After completing this chapter, the student will be able to:

Transcription

1 Fundamentals of Electricity OBJECTVES After completing this chapter, the student will be able to: Define atom, matter, element, and molecule. List the parts of an atom. Define the valence shell of an atom. dentify the unit for measuring current. Draw the symbol used to represent current flow in a circuit. Describe the difference between conductors, insulators, and semiconductors. Define difference of potential, electromotive force, and voltage. Draw the symbol used to represent voltage. dentify the unit used to measure voltage. Define resistance. dentify characteristics of resistance in a circuit. dentify the unit for measuring resistance. Draw the symbol used to represent resistance in a circuit. See accompanying CD for interactive presentations and tutorials relating to Chapter 1. Everything, whether natural or man-made, can be broken down into either an element or a compound. However, the smallest part of each of these is the atom. The atom is made up of protons, neutrons, and electrons. The protons and neutrons group together to form the center of the atom called the nucleus. The electrons orbit the nucleus in shells located at various distances from the nucleus. When appropriate external force is applied to electrons in the outermost shell, they are knocked loose and become free electrons. The movement of free electrons is called current. The external force needed to create this current is called voltage. 3

2 SECTON 1 DC CRCUTS As it travels along its path, the current encounters some opposition, called resistance. This chapter looks at how current, voltage, and resistance collectively form the fundamentals of electricity. 11 MATTER, ELEMENTS, AND COMPOUNDS Matter is anything that occupies space and has weight. t may be found in anyone of three states: solid, liquid, or gas. Examples of matter include the air we breathe, the water we drink, the clothing we wear, and ourselves. Matter may be either an element or a compound. An element is the basic building block of nature. t is a substance that cannot be reduced to a simpler substance by chemical means. There are now over 100 known elements (Appendix 2). Examples of elements are gold, silver, copper, and oxygen. The chemical combination of two or more elements is called a compound (Figure 1-1). A compound can be separated by chemical but not by physical means. Examples of compounds are water, which consist of hydrogen and oxygen, and salt. which consists of sodium and chlorine. The smallest part of the compound that still retains the properties of the compound is called a molecule. A molecule is the chemical combination of two or more atoms. An atom is the smallest particle of an element that retains the characteristic of the element. The physical combination of elements and compounds is called a mixture. Examples of mixtures include air, which is made up of oxygen, nitrogen, carbon dioxide, and other gases, and salt water, which consists of salt and water. FGURE 1-1 The chemical combination of two or more elements is called a compound. A molecule is the chemical combination of two or more atoms. Examples are water (H 2 0) and salt (NaC!). ELEMENT 1 ELEMENT 2 COMPOUND

3 CHAPTER1 FUNDAMENTALS OF ELECTRCTY "1-1 QUESTONS 1. n what forms can matter be found? 2. What is a substance called that cannot be reduced to a simpler substance by chemical means? 3. What is the smallest possible particle that retains the characteristic of a compound? 4. What is the smallest possible particle that retains the characteristic of an element? 1m A CLOSER LOOK AT ATOMS As previously stated, an atom is the smallest particle of an element. Atoms of different elements differ from each other. f there are over 100 known elements, then there are over 100 known atoms. Every atom has a nucleus. The nucleus is located at the center of the atom. t contains positively charged particles called protons and uncharged particles called neutrons. Negatively charged particles called electrons orbit around the nucleus (Figure 1-2). The number of protons in the nucleus of the atom is called the element's atomic number. Atomic numbers distinguish one element from another. Each element also has an atomic weight. The atomic weight is the mass of the atom. t is determined by the total number of protons and neutrons in the nucleus. Electrons do not contribute to the total mass of the atom; an electron's mass is only 1/1845 that of a proton and is not significant enough to consider. The electrons orbit in concentric circles about the nucleus. Each orbit is called a shell. These shells are filled in sequence; K is filled first, then L, M, N, and so on (Figure 1-3). The maximum number of electrons that each shell can accommodate is shown in Figure 1-4. The outer shell is called the valence shell and the number of electrons it contains is the valence. The farther the valence shell is from the nucleus, the less attraction the nucleus has on each valence electron. Thus the potential for the atom to gain or lose electrons increases if the valence shell is not full and is located far enough away from the FGURE 1 3 The electrons are held in shells around the nucleus. FGURE 1 2 Parts of an atom. ELECTRON PROTON NEUTRON ORBT NUCLEUS

