Introduction to Electricity

Transcription

1 Introduction to Electricity OBJECTIVE The learner will be able to:» Explain the importance of electricity and how it is produced» Describe the two different types of electricity» Define terms related to electricity» Compare and contrast the fundamental laws governing the behavior of electricity in a circuit ORIENTING QUESTIONS ü What does the electron theory stipulate about all matter? ü Why is electricity released in the form of static electricity at times rather than in the form of a current? ü How are electrons related to an electrical current? ü What is the difference between Kirchhoff s Current Law and Kirchhoff s Voltage Law? INTRODUCTION Electricity is one of the most commonly used forms of energy in the modern world. This module presents important concepts related to electricity and the use of it in our world today. First, it will provide a basic understanding of how electricity is produced along with an introduction to the different types of electricity. The module will then provide technical definitions and other information related to concepts about electricity, as well as the fundamental laws governing the behavior of electricity in a circuit. Version 1 :: CUCWD 102 :: Rev OPEN TEXT Figure 1. Electric Power Lines 1

2 1 Fundamentals of Electricity Electricity is the energy resulting from the movement of charged particles. An understanding of the Electron Theory provides insight into the electrical properties of substances. This theory stipulates that all matter is made up of molecules, which are made up of atoms, and that each atom is a combination of electrons, protons, and neutrons. Matter can be defined as a material that has measurable mass and occupies space. This terminology incorporates substances in all states including liquid, solid, and gas. Scientists did not describe matter in terms of atoms until the beginning of the twentieth century. Matter is made up of molecules. A molecule is the smallest unit of matter that can be physically displayed and take part in a chemical reaction. The movement of molecules is related to the temperature of the matter. Molecules become more agitated as the temperature goes higher. Molecules are made up of atoms. A water molecule is shown in Figure 2. Figure 2. Water Molecule in 3D An atom is the basic unit of a molecule and is the smallest entity of matter. Atoms carry the chemical and physical properties of that matter. Atoms consist of a nucleus and electrons which gravitate around the nucleus. The nucleus of an atom is typically made of positively charged protons and electrically neutral neutrons. The electrons are negatively charged and normally in equal proportion to the protons in the atom, which makes the atom neutral. Activity 1.1» The Electron Theory stipulates that all matter is made up of molecules, which are made up of atoms, and that each atom is a combination of electrons, protons, and neutrons. Define the following terms: molecule, atom, and electron. 2 Version 1 :: CUCWD 102 :: Rev OPEN TEXT

3 2 Electron Flow Atoms from some substances release one or more electrons when placed in contact with other materials. This movement of electrons from one atom to the next is called electron flow and often results in electricity. Electricity can either flow as a current or be released in the form of static electricity, depending on the availability of an escape route for the electron flow. Static electricity is an electric charge trapped on a surface that is usually created by the friction between two surfaces that are highly resistant to electrical current. Static electricity can be created in a variety of ways. For instance, it can be produced by applying mechanical stress to certain crystals or ceramics through heat. It can also be produced by bringing an electronically charged object close to a neutral one. The two notations for electrical flow are conventional flow and electron flow. These two types of flow both describe the movement of electrical charges through a circuit with electricity flowing from a point carrying a surplus charge to another point with a deficiency in charge. With a conventional flow notation, a technician considers the point of origin of electron flow as positive because it carries a surplus charge. The receiving end for the electron flow is described as negative because it is deficient in electrical charge. Conventional flow is represented by the red line in Figure 3. Figure 3. Current notation An electron flow notation, represented by the green line in Figure 3, is a description of the actual movement of electrons from the point in a circuit with a surplus of electrons (the negative side) to the point in the circuit that is deficient in electrons (the positive side). Activity 2.1» Differentiate between conventional flow and electron flow, the two notations for electrical flow. 3 Version 1 :: CUCWD 102 :: Rev OPEN TEXT

4 3 Definitions of Terms Related to Electricity The following terms related to electricity are explained in this section: voltage, magnetic field, electron, current, resistance, conductor, insulator, and semiconductor. Voltage is the difference in the electrical charge between two points that can push electrical current between the points. Voltage is also known as electrical pressure. When voltage is high between two points, it is possible to obtain a high electrical flow. Voltage can be created by the:» Use of chemicals (example is car battery)» Use of a magnetic field (an area of force around an object that happens due to changing electric fields created by the movement of electrical charges)» Use of heat or light (example is solar cell). Regardless of the method used, voltage occurs only when a negatively charged item is in the presence of a positively charged item. Voltage is measured in volts. An electron is an elementary particle that is negatively charged. A current is the movement of electrons resulting from a difference in voltage, or pushing force. A current is also known as a flow of charge and a closed circuit is usually required. Electrons tend to flow from a point of lower potential charge to a point of higher potential charge because they are negatively charged. Current is a measurement of the number of electrons that go through a specific point in the circuit in a second. This measurement is expressed in ampere or amps and the symbol is A. Current flows from negative to positive outside of the power source, but it flows from positive to negative inside the source. All materials have some opposition to the flow of current due to the friction created by the movement of electrons. This opposition is called resistance and is measured in ohms (Ω). The symbols that represent resistance are the three jagged lines in the diagram shown in Figure 4. Figure 4. Resistance in Electricity 4 The wires carrying current from the source to another point in the circuit will also have some resistance. This resistance will vary proportionally with the length of the wire and inversely with the diameter of the wire and the larger the surface area, the less the resistance will be. The composition of the material and the temperature can also affect resistance. Version 1 :: CUCWD 102 :: Rev OPEN TEXT

