Matter, Atoms, and Electrical Materials

12-1 Fundamentals of Electricity & Electronics Chapter 12

Introduction

This chapter addresses the fundamental concepts that are the building blocks for advanced electrical knowledge and practical troubleshooting. Some of the questions addressed are: How does energy travel through a copper wire and through space? What is electric current and electromotive force? What makes a landing light turn on or a hydraulic pump motor run? Each of these questions requires an understanding of many basic principles. By adding one basic idea on top of other basic ideas, it becomes possible to answer most of the interesting and practical questions about electricity or electronics.

Our understanding of electrical current must begin with the nature of matter. All matter is composed of molecules. All molecules are made up of atoms, which are themselves made up of electrons, protons, and neutrons.

General Composition of Matter

Matter

Matter can be defined as anything that has mass and volume and is the substance of which physical objects are composed. Essentially, it is anything that can be touched. Matter is what all things are made of; whatever occupies space, has mass, and is perceptible to the senses in some way. Weight is an indirect method of determining mass, but it is not the same. Weight is a measure of the pull of gravity acting on the mass of an object. The more mass an object has, the more it weighs under the earth’s force of gravity. Mathematically, weight can be stated as follows: Weight = Mass × Gravity Categories of matter are ordered by molecular activity.

The four categories or states are: solids, liquids, gases, and plasma. For the purposes of the aircraft technician, only solids, liquids, and gases are considered. Element An element is a substance that cannot be reduced to a simpler form by chemical means. Iron, gold, silver, copper, and oxygen are examples of elements. Beyond this point of reduction, the element ceases to be what it is. Compound A compound is a chemical combination of two or more elements. Water is one of the most common compounds and is made up of two hydrogen atoms and one oxygen atom. Molecule The smallest particle of matter that can exist and still retain its identity, such as water (H 2O), is called a molecule.

[Figure 12-1] Substances composed of only one type of atom are called elements. But most substances occur in nature as compounds, that is, combinations of two or more types of atoms. It would no longer retain the characteristics of water if it were compounded of one atom of hydrogen and two atoms of oxygen. If a drop of water is divided and then divided again and again until it cannot be divided any longer, it is still water. Atom The atom is considered to be the most basic building block of all matter. Atoms are composed of three subatomic particles: protons, neutrons, and electrons. These three particles determine the properties of the specific atoms.

Elements are substances composed of the same atoms with specific properties. Oxygen is an example of this. The main property that defines each element is the number of neutrons, protons, and electrons. Hydrogen and helium are examples of elements. Both of these elements have neutrons, protons, and electrons but differ in the number of those items. This difference alone accounts for the variations in chemical and physical properties of these two different elements. There are over 100 known elements in the periodic table. [Figure 12-2] They are categorized according to their properties on that table. The kinetic theory of matter also states that the particles that make up the matter are always moving. Thermal expansion is considered in the kinetic theory and explains why matter contracts when it is cool and expands when it is hot, with the exception of water/ice.

Electrons, Protons, & Neutrons At the center of the atom is the nucleus, which contains protons and neutrons. Protons are positively-charged particles, and neutrons are neutrally-charged particles. A neutron has approximately the same mass as the proton. The third particle of the atom is the electron that is a negatively- 12-2 Oxygen atom Hydrogen atoms Nucleus Electrons charged particle with a very small mass compared to the proton. The proton’s mass is approximately 1,837 times greater than the electron. Due to the proton and the neutron location in the central portion of the atom (nucleus) and the electron’s position at the distant periphery of the atom, it is the electron that undergoes the change during chemical reactions. Since a proton weighs approximately 1,845 times as much as an electron, the number of protons and neutrons in its nucleus determines the overall weight of an atom. The weight of an electron is not considered in determining the weight of an atom. Indeed, the nature of electricity cannot be defined clearly because it is not certain whether the electron is a negative charge with no mass (weight) or a particle of matter with a negative charge.

Hydrogen represents the simplest form of an atom. [Figure 12-3] At the nucleus of the hydrogen atom is one proton and at the outer shell is one orbiting electron. At a more complex level is the oxygen atoms, which has eight electrons in two shells orbiting the nucleus with eight protons and eight neutrons. [Figure 12-4] When the total positive charge of the protons in the nucleus equals the total negative charge of the electrons in orbit around the nucleus, the atom is said to have a neutral charge. Electron Shells & Energy Levels Electrons require a certain amount of energy to stay in an orbit. This particular quantity is called the electron’s energy level. By its motion alone, the electron possesses kinetic energy, while the electron’s position in orbit determines its potential energy. The total energy of an electron is the main factor that determines the radius of the electron’s orbit.

Electrons of an atom appear only at certain definite energy levels (shells). The spacing between energy levels is such that when the chemical properties of the various elements are cataloged, it is convenient to group several closely spaced permissible energy levels together into electron shells. The maximum number of electrons that can be contained in any shell or sub-shell is the same for all atoms and is defined as electron capacity = 2n 2. In this equation, n represents the energy level in question. The first shell can only contain two electrons; the second shell can only contain eight electrons; the third, 18, and so on until we reach the seventh shell for the heaviest atoms, which have six energy levels. Because the innermost shell is the lowest energy level, the shell begins to fill up from the shell closest to the nucleus and fill outward as the atomic number of the element increases. However, an energy level does not need to be completely filled before electrons begin to fill the next level. The Periodic Table of Elements should be checked to determine an element’s electron configuration.

