Electrical Theory Lesson 1: Electricity and Electronics

Transcription

1 Page 1: Welcome to Lesson 1 of Electrical Theory. This lesson covers the following objectives: Identify the relationship between elements and compounds. Construct a model of an atom. Discuss the concepts of atomic weight and atomic number. State the law of charges and explain it using several examples. Explain what is meant by electric current, voltage, and resistance. Describe the two theories of current direction. Distinguish between conductors, insulators, and semiconductors. State and explain Ohm s Law Page 2: Everything in the universe is made up of matter. Matter can be defined as anything that occupies space or has mass. Matter can be found in the form of solids, liquids, and gases. Water is usually found in liquid form. Yet water can be readily changed to a solid or a vapor form by changing its temperature. Page 3: To truly identify a substance, the substance must be broken down into its smallest parts. The substance must be described in terms of its atomic structure. A substance has been broken down to its purest form when breaking it down further will change its atomic characteristics. This form is called an element. There are over 100 elements. Most of these elements occur naturally in our universe. Some common examples of naturally occurring elements are iron, copper, gold, aluminum, carbon, and oxygen. Page 4: Salt is a chemical compound composed of two different elements. The elements are sodium and chlorine. Each of these elements, sodium and chlorine, are deadly poisons to the human body. When combined, sodium and chorine become the harmless compound know as common table salt. If it were possible for you to smash that crystal of salt into its smallest possible piece, you would have one molecule of salt. A molecule is the smallest part of a compound that still retains all the characteristics of that compound. The smallest form of an element is known as the atom. The atom is so small that it is difficult to visualize. If we attempted to fill a matchbox with atoms at a rate of ten million per second, it would take over a billion years to fill the box! The atom is the smallest for any material can assume without changing its characteristics. Page 5: To understand the mystery of electricity we must have a basic understanding of the structure and forces that make up the atom; that atoms are composed of many minute particles. The structure of the atom consists of a nucleus in the center. Electrons whirl around this nucleus and are held in their orbit by their attractions to the nucleus. The electrons orbiting around the nucleus display a negative charge. The nucleus displays a positive charge because it is composed of positively charged protons and neutrally charged neutrons. The number of electrons and protons that make up a particular atom are usually equal in number. This equal number creates a canceling effect between the negative and 1

2 positive charges. Each element has a fixed number of electrons in orbit. Examples of the atomic structure of two common elements are displayed here. Page 6: All elements are arranged in the Periodic Table of Elements according to their atomic number. The atomic number of an element refers to the number of protons or electrons that make up an atom of that element. The order of elements may also be arranged by atomic weight. The atomic weight of an element refers to the approximate number of protons and neutrons in the nucleus. For example, the atomic weight of oxygen is sixteen; its atomic number is eight. Page 7: An atom remains in its normal state unless energy is added by some exterior force such as heat, friction, or bombardment by other electrons. If the exterior force is of sufficient strength, electrons in the atom s outer rings or orbits can leave their orbit. If electrons leave the outer orbit, the atom becomes out of balance electrically. When the electron leaves the outer orbit, the atom becomes ionized. An ionized atom is electrically unbalanced. An atom that loses an electron from its outer orbit has more protons in the nucleus than electrons in orbit around the atom. The atom becomes a positive ion. When an atom gains an extra electron, it becomes a negative ion. The electron that has broken out of its orbit is negatively charged. This concept of negative and positive ions is a key building block to understanding electronic theory. Page 8: None Page 9: The word static means at rest. Electricity can be at rest. Static electricity can be demonstrated when stroking the fur of a cat. As you stroke you will notice that its fur is attracted to your hand as you bring your hand back over the cat. If this is done at night, you may see and hear tiny sparks. The sound is caused by the discharge of static electricity. When we stroke the fur with our hand, the friction between the cat s fur and our hand excites the atoms. Some atoms lose electrons while others gain electrons. The sparks are created as the atoms attempt to neutralize themselves by gaining back the lost electrons. You can generate a static charge of electricity by walking across a wool or nylon rug with plastic soled shoes. After walking across such a rug, you receive the surprising experience of discharging several thousand volts of static electricity to a metallic object such as a door handle. Page 10: One of the fundamental laws in the study of electricity is the law of charges. The law of charges states that: Like charges repel each other and unlike charges attract each other. The power of attraction can be seen when you run a comb through your hair several times. The comb will attract some of the hair toward itself. Page 11: The force of attraction and repulsion of charged particles was studied by the French scientist, Charles A. Coulomb. Because electrons are so very small, a charge of just a few electrons, say a dozen, is almost impossible to measure. Coulomb developed a practical unit for measurement of an amount of electricity. It is known as the coulomb. One coulomb represents approximately 6.24 x electrons or 2