5 A bridge is a two-branch circuit that reaches balance when the difference in the potential charge between a point in one branch and the same point in the other branch is null. The Wheatstone Bridge was created in the early ninetieth century and this type of circuit is used to measure unknown resistance. It consists of four resistances, one of which is the unknown, interconnected in a specific fashion. Figure 5. Wheatstone Bridge An example of this type of circuit is shown in Figure 5. If R 1, R 2, and R 3 are the known resistances used in the bridge, R x represents the unknown resistance. G is the galvanometer and it indicates the voltage across the branches. The bridge will be in a state of balance only if the value displayed by V is null. In accordance with the balance principle of bridges, the ratios of R 2 with R 1 (R 2 /R 1 ) and R x with R 3 (R x / R 3 ) will only be equal when G points to zero. A conductor is a substance or object with the intrinsic ability to carry electric current. The atomic structure of a substance is a good way to recognize the associated electrical properties. For instance, some materials have loosely bound electrons that are called free electrons. A material with loosely bound electrons is a fairly good conductor of electricity and conduction capabilities increase when the number of free electrons in a substance increases. All conductors offer some resistance to electrical current regardless of the associated electron count. An insulator is a material or substance such as glass that has a minimal number of free electrons, thus causing resistance to the current flow. Typically, chemical compounds that are made of different chemical elements are insulators. Examples of insulators are Teflon, polyvinyl chloride (PVC), mica, quartz and glass. A semiconductor cannot be classified as either a conductor or insulator because it has exactly four electrons in the outer sphere. Semiconductors allow current flow only in one direction. However, for semiconductors to allow current flow, they should be connected in the same direction as the flow. A diode is an example of semiconductor. Activity 3.1» Explain the following terms that are related to electricity: voltage, magnetic field, electron, current, resistance, conductor, insulator, and semiconductor. 5 Version 1 :: CUCWD 102 :: Rev OPEN TEXT

6 4 Ohm s Law and Kirchhoff s Laws Voltage, current, and resistance are all tied together by a simple equation: Voltage = Current * Resistance (V= I * R) which is called Ohm s Law. George Simon Ohm, a German physicist, established this law and the underlying principle. This equation and other derivatives have simplified the understanding of electrical concepts. One example of a concept that is easier to understand based on Ohm s Law is that high voltage applied across a material with a small resistance will lead to a strong current. This law also implies that neither voltage nor current can produce an isolated change. For example, if the resistance in a circuit is kept constant, a change in voltage will result in a similar change in current and vice-versa. On the other hand, a change in a circuit s resistance will result in an inverse change in the current for the same voltage. It is important to note that the application of this law and the associated conclusions are valid only with consistent temperature conditions. Gustav Kirchhoff established two equations dealing with the conservation of charge and energy in electrical circuits. These two equations are called Kirchhoff s Current Law and Kirchhoff s Voltage Law. Based on Kirchhoff s Current Law (KCL), the sum of the current flowing in is equal to the sum of the current flowing out at any given junction in an electrical circuit. KCL is also known as the First Law and is shown in Figure 6. Kirchhoff s Voltage Law (KVL) is also an equality equation, but it deals with energy. Based on KVL, the directed sum of voltages around any closed circuit is null. In other words, the difference of potential charge between two points in a closed circuit is the same regardless of the path chosen. For example, in Figure 7, the voltage between the points a and d is equal to the generated V 4 and also to the sum (V 1 + V 2 +V 3 ). KVL is also known as the Second Law. Figure 6. Kirchhoff s Current Law (KCL) Figure 7. Kirchhoff s Voltage Law (KVL) Activity 4.1» Compare and contrast Ohm s Law, Kirchhoff s Current Law, and Kirchhoff s Voltage Law. 6 Version 1 :: CUCWD 102 :: Rev OPEN TEXT