4 SECTON DC CRCUTS FGURE 1-4 The number of electrons each shell can accommodate. FGURE 1-6 Copper has a valence of 1. SHE;LL DESGNATON K L M N o P Q TOTAL NUMBEAOF ELECTRONS NUCLEUS nucleus. Conductivity of an atom depends on its valence band. The greater the number of electrons in the valence shell, the less it conducts. For example, an atom having seven electrons in the valence shell is less conductive than an atom having three electrons in the valence shell. Electrons in the valence shell can gain energy. f these electrons gain enough energy from an external force, they can leave the atom and become free electrons, moving randomly from atom to atom. Materials that contain a large number of free electrons are called conductors. Figure 1-5 compares the conductivity of various metals used as conductors. On the chart, silver, copper, and gold have a valence of 1 (Figure 1-6). However, silver is the best conductor because its free electron is more loosely bonded. nsulators, the opposite of conductors, prevent the flow of electricity, nsulators are stabilized by absorbing valence electrons from other atoms to fill their valence shells, thus eliminating free electrons. FGURE 1-7 nsulation properties of various materials used as insulators. FGURE 1-5 Conductivity of various metals used as conductors. Silver High Copper Gold Aluminum Tungsten ron Nichrome Low Mica Glass Teflon Paper (Paraffin) Rubber Bakelite Oils Procelain Air High Low

5 CHAPTER 1 FUNDAMENTALS OF ELECTRCTY Materials classified as insulators are compared in Figure 1-7. Mica is the best insulator because it has the fewest free electrons in its valence shell. A perfect insulator will have atoms with full valence shell. This means it cannot gain electrons. Halfway between conductors and insulators are semiconductors. Semiconductors are neither good conductors nor good insulators but are important because they can be altered to function as conductors or insulators. Silicon and germanium are two semiconductor materials. An atom that has the same number of electrons and protons is said to be electrically balanced. A balanced atom that receives one or more electrons is no longer balanced. t is said to be negatively charged and is called a negative ion. A balanced atom that loses one or more electrons is said to be positively charged and is called a positive ion. The process of gaining or losing electrons is called ionization. onization is significant in current flow. 1-2 QUESTONS l. What atomic particle has a positive charge and a large mass? 2. What atomic particle has no charge at all? 3. What atomic particle has a negative charge and a small mass? 4. What does the number of electrons in the outermost shell determine? 5. What is the term for describing the gaining or losing of electrons? lb CURRENT Given an appropriate external force, the movement of electrons is from negatively charged atoms to positively charged atoms. This flow of electrons is called current (). The symbol is used to represent current. The amount of current is the sum of the charges of the moving electrons past a given point. An electron has a very small charge, so the charge of 6.24 X electrons is added together and called a coulomb (C). When one coulomb of charge moves past a single point in one second it is called an ampere (A). The ampere is named for a French physicist named Andre Marie Ampere ( ). Current is measured in amperes. 1-3 QUESTONS l. What action causes current in an electric circuit? 2. What action results in an ampere of current? 3. What symbol is used to represent current? 4. What symbol is used to represent the unit ampere? ld VOLTAGE When there is an excess of electrons (negative charge) at one end of a conductor and a deficiency of electrons (positive charge) at the opposite end, a current flows between the two ends. A current flows through the conductor as long as this condition persists. The source that creates this excess of electrons at one end and the deficiency at the other end represents the potential. The potential is the ability of the source to perform electrical work. The actual work accomplished in a circuit is a result of the difference of potential available at the two ends of a conductor. t is this difference of potential that causes electrons to move or flow in a circuit (Figure 1-8). The difference of potential is referred to as electromotive force (emf) or voltage. Voltage is the force that moves the electrons in the circuit. Think of voltage as the pressure or pump that moves the electrons. The symbol Eis used in electronics to represent voltage. The unit for measuring voltage is the volt (V), named for Count Alessandro Volta ( ), inventor of the first cell to produce electricity.