3 6,240,000,000,000,000,000. While the coulomb is used to describe the flow of electricity, it is not used to describe static charges. Page 12: The field of force surrounding a charged body is called the electrostatic field or dielectric field. The field can exhibit a positive or negative charge depending on a gain or loss of electrons. Two charged masses are shown here. Lines represent the electrostatic fields of opposite polarity and the attractive force existing between the masses. In this figure, two charged masses are shown with like polarities. A repulsive force exists between the charged masses due to the electrostatic fields. This figure illustrates that when two electrostatic fields are joined together, the electrons flow from the mass with an excess of electrons to the mass that has a need of electrons. The excess electrons flow from the body that is negatively charged to the positively charged body that has the electron deficiency. This transfer of electrons can be accomplished by touching the two bodies together of by connecting them with a material that supports the flow of electrons, which we call a conductor because it conducts electricity. Page 13: Charges can be transferred two ways. One way is by direct contact. When a charged body such as a glass rod touches another body such as the top of an electroscope, the electroscope takes on part of the charge of the rod. Another way of transferring a charge is induction. A charge is induced by bringing a charged object near another object. The glass rod need only be brought near the top of the electroscope to charge it. When an object is charged by induction, the object takes on the opposite charge as the rod. When the rod touches the object, the object takes the same charge as the rod. Page 14: None Page 15: A basic electrical circuit consists of three main parts, a source of voltage, a load, and conductors. This image shows a basic circuit that consists of a battery as the source of electrical energy, a lamp as the electrical load, and two wires as the conductors connecting the battery to the lamp. In the source of this circuit, the battery, a chemical reaction takes place that results in ionization. This ionization produces an excess of electrons, a negative charge, and a depletion of electrons, a positive charge. The battery has two terminals. These terminals are connection points for the two conductors. One terminal is marked with a plus sign and the other a negative sign. These two markings are referred to as polarity markings. Page 16: A load is created when the electrical energy produced in a circuit is converted to some other form of energy such as heat, light, or magnetism. The load in this simple circuit is a lamp that produces light. The source and load should match according to the voltage rating. If the lamp is rated at six volts, then the battery should also be rated at six volts. If the battery is rated at a lower voltage rating, the lamp will appear dim or not light. If the battery is rated at a much higher voltage, the lam will be damaged by the excess electrical energy. The conductors are two copper wires covered with a plastic insulation coating. The copper wire provides a path through which the electrical energy can flow, while 3