7 SUMMARY Key Concepts» Electricity is one of the most commonly used forms of energy in the modern world.» The Electron Theory stipulates that all matter is composed of molecules that are composed of atoms, which in turn are made up of electrons, neutrons and protons.» The two notations used to describe the current flow are the conventional flow and the electron flow.» Voltage is a type of pressure which pushes electrons to flow through a material.» A current is the movement of electrons resulting from a difference in voltage between two points. Current is a measurement of the number of electrons that go through a specific point in the circuit in a second.» A conductor is a substance or object with the intrinsic ability to carry electric current whereas an insulator has very high resistance and does not carry an electric current. A semiconductor can conduct electricity, but only in one direction.» The discoveries of the fundamental laws that govern the behavior of electricity have helped to simplify understanding of electrical concepts. Key Terms 1 Electricity pg.2 Matter pg.2 Molecule pg.2 Atom pg.2 Nucleus pg.2 2 Electron Flow pg.3 Static Electricity pg.3 Further Study Books: 3 Voltage pg.4 Magnetic Field pg.4 Electron pg.4 Current pg.4 Resistance pg.4 Wheatstone Bridge pg.5 Conductor pg.5 Insulator pg.5 Semiconductor pg.5 4 Ohm s Law pg.6 Kirchhoff s Current Law (KCL) pg.6 Kirchhoff s Voltage Law (KVL) pg.6 1. Gibilisco, S., & NetLibrary, I. (2002). Teach Yourself Electricity and Electronics. New York: McGraw-Hill. 2. Thompson, Lawrence M. (2006). Basic Electricity and Electronics for Control - Fundamentals and Applications (3rd Edition). ISA. ( browse/display?_ext_knovel_display_bookid=1816&verticalid=0) Online: 7 Version 1 :: CUCWD 102 :: Rev OPEN TEXT

8 GLOSSARY» Atom: basic unit of a molecule and smallest entity of matter carrying the chemical and physical properties of that matter» Conductor: substance or object with the intrinsic ability to carry electric current» Current: flow of electrical charges» Electricity: energy resulting from the movement of charged particles» Electron: elementary particle that is negatively charged» Electron flow: movement of electrons from one atom to the next» Insulator: Object or matter with atomic structure that does not allow it to carry an electric current» Kirchhoff s Current Law: based on this law, the sum of the current flowing in is equal to the sum of the current flowing out at any given junction in an electrical circuit» Kirchhoff s Voltage Law: based on this law, the directed sum of voltages around any closed circuit is null; in other words, the difference of potential charge between two points in a closed circuit is the same regardless of the path chosen» Magnetic field: area of force around an object that happens due to changing electric fields created by the movement of electrical charges» Matter: physical substance that occupies space» Molecule: smallest physical entity of matter» Nucleus: part of the structure of the atom that is composed of protons that are positively charged and neutrons that have no electrical charge» Ohm s Law: mathematical law that describes the relationship between the current flowing through an object, the resistance, and the voltage at the terminal or terminals of the object» Resistance: opposition to current found in an object or portion of circuit» Semiconductor: object that can only carry electric current under specific conditions» Static electricity: happens due to an accumulation of charges on an insulator and is the only form of electricity in which motion is irrelevant» Voltage: electrical pressure that pushes electrons to move across matter and is the difference of electrical potential between two points» Wheatstone bridge: Circuit used to determine the value of an unknown resistance This material is based upon work supported by the National Science Foundation under Grant Number DUE Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation. 8 Copyright 2013 Clemson University Center for Workforce Development. All Rights Reserved. Version 1 :: CUCWD 102 :: Rev OPEN TEXT

9 Attribution Tables Author/s Title Source License Angelo Power lines CC-BY-2.0 DeSantis N00/ / Eric Skiff Hand-cranked generator Chemitorium Water Molecule in 3D Ingo Hoffman Flekstro Static Electrticity Current Notation CORE-Matreials Zn-Mg-Ho Diffraction Pasquale. Carelli PACO Phatency Omegatron Microsoft Clipart Rhys A. Resistance in Electricity Wheatstone Bridge Kirchhoff's Current Law Kirchhoff s Voltage Law City at night (PPT Lecture 2) Danger: High voltage (PPT Lecture 2) Windell Oskay Magnetic Fields -15 Windell Oskay Diodes ericskiff/ / File:Water_Molecule_3D_X_3.jpg la_sombra/ / php?title=file:current_notation.svg&page=1 core-materials/ / File:3Resistance.png#filelinks File:PWheatstone.png File:Kirchhoff%27s_first_law_example.png png#filehistory images/results.aspx?qu=city&ex=1#ai: MP rhysasplundh/ / oskay/ / N00/ / CC-BY-SA-2.0 Public domain CC-BY-2.0 CC-BY-SA 3.0 CC-BY-SA-2.0 Public domain CC-BY-SA 3.0 CC-BY-SA 3.0 CC-BY-SA 3.0 With permission from Microsoft CC-BY-2.0 CC-BY-2.0 CC-BY Version 1 :: CUCWD 102 :: Rev OPEN TEXT

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