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3.1 Electron Theory

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Category A, Bl, 82 and C Aircraft MaintenanceLicence

Basic knowledge for categories A, B1 and 82 are indicated by the allocation o{ knowledge levels indicators (1, 2 or3) against each applicable subject. Category C applicants must meet either the category Bt or the category 82basic knowledge levels.The knowledge level indicators are defined as lollows:

LEVEL 1A familiarisalion with the principal elements of the subject.

Objectives:The applicant should be familiar with the basic elements of the subject.The applicant should be able to give a simple description of the whole subject, using common words andexamples.The applicant should be able lo use typical terms.

LEVEL 2A general knowledge of the theoretical and practical aspects of the subject.An ability to apply that knowledge.

Objectives:The applicant should be able to understand the theoretical fundamentals ol the subject.The applicant should be able to give a general description of the subject using, as appropriate, typicalexamples.The applicant should be able to use mathematical lormulae in conjunclion with physical laws describing thesubject.The applicant should be able to read and understand sketches, drawings and schematics describing thesubject.The applicant should be able to apply his knowledge in a practical manner using detailed procedures.

LEVEL 3A detailed knowledge of the theoretical and practical aspects ol the subjecl.A capacity to combine and apply the separate elements of knowledge in a logical and comprehensivemanner.

Objectives:The applicant should know the theory of the subject and interrelationships with other subjects.The applicant should be able to give a detailed description of the subject using theoreticil fundamentalsand specific examples.The applicant should understand and be able to use mathematical formulae related to the subject.The applicant should be able to read, understand and prepare sketches, simple drawings and schematrcsdescribing the subject.The applicant should be able to apply his knowledge in a practical manner using manufacturersinstructions.The applicant should be able to interpret results from various sources and measurements and applycorrective action where appropriate.

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Module 3.1 Electron Theory

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Table of Contents

Module 3.1 Electron TheoryMatterElements and CompoundsMolecu lesAtomsEnergy LevelsShells and Sub-shellsValenceCompoundslonisation

11111111Conductors, Semiconductors, and lnsulators

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Structure and distribution of eleckical chargeswithin: atoms. molecules. ions. comoounds


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Matter',latler is defined as anything that occupies space and has weight; that is, the weight and:'nensions of matter can be measured. Examples of matter are air, water, automobiles,: cthing. and even our own bodies" Thus, we can say that matter may be found in any one of:^ree states: solid, liquid, and gaseous.

Elements and Compounds:- ELEMENT is a substance which cannot be reduced to a simpler substance by chemical-eans. Examples of elements with which you are in everyday contact are iron, gold, silver,::lper. and oxygen. There are now over 100 known elements. All the different substances we.1":','; about are composed of one or more of these elements.

,', 'en hvo or more elements are chemically combined, the resulting substance is called acompound. A compound is a chemical combination of elements which can be separated by:-:nical but not by physical means. Examples of common compounds are water which.:^s sts of hydrogen and oxygen, and table salt, which consists of sodium and chlorine. Amixture, on the other hand, is a combination of elements and compounds, not chemically::-c,ned. that can be separated by physical means. Examples of mixtures are air, which is--:a up of nitrogen, oxygen, carbon dioxide, and small amounts of several rare qases, and sea'r a:e'. yrhich consists chiefly of salt and water.

Molecules: molecule is a chemical combination of two or more atoms, (atoms are described in the next::'a3raph). In a compound the molecule is the smallest particle that has all the characteristics:' :-: compound.

I : - s cer water, f or example" Water is matter, since it occupies space and has weight.l::erding on the temperature, it may exist as a liquid (water), a solid (ice), or a gas (steam).=::ai-d ess ol the temperature, it will still have the same composition. lf we start with a quantity:' ,,, a:er. divide this and pour out one half , and continue this process a sufficient number of: -:s. ,1'e will eventually end up with a quantity of water which cannot be further divided without::ls r'J io be water. This quantity is called a molecule of water. lf this molecule of water: , r:r. instead of two pafts of water, there will be one parl of oxygen and two parts of- -.- -^^ I LJ^n \'- '-c , :\J./. DIT t[braryAtoms

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'-f v z- J\-, rJ rr.t: 3:- es are made up of smaller parlicles called atoms. An atom is the smallest particle o{ an::-er: ihal retains the characteristics of that element. The atoms oi one element, however,: -:' '-3''n the atoms of all other elements. Since there are over '100 known elements, there-

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s: :e o,rer 100 different atoms, or a different atom for each element. Just as thousands of

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Any particle that is a chemical combination of two or more atoms is called a molecule. The 1oxygen molecule consists of two atoms of oxygen, and the hydrogen molecule consists of two rEatomsofhydrogen.Sugar,ontheotherhand,isacompoundcomposedofatomSofcarbon,hydrogen, and oxygen. These atoms are combined into sugar molecules. Since the sugar

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molecules can be broken down by chemical means into smaller and simpler units, we cannot -

have sugar atoms.

The atoms of each element are made up of electrons, protons, and, in most cases, neutrons, -which are collectively called subatomic particles. Furthermore, the electrons, protons, and -neutrons of one element are identical to those of any other element. The reason that there are --..different kinds of elements is that the number and the arrangement of electrons and protons --

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words can be made by combining the proper letters of the alphabet, so thousands of differentmaterials can be made by chemically combining the proper atoms.