Valence Electrons Valence is the number of chemical bonds an atom can form. Valence electrons are electrons that can participate in chemical bonds with other atoms. The number of electrons in the outermost shell of the atom is the determining factor in its valence. Therefore, the electrons contained in this shell are called valence electrons. Ions Ionization is the process by which an atom loses or gains electrons. Dislodging an electron from an atom causes the atom to become positively charged. This net positively- charged atom is called a positive ion or a cation. An atom that has gained an extra number of electrons is negatively charged and is called a negative ion or an anion. When atoms are neutral, the positively-charged proton and the negatively- charged electron are equal.

Free Electrons Valence electrons are found drifting midway between two nuclei. Some electrons are more tightly bound to the nucleus of their atom than others and are positioned in a shell or sphere closer to the nucleus, while others are more loosely bound and orbit at a greater distance from the nucleus. These outermost electrons are called “free” electrons because they can be easily dislodged from the positive attraction of the protons in the nucleus. Once freed from the atom, the electron can then travel from atom to atom, becoming the flow of electrons commonly called current in a practical electrical circuit.

Electron Movement Conductors, Insulators, and Semiconductors The valence of an atom determines its ability to gain or lose an electron, which ultimately determines the chemical and electrical properties of the atom. These properties can be categorized as being a conductor, semiconductor, or insulator, depending on the ability of the material to produce free electrons. When a material has a large number of free 12-3 Electron Nucleus (1 Proton) 8 Protons 8 Neutrons electrons available, a greater current can be conducted in the material. Conductors Elements such as gold, copper, and silver possess many free electrons and make good conductors. The atoms in these materials have a few loosely bound electrons in their outer orbits. Energy in the form of heat can cause these electrons in the outer orbit to break loose and drift throughout the material. Copper and silver have one electron in their outer orbits. At room temperature, a piece of silver wire has billions of free electrons.

Insulators Insulators are materials that do not conduct electrical current very well or not at all, such as glass, ceramic, and plastic. 12-4 Under normal conditions, atoms in these materials do not produce free electrons. The absence of free electrons means that electrical current cannot be conducted through the material. Only when the material is in an extremely strong electrical field will the outer electrons be dislodged. This action is called breakdown and usually causes physical damage to the insulator. Semiconductors The material properties of semiconductors fall in between conductors and insulators. In their pure state, they are not good at conducting or insulating. Semiconductors can operate like a conductor or insulator depending on what external load is placed on the material. Semiconductors are used to make transistors and integrated circuits. Silicon and germanium are the most widely used semiconductor materials. For a more detailed explanation on this topic, refer to page 12-98 in this chapter.

Metric Based Prefixes Used for Electrical Calculations In any system of measurements, a single set of units is usually not sufficient for all the computations involved in electrical repair and maintenance. Small distances, for example, can usually be measured in inches, but larger distances are more meaningfully expressed in feet, yards, or miles. Since electrical values often vary from numbers that are a millionth part of a basic unit of measurement to very large values, it is often necessary to use a wide range of numbers to represent the values of units, such as volts, amperes, or ohms. A series of prefixes that appear with the name of the unit have been devised for the various multiples or submultiples of the basic units. There are 12 of these prefixes, which are also known as conversion factors. Four of the most commonly used prefixes in electrical work are: Mega (M) means one million (1,000,000).

Kilo (k) means one thousand (1,000). Milli (m) means one-thousandth (1⁄1,000). Micro (μ) means one-millionth (1⁄1,000,000). Kilo is one of the most extensively used conversion factors. It explains the use of prefixes with basic units of measurement. Kilo means 1,000, and when used with volts, is expressed as kilovolt, meaning 1,000 volts. The symbol for kilo is the letter “k.” Thus, 1,000 volts is one kilovolt or 1 kV . Conversely, one volt would equal one-thousandth of a kV , or 1 ⁄1,000 kV . This could also be written 0.001 kV . Similarly, the word “milli” means one-thousandth, and thus, 1 millivolt equals one-thousandth (1⁄1000) of a volt.

express electrical quantities, together with the prefixes and symbols used to represent each number.

Static Electricity

Electricity is often described as being either static or dynamic. The difference between the two is based simply on whether the electrons are at rest (static) or in motion (dynamic). Static electricity is a buildup of an electrical charge on the surface of an object. It is considered “static” due to the fact that there is no current flowing as in alternate current (AC) or direct current (DC) electricity. Static electricity is usually caused when non-conductive materials, such as rubber, plastic, or glass, are rubbed together causing a transfer of electrons, which results in an imbalance of charges between the two materials. The fact that there is an imbalance of charges between the two materials means that the objects will exhibit an attractive or repulsive force.