4 the plastic coating restricts the electrical energy to the copper wire making the conductor pathway safe for personnel. Page 17: Ionization can be caused by forces such as heat, light, magnetism, chemical action, or mechanical pressure. This results in the creation of an electrical voltage. What is voltage? Voltage is the force behind electron flow. In the simple circuit just described, the battery was the source of electrical energy. This battery has a rating of six volts. The volt is the electrical unit used to express the amount of electrical pressure present, or the amount of electrical force produced by the chemical action inside the battery. Electrical pressure or voltage can also be expressed as potential, potential difference, or as electromotive force. For our purposes, these terms mean the same thing. Voltage is usually represented by the capital letter E or V. In this course, we will represent voltage using the letter E. Page 18: Electrical current is the flow of electrons. The amount of electrons flowing past any given point one second is rated in the electrical unit ampere. The ampere is expressed using the letter I. A coulomb is a quantity of electrons. The ampere describes the rate of flow of the electrons past any given point in a circuit. One ampere is equal to one coulomb of charge flowing past a point in one second. In the battery, the amount of electrical pressure inside the battery is expressed as the voltage rating of the battery. The current from the battery in the electrical circuit is the volume of electron flow past a given point, and is rated in amperes or amps. The electron flow can continue as long as there is voltage or electrical pressure present in the battery. Page 19: All electrical circuits have resistance. Resistance is the opposition to the flow of electrons. Resistance is measured in ohms, and the electrical symbol for ohm is the Greek letter omega. The resistance values of elements and compounds differ according to the atomic structure of the material. A good conductor of electricity is anything that permits the free flow of electrons. A poor conductor of electricity is a material that will not permit the free flow of electrons. Extremely poor conductors are referred to as insulators. A semiconductor is a material that limits the flow of free electrons. A semiconductor is considered neither a good conductor nor poor conductor of electricity. Page 20: Moisture can affect the electrical conducting ability of many materials. It can even cause an insulator to become a good conductor. When wood is dry, it is classified as an insulator, but when wood becomes wet or moist, it behaves more like a semiconductor. It is the outer ring of an atom that determines whether an element is a good or poor conductor. If the outer ring has only one electron, that electron can be freed from its orbit rather easily by an outside force. If there are many electrons in the outer orbit, the electrons are held tighter in orbit. They are harder to free from the atom. Elements that do not readily give up an electron are insulators. Copper is an excellent conductor of electricity. Page 21: There are two types of electrical current, direct current or DC and alternating current or AC. The difference between these currents is how they flow through an electrical circuit. Direct current flows in only one direction through an electrical circuit. An example of direct current is a standard 4

5 battery. The battery has a set polarity (positive and negative terminals) and will produce an electric current in only one direction. On the other hand, alternating current, as its name implies, flows in both directions. First it flows in one direction, and then it reverses its flow to the opposite direction. There are no positive or negative polarity markings in alternating current because the polarity changes so rapidly in the typical AC electrical circuit. The terms cycle and hertz are used to describe how fast the current is alternating or changing direction in the circuit. A 60 cycle AC circuit, operating at 60 hertz, changes direction 120 times per second. This is the standard for alternating current in the United States. Page 22: Approximately 200 years ago, scientists theorized that electricity had both positive and negative polarities. At that time they arbitrarily decided that electrical current flowed from positive to negative. This theory is known as the conventional current flow theory. As our knowledge of science progressed, and with the discovery of the atom and semiconductor electronics, it became apparent that the conventional current flow theory was incorrect. It is widely accepted that it is the electrons that actually move, flowing from negative to positive, not from positive to negative. This newer theory is known as electron flow theory. Most of the images in this module show electron flow. The most important point is that the correct polarity must be maintained when building circuits with devices that require a definite polarity. Page 23: There are two ways a component can be connected in a circuit, either series or parallel. This circuit has three lamps connected to a battery. In this circuit there is only one path over which the electrons can flow. When electrons only have one circuit path to follow, that circuit is called a series circuit. The lamps are said to be wired in series with respect to each other. In this image there are three lamps connected in parallel. The circuit has three different paths for the electrons to follow from battery terminal to battery terminal. Both the series and parallel circuits have advantages and disadvantages which will be discussed later. Page 24: The possibility of fire is always a threat in an electronics laboratory. When working with electrical circuits, you should have a basic understanding of fire extinguisher types and know how to use them. Fire extinguishers are divided into the following five classes. It s important to know that a fire extinguisher with all the letters, A, B, C, would be suitable for a class A, B, or C fire, whereas water are only appropriate for Class A fires. Page 25: The acronym PASS explains the proper procedure for using a fire extinguisher. PASS was developed to match the recommended procedures by the National Fire Protection Agency for fire extinguisher use. While standing approximately six to eight feet from the fire you should follow these procedures. Page 26: None 5

