Basic Electricity
No Image
FIGURE 36-1
Parts of an atom.

All questions about the nature of electricity lead to the composition of matter. All matter is made up of atoms Figure 36-1. Every atom has a nucleus, with at least one positively charged proton and, in most atoms, at least one neutron that has no charge. Moving around the nucleus are one or more negatively charged electrons. Electrons travel in different rings or shells around the nucleus. Each ring or shell can contain a specific maximum number of electrons. Any additional electrons must fit into the next higher ring or shell. With equal numbers of protons and electrons, the charges within an atom cancel each other out, leaving the atom with no overall charge. In this state, the electrons and protons are content to stay in the atom as they are.

An atom with more electrons than protons has an overall negative charge and is called a negative ion Figure 36-2. Ion simply means the atom has an imbalance of electrical charges due to the gain or loss of electrons. This negative ion is not balanced, so it is looking for a change. Since the electrons have the same negative charge, they repel one another, and the repelling force wants to push one of the electrons away from the atom.

No Image
FIGURE 36-2
Ions. A., Negative ion. B. Positive ion.

A deficiency of electrons gives the atom an overall positive charge. This atom is called a positive ion. It is also not balanced and is looking for a change. In this case, it is exerting an attracting force on electrons to try to pull one into its atom from another atom. If a negative ion and positive ion are close enough, the negative charge of the negative ion exerts a repelling force on the extra electron, causing it to be pushed away from its atom; at the same time, the positive ion exerts an attracting force on the extra electron. These repelling and attracting forces cause the electron to be pushed and pulled from the negative atom to the positive atom, balancing both atoms out. The flow of electrons from atom to atom is called current flow and is the basic concept of electricity.

Not all atoms can give up or accept electrons easily. Materials that can do so easily are called conductors, while those that cannot do so easily are called insulators Figure 36-3. The explanation of what makes a good conductor or a good insulator is quite complex and is found in the theories of quantum mechanics, which address the arrangement and behavior of electrons around the nuclei of atoms. To simplify matters, it is safe to say that in some materials there are electrons, called free electrons, located on the outer ring, called the valence ring. These electrons are only loosely held by the nucleus and are free to move from one atom to another when an electrical potential (pressure) is applied. In fact, atoms with fewer electrons in the valence ring are the best conductors, one electron being the best conductive material. This is because the single electron by itself in the valence ring is held the most loosely by the nucleus. Materials made up of atoms with one to three valance ring electrons are considered conductors. The more atoms that have free electrons a particular material has, the better it can conduct electrons. Metals typically have lots of free electrons because of the atoms’ structure and are therefore good conductors.

No Image
FIGURE 36-3
A. Conductor. B. Insulator. C. Semiconductor.

Every substance, even air, will conduct an electrical current if enough electrical pressure (voltage) is applied to it, but the word conductor normally is used for materials that allow current flow with little resistance. Most metals are good conductors. The most common conductor used in automobiles is copper. It is used in virtually all of the wiring that connects automotive components together. The more electrons a conductor must carry, the heavier the gauge or thickness the wire needs to be.

Materials that do not conduct electrons easily are called insulators. Most plastics are good insulators. The plastic covering on a wire is an example of this. The ceramic portion of a spark plug is also a good insulator. In insulators, electrons in the valence ring are bound much more tightly to the nucleus. A good insulator does not support current flow because it has no or very few free electrons, and the electrons it does have cannot move freely; therefore, an insulator prevents the movement of electrons when an electrical potential is applied. Insulators are made up of atoms that have five to eight valence ring electrons. The greater the number of valence ring electrons, the better the insulator (Figure 36-3).

Semiconductors are materials that conduct electricity more easily than insulators but not as well as conductors. Semiconductors such as silicon are crucial in electronics. They are used to make electronic components, such as transistors and microchips, that can switch the material from a conductor to an insulator and back again very quickly and without mechanical means. Atoms that have four valance ring electrons are considered semiconductors (Figure 36-3). Note that because the semiconductor material has precisely four electrons, it is only one electron away from becoming an insulator or a conductor. If an electron is added, it becomes an insulator; if an electron is removed, it becomes a conductor. Thus, the semiconductor material can be used as a switch to control whether electrons flow though the semiconductor material or are stopped by it. All we have to do is add or subtract electrons from the semiconductor material, which we will explore further in a later section.