Attractive and Repulsive Forces One of the most fundamental laws of static electricity, as well as magnetics, deals with attraction and repulsion. Like charges repel each other and unlike charges attract each other. All electrons possess a negative charge and as such repel each other. Similarly, all protons possess a positive charge and as such repel each other. Electrons (negative) and protons (positive) are opposite in their charge and attract each other. For example, if two pith balls are suspended, as shown in rod, some of the charge from the rod is transferred to the balls. The balls now have similar charges and, consequently, repel each other as shown in part B of Figure 12-6. If a plastic rod is rubbed with fur, it becomes negatively charged and the fur is positively charged. By touching each ball with these differently charged sources, the balls obtain opposite charges and attract each other as shown in part C of Figure 12-6.

Although most objects become charged with static electricity by means of friction, a charged substance can also influence objects near it by contact. [Figure 12-7] If a positively-charged rod touches an uncharged metal bar, it draws electrons from the uncharged bar to the point of contact. Some electrons enter the rod, leaving the metal bar with a deficiency of electrons (positively charged) and making the rod less positive than it was or, perhaps, even neutralizing its charge completely. A method of charging a metal bar by induction is demonstrated in Figure 12-8. A positively-charged rod is brought near, but does not touch, an uncharged metal bar. Electrons in the metal bar are attracted to the end of the bar nearest the positively- charged rod, leaving a deficiency of electrons at the opposite end of the bar. If this positively-charged end is touched by a neutral object, electrons will flow into the metal bar and 12-5 Number Prefix Symbol 1,000,000,000,000 tera t 1,000,000,000 giga g 1,000,000 mega M 1,000 kilo k 100 hecto h 10 deka dk 0.1 deci d 0.01 centi c 0.001 milli m 0.000001 micro µ 0.000000001 nano n 0.000000000001 pico pFigure 12-5. Prefixes and symbols for multiples of basic quantities.

neutralize the charge. The metal bar is left with an overall excess of electrons. Electrostatic Field A field of force exists around a charged body. This field is an electrostatic field (sometimes called a dielectric field) and is represented by lines extending in all directions from the charged body and terminating where there is an equal and opposite charge. To explain the action of an electrostatic field, lines are used to represent the direction and intensity of the electric field of force. As illustrated in Figure 12-9, the intensity of the field is indicated by the number of lines per unit area, and the direction is shown by arrowheads on the lines pointing in the direction in which a small test charge would move (or tend to move) if acted upon by the field of force.

Either a positive or negative test charge can be used, but it has been arbitrarily agreed that a small positive charge is always used in determining the direction of the field. Thus, the direction of the field around a positive charge is always away from the charge because a positive test charge would be repelled. [Figure 12-9] On the other hand, the direction of the lines about a negative charge is toward the charge, since a positive test charge is attracted toward it. charges. Positive charges are shown, but regardless of the type of charge, the lines of force would repel each other if the charges were alike. The lines terminate on material objects and always extend from a positive charge to a negative charge. These are imaginary lines used to show the direction a real force takes.

It is important to know how a charge is distributed on an object. Figure 12-11 shows a small metal disk on which a concentrated negative charge has been placed. By using an electrostatic detector, it can be shown that the charge is spread evenly over the entire surface of the disk. Since the metal disk provides uniform resistance everywhere on its surface, the mutual repulsion of electrons results in an even distribution over the entire surface. Another example, shown in Figure 12-12, is the charge on a hollow sphere. Although the sphere is made of conducting material, the charge is evenly distributed over the outside surface. The inner surface is completely neutral. This phenomenon is used to safeguard operating personnel of the large Van de Graaff static generators used for atom smashing.

The safest area for the operators is inside the large sphere, where millions of volts are being generated. The distribution of the charge on an irregularly-shaped object differs from that on a regularly-shaped object. Figure 12-13 shows that the charge on such objects is not evenly distributed. The greatest charge is at the points, or areas of sharpest curvature, of the objects. Electrostatic Discharge (ESD) Considerations One of the most frequent causes of damage to a solid- state component or integrated circuits is the electrostatic discharge (ESD) from the human body when one of these devices is handled. Careless handling of line replaceable units (LRUs), circuit cards, and discrete components can cause unnecessarily time consuming and expensive repairs.

This damage can occur if a technician touches the mating pins for a card or box. Other sources for ESD can be the top of a toolbox that is covered with a carpet. Damage can be avoided by discharging the static electricity from your body by touching the chassis of the removed box, by wearing a grounding wrist strap, and exercising good professional handling of the components in the aircraft. This can include placing protective caps over open connectors and not placing an ESD-sensitive component in an environment that causes damage. Parts that are ESD sensitive are typically shipped in bags specially designed to protect components from electrostatic damage.

Other precautions that should be taken with working with electronic components are: 1. Always connect a ground between test equipment and circuit before attempting to inject or monitor a signal. 2. Ensure test voltages do not exceed maximum allowable voltage for the circuit components and transistors. 3. Ohmmeter ranges that require a current of more than one milliampere in the test circuit should not be used for testing transistors.