6 Page 27: Electrical circuits that are built correctly will be in perfect electrical balance. The current through the resistance is directly related to the amount of electrical pressure or voltage applied to the circuit. This balance of the three factors, voltage (E), resistance (R), and current (I) can be expressed by Ohm s Law. Ohm s Law is named for the 19 th century German scientist George Simon Ohm. The relationship expressed by Ohm s Law is the basic formula that is used more extensively than any other electrical formula you will encounter in your study of electricity and electronics. Ohm s law states that the current measured in amperes (I) in a circuit is equal to the applied voltage (E) divided by the resistance (R). Ohm s law is expressed in the three formulas shown here. A memory device commonly used to assist you in learning Ohm s law is illustrated here. Cover the unknown quantity and the remaining letters show the correct equation. For example, I is equal to E divided by R. Page 28: Here s an example of Ohm s Law. In this diagram, a lamp with a resistance of 4 ohms has been connected to a 12 volt source. The current is unknown. By applying Ohm s law you can determine the current to be equal to 3 amperes. In this diagram, a 24 ohm resistance heater works most efficiently when using 5 amps of current. How much voltage is required for the heater to operate at 5 amps? By applying Ohm s law, the amount of electrical pressure needed to conduct 5 amps through a 24 ohm resistance is 120 volts. In this final diagram, a resistor is connected to a 24 volt source. A meter that measures current indicates there are 3 amperes present in the circuit. By applying Ohm s law the amount of resistance needed to limit the current value to 3 amperes is found to be 8 ohms. Page 29: Measurements of electrical quantities vary from small amounts to large amounts. To make it easier to label electronic parts and equipment, and to make electronic calculation easier, a system of prefixes is used when expressing electrical quantities. Without this system, we would need to use an excessive amount of zeros to the left and right of a decimal point. Here is a chart listing the most common prefixes used in the electronics industry today. When an electrical quantity such as voltage is written, it is expressed in units such as kilovolt, megavolt, millivolt, and microvolt to avoid using an awkward numerical form. For example, the quantity 5,000,000 volts would be written as 5 MV. If the quantity was volts, it would be written as 5 mv. When voltage is expressed with a capital letter M, it represents millions of volts, but when voltage is expressed using a lower case m, it represents the fractional unit of thousandths. Page 30: Here s a review of what you need to know about electrical prefixes by way of a short tutorial. This chart shows important information about the Giga, Mega, Kilo, milli, and micro prefixes. Take a moment to review it. Remember, any number raised to the zero power is equal to 1. For example 10 0 is equal to 1. Also, it s important to remember that as power increases the decimal point of the number moves to the left as depicted in this image. And when power decreases, the decimal point moves to the right. 6

7 Page 31: In this first example we ll look at how to express 10,000 volts. If we want to express it as kilovolts it would be written as 10 kv which means 10 kilovolts. Now, let s go from kilovolts to megavolts. As you can see by the image, we are moving from 103 volts to 106 volts, as the power increases the decimal point will move to the left. To convert 10 kilovolts to megavolts you would move the decimal point three spaces to the left and it would result in.01 megavolts. Page 32: In this example we will convert 2 Amps (10 0 A) into milliamps (10 3 A). Since power is decreasing, the decimal point moves to the right. In this case the decimal moves three spaces to the right and becomes 2000 milliamps. Page 33: In this next example we will convert 3.5 ohms (10 0 ohms) into kilo ohms (10 3 ohms). Since power is increasing, the decimal point will move to the left three spaces and the resulting answer is that 3.5 ohms equals.0035 kilo ohms. Page 34: In this example we will convert 2500 (10 6 A) micro amps into amps (10 0 A). Since power is increasing, the decimal point will move to the left six positions and the resulting answer is that 2500 micro amps equals.0025 amps. Page 35: In this final example we will convert one milliamp (10 3 A) into kilo Amp (10 3 A). Since power is increasing we move the decimal point to the left which results in one milliamp equaling kilo Amp. Page 36: In this lesson you covered the following objectives. 7