spheres, one highly charged and the other electrically neutral. If we place a metal object so that it touches both spheres, the previously uncharged sphere quickly becomes charged.
If, instead, we had connected the two spheres by a wooden rod, the uncharged ball would not become noticeably charged.
From the atomic point of view, the electrons in an insulating material are bound very tightly to the nuclei. In a good conductor, some of the electrons are bound very loosely and can move about freely within the material (although they cannot leave the object easily) and are often referred to as free electrons or conduction electrons.
When a positively charged object is brought close to or touches a conductor, the free electrons in the conductor are attracted by this positively charged object and move quickly toward it. In a semiconductor, there are many fewer free electrons, and in an insulator, almost none.