Basic Physical Concepts

IT IS IMPORTANT TO UNDERSTAND SOME SIMPLE, GENERAL PHYSICS PRINCIPLES in order to have a full grasp of electricity and electronics. It is not necessary to know high-level mathematics. In science, you can talk about qualitative things or about quantitative things, the “what” versus the “how much.” For now, you need only be concerned about the “what.” The “how much” will come later.

Atoms
All matter is made up of countless tiny particles whizzing around. These particles are extremely dense; matter is mostly empty space. Matter seems continuous because the particles are so small, and they move incredibly fast.

Even people of ancient times suspected that matter is made of invisible particles. They deduced this from observing things like water, rocks, and metals. These substances are much different from each other. But any given material—copper, for example—is the same wherever it is found. Even without doing any complicated experiments, early physicists felt that substances could only have these consistent behaviors if they were made of unique types, or arrangements, of particles. It took centuries before people knew just how this complicated business works. And even today, there are certain things that scientists don’t really know. For example, is there a smallest possible material particle?

There were some scientists who refused to believe the atomic theory, even around the year of 1900. Today, practically everyone accepts the theory. It explains the behavior of matter better than any other scheme.

Eventually, scientists identified 92 different kinds of fundamental substances in nature, and called them elements. Later, a few more elements were artificially made.

Each element has its own unique type of particle, known as its atom. Atoms of different elements are always different. The slightest change in an atom can make a tremendous difference in its behavior. You can live by breathing pure oxygen, but you can’t live off of pure nitrogen. Oxygen will cause metal to corrode, but nitrogen will not. Wood will burn furiously in an atmosphere of pure oxygen, but will not even ignite in pure nitrogen. Yet both are gases at room temperature and pressure; both are colorless, both are odorless, and both are just about of equal weight. These substances are so different because oxygen has eight protons, while nitrogen has only seven.

There are many other examples in nature where a tiny change in atomic structure makes a major difference in the way a substance behaves.

Protons, neutrons, and the atomic number
The part of an atom that gives an element its identity is the nucleus. It is made up of two kinds of particles, the proton and the neutron. These are extremely dense. A teaspoonful of either of these particles, packed tightly together, would weigh tons. Protons and neutrons have just about the same mass, but the proton has an electric charge while the neutron does not.

The simplest element, hydrogen, has a nucleus made up of only one proton; there are usually no neutrons. This is the most common element in the universe. Sometimes a nucleus of hydrogen has a neutron or two along with the proton, but this does not occur very often. These “mutant” forms of hydrogen do, nonetheless, play significant roles in atomic physics.
The second most abundant element is helium. Usually, this atom has a nucleus with two protons and two neutrons. Hydrogen is changed into helium inside the sun, and in the process, energy is given off. This makes the sun shine. The process, called fusion, is also responsible for the terrific explosive force of a hydrogen bomb.

Every proton in the universe is just like every other. Neutrons are all alike, too. The number of protons in an element’s nucleus, the atomic number, gives that element its identity. The element with three protons is lithium, a light metal that reacts easily with gases such as oxygen or chlorine. The element with four protons is beryllium, also a metal. In general, as the number of protons in an element’s nucleus increases, the number of neutrons also increases. Elements with high atomic numbers, like lead, are therefore much denser than elements with low atomic numbers, like carbon. Perhaps you’ve compared a lead sinker with a piece of coal of similar size, and noticed this difference.

Isotopes and atomic weights
For a given element, such as oxygen, the number of neutrons can vary. But no matter what the number of neutrons, the element keeps its identity, based on the atomic number. Differing numbers of neutrons result in various isotopes for a given element.

Each element has one particular isotope that is most often found in nature. But all elements have numerous isotopes. Changing the number of neutrons in an element’s nucleus results in a difference in the weight, and also a difference in the density, of the element. Thus, hydrogen containing a neutron or two in the nucleus, along with the proton, is called heavy hydrogen.

The atomic weight of an element is approximately equal to the sum of the number of protons and the number of neutrons in the nucleus. Common carbon has an atomic weight of about 12, and is called carbon 12 or C12. But sometimes it has an atomic weight of about 14, and is known as carbon 14 or C14. The table lists all the known elements in alphabetical order, with atomic numbers in one column, and atomic weights of the most common isotopes in another column. The standard abbreviations are also shown.