6 SECTON 1 DC CRCUTS FGURE 1-8 Electrons flow in a circuit because of the difference of potential. 1"'-.~-VDFFERENCE OF POTENTAL + -..= LOAD _...J L 1-4 QUESTONS 1. What force moves electrons in a circuit? 2. What is the term that represents the potential between the two ends of a conductor? 3. What symbol is used to represent voltage? 4. What symbol is used to represent the unit volt? lj RESSTANCE As the free electrons move through the circuit, they encounter atoms that do not readily give up electrons. This opposition to the flow of electrons (the current) is called resistance (R). Every material offers some resistance or opposition to current flow. The degree of resistance of a material depends on its size, shape, and temperature. Materials with a low resistance are called conductors. Conductors have many free electrons and offer little resistance to current flow. As previously mentioned, silver, copper, gold, and aluminum are examples of good conductors. Materials with a high resistance are called insulators. nsulators have few free electrons and offer a high resistance to current flow. As previously mentioned, glass, rubber, and plastic are examples of good insulators. Resistance is measured in ohms, a unit named for the German physicist George Simon Ohm ( ). The symbol for the ohm is the Greek letter omega (Q). 1-5 QUESTONS 1. What is the term used to describe opposition to current flow? 2. What is the main difference between conductors and insulators? 3. What is the symbol used to represent resistance? 4. What is the symbol used to represent the unit of resistance? SUMMARY ~~. Matter is anything that occupies space. Matter can be an element or compound. An element is the basic building block of nature. A compound is a chemical combination of two or more elements. A molecule is the smallest unit of a compound that retains the properties of the compound. An atom is the smallest unit of matter that retains the structure of the element. An atom consists of a nucleus, which contains protons and neutrons. t also has one or more electrons that orbit around the nucleus. Protons have a positive charge, electrons have a negative charge, and neutrons have no charge.

7 CHAPTER 1 FUNDAMENTALS OF ELECTRCTY The atomic number of an element is the number of protons in the nucleus. The atomic weight of an atom is the sum of protons and neutrons. The orbits of the electrons are called shells. The outer shell of an atom is called the valence shell. The number of electrons in the valence shell is called the valence. An atom that has the same number of protons as electrons is electrically balanced. The process by which atoms gain or lose electrons is called ionization. The flow of electrons is called current. Current is represented by the symbol. The charge of 6,240,000,000,000,000,000 (or 6.24 X ) electrons is called a coulomb. An ampere of current is measured when one coulomb of charge moves past a given point in one second. Ampere is represented by the symbol A. Current is measured in amperes. An electric current flows through a conductor when there is an excess of electrons at one end and a deficiency at the other end. A source that supplies excess electrons represents a potential or electromotive force. The potential or electromotive force is referred to as voltage. Voltage is the force that moves electrons in a circuit. The symbol E is used to represent voltage. A volt (V) is the unit for measuring voltage. Resistance is the opposition to current flow. Resistance is represented by the symbol R. All materials offer some resistance to current flow. The resistance of a material is dependent on the material's size, shape, and temperature. Conductors are materials with low resistance. nsulators are materials with high resistance. Resistance is measured in ohms. The Greek letter omega (Q) is used to represent ohms. CHAPTER 1 SELF-TEST.. '~ ~~, i ~J' 1. What criteria determine whether an atom is a good conductor? 2. What determines whether a material is a conductor, semiconductor, or insulator? 3. Why is it essential to understand the relationship between conductors, semiconductors, and insulators? 4. Explain the difference between current, voltage, and resistance. 5. Describe how the resistance of a material is determined. 6. Name one standard that can be a resource to you in evaluating the safety compliance of an electric drill? 7. Where can you determine what health hazard, if any, solder in your lab poses? 8. n comparing electronics laboratory practices with applicable MSDS information (such as soldering practices), identify changes that could be made to improve safety in the lab.