Electrons
Surrounding the nucleus of an atom are particles having opposite electric charge from the protons. These are the electrons. Physicists arbitrarily call the electrons’ charge negative, and the protons’ charge positive. An electron has exactly the same
charge quantity as a proton, but with opposite polarity. The charge on a single electron or proton is the smallest possible electric charge. All charges, no matter how great, are multiples of this unit charge.

One of the earliest ideas about the atom pictured the electrons embedded in the nucleus, like raisins in a cake. Later, the electrons were seen as orbiting the nucleus, making the atom like a miniature solar system with the electrons as the planets (Fig. 1-1).

Still later, this view was modified further. Today, the electrons are seen as so fastmoving, with patterns so complex, that it is not even possible to pinpoint them at any given instant of time. All that can be done is to say that an electron will just as likely be inside a certain sphere as outside. These spheres are known as electron shells. Their centers correspond to the position of the atomic nucleus. The farther away from the nucleus the shell, the more energy the electron has (Fig. 1-2).

Electrons can move rather easily from one atom to another in some materials. In other substances, it is difficult to get electrons to move. But in any case, it is far easier to move electrons than it is to move protons. Electricity almost always results, in some way, from the motion of electrons in a material.

Electrons are much lighter than protons or neutrons. In fact, compared to the nucleus of an atom, the electrons weight practically nothing.

Generally, the number of electrons in an atom is the same as the number of protons. The negative charges therefore exactly cancel out the positive ones, and the atom is electrically neutral. But under some conditions, there can be an excess or shortage of electrons. High levels of radiant energy, extreme heat, or the presence of an electric field (discussed later) can “knock” or “throw” electrons loose from atoms, upsetting the balance.

Ions
If an atom has more or less electrons than neutrons, that atom acquires an electrical charge. A shortage of electrons results in positive charge; an excess of electrons gives a negative charge. The element’s identity remains the same, no matter how great the excess or shortage of electrons. In the extreme case, all the electrons might be removed from an atom, leaving only the nucleus. However it would still represent the same element as it would if it had all its electrons.

A charged atom is called an ion. When a substance contains many ions, the material is said to be ionized.

A good example of an ionized substance is the atmosphere of the earth at high altitudes. The ultraviolet radiation from the sun, as well as high-speed subatomic particles from space, result in the gases’ atoms being stripped of electrons. The ionized gases tend to be found in layers at certain altitudes. These layers are responsible for long-distance radio communications at some frequencies.

Ionized materials generally conduct electricity quite well, even if the substance is normally not a good conductor. Ionized air makes it possible for a lightning stroke to take place, for example. The ionization, caused by a powerful electric field, occurs along a jagged, narrow channel, as you have surely seen. After the lightning flash, the nuclei of the atoms quickly attract stray electrons back, and the air becomes electrically neutral again.

An element might be both an ion and an isotope different from the usual isotope. For example, an atom of carbon might have eight neutrons rather than the usual six, thus being the isotope C14, and it might have been stripped of an electron, giving it a positive unit electric charge and making it an ion.

Compounds
Different elements can join together to share electrons. When this happens, the result is a chemical compound. One of the most common compounds is water, the result of two hydrogen atoms joining with an atom of oxygen. There are literally thousands of different chemical compounds that occur in nature.

A compound is different than a simple mixture of elements. If hydrogen and oxygen are mixed, the result is a colorless, odorless gas, just like either element is a gas separately. A spark, however, will cause the molecules to join together; this will liberate energy in the form of light and heat. Under the right conditions, there will be a violent explosion, because the two elements join eagerly. Water is chemically illustrated in Fig. 1-3.

Compounds often, but not always, appear greatly different from any of the elements that make them up. At room temperature and pressure, both hydrogen and oxygen are gases. But water under the same conditions is a liquid. If it gets a few tens of degrees colder, water turns solid at standard pressure. If it gets hot enough, water becomes a gas, odorless and colorless, just like hydrogen or oxygen.

Another common example of a compound is rust. This forms when iron joins with oxygen. While iron is a dull gray solid and oxygen is a gas, rust is a maroon-red or brownish powder, completely unlike either of the elements from which it is formed.

Source: “Book Teach Yourself Electricity and Electronics” – McGraw-Hill