The electron is considered to be a small negative charge of electricity. The proton has a positivecharge of electricity equal and opposite to the charge of the electron. Scientists have measuredthe mass and size of the electron and proton, and they know how much charge eachpossesses. The electron and proton each have the same quantity of charge, although the massof the proton is approximately 1837 times that of the electron. ln some atoms there exists aneutral padicle called a neutron. The neutron has a mass slightly greater than that of a proton,but it has no electrical charge. According to a popular theory, the electrons, protons, andneutrons of the atoms are thought to be arranged in a manner similar to a miniature solarsystem. The protons and neutrons form a heavy nucleus with a positive charge, around whichthe very light electrons revolve.

Figure 1.'1 shows one hydrogen and one helium atom. Each has a relatively simple structure.The hydrogen atom has only one proton in the nucleus with one electron rotating about it. Thehelium atom is a little more complex. lt has a nucleus made up of two protons and two neutrons,with two electrons rotating about the nucleus. Elements are classified numerically according tothe complexity of their atoms. The atomic number of an atom is determined by the number ofprotons in its nucleus.

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Structule of Hydrogen and Helium

ln a neutral state, an atom contains an equal number of protons and electrons. Therefore, anatom of hydrogen - which contains one proton and one electron - has an atomic number of 1;and helium, with two protons and two electrons, has an atomic number of 2. The complexity ofatomic structure increases with the number of protons and electrons.



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HYEMGEN


Energy Levelssince an electron in an atom has both mass and motion, it contains two types of energy' Byvirtue of its motion the electron contains kinetic energy. Due to its position it also containspot"nti"iln"tgy. The totai energy contained by an electron (kinetic plus potential) is the factorwhich determines the radius of th"e electron orbii' ln order for an electron to remain in this orbit'it must neither GAIN nor LOSE energy.

Figure 1.2 -

Energy levels in an atom

Once the electron has been elevated to an energy level higher than the lowest possible energyl";;i th" atom is said to be in an excited state. ihe electron will not remain in this excitedaonOition for more than a fraction of a second before it will radiate the excess energy and returnto a lower energy orbit. To illustrate this principle, assume that a normal electron has

justreceived a phoion ot energy iufficient to'raise it from the first to the third energy level' ln a short

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one accepted theory proposes the existence of light as tiny packets of energy.called photons'photons can contain uuriou" quuniitles of energy.-The amount depends upon the colour of theiigr'1-r"otu;0. Should a ptroioii ot sufficient en6igy collide with an orbital electron, the electron*]ft aOsorU the photon's energy, as shown in figure t.Z. The electron, which now has a greaterthan normal amount ot energ-y, witt lump to a nlw orbit farther from the nucleus' The first neworbit to which the electron

"Ji'iutnri r-1ui a radius four times as large as the radius of the original

orbit. Had the electron ,"""V"i "

ft"utut amount of energy, the next possible orbit to which itcould jump would have u ruJir. niie times the original. Thus, each orbit may be considered toiepresent'one of a large nrrb"r of energy levels that the electron may attain' lt must beemphasized that the e]ectron cannot jum-[ to iusl any orbit' The electron will remain in its lowestorbit until a sufficient amou;t of energy is auuil"bl", at which time the electron will accept the

"n"igy unO jump to one of a series oipermissible orbits. An electron cannot exist in the space

netwL'en enLrgy levels. This indicates that the electron will not accept a photon of energy unlessit contains enough energy to elevate itself to one of the higher energy levels. Heat energy andcollisions with oi-her partictes can also cause the electron to jump orbits.

It is well known that light is a form of energy, but the physical form in which this energy exists is -not known. ' L-\--

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period of time the electron may jump back to the first level emitting a new photon identical to theone it received.

A second alternative would be for the electron to return to the lower level in two jumps; from thethird to the second, and then from the second to the first. ln this case the electron would emittwo photons, one for each jump. Each of these photons would have less energy than theoriginal photon which excited the electron.

This principle is used in the fluorescent light where ultraviolet light photons, which are not visibleto the human eye, bombard a phosphor coating on the inside of a glass tube. The phosphorelectrons, in returning to their normal orbits, emit photons of light that are visible. By using theproper chemicals for the phosphor coating, any colour of light may be obtained, including white.

This same principle is also used in lighting up the screen of a television picture tube.The basic principles just developed apply equally well to the atoms of more complex elements.ln aioms containing two or more electrons, the electrons interact with each other and the exactpath of any one electron is very difficult to predict. However, each electron lies in a specilicenergy band and the orbits will be considered as an average of the electron's position.

Shells and Sub-shellsThe ditference between the atoms, insofar as their chemical activity and stability are concerned,is dependent upon the number and position of the electrons included within the atom. How arethese electrons positioned within the atom? ln general, the electrons reside in groups of orbitscalled shells. These shells are elliptically shaped and are assumed to be located at fixedintervals. Thus, the shells are arranged in steps that correspond to fixed energy levels. Theshells, and the number of electrons required to fill them, may be predicted by the employment ofPauli's exclusiol principle. Simply stated, this principle specifies that each shell will contain amaximum of 2n'electrons, where n corresponds to the shell number starling with the oneclosest to the nucleus. By this principle, the second shell, for example, would contain 2(2\2 or Ielectrons when f ull.

ln addition to being numbered, the shells are also given letter designations, as pictured in figure'1-3. Starting with the shell closest to the nucleus and progressing outward, the shells arelabelled K, L, M, N, O, P, and Q, respectively. The shells are considered to be full, or complete,when they contain the following quantities ol electrons: two in the K shell, eight in the L shell, 18in the M shell, and so on, in accordance with the exclusion principle.

Each of these shells is a major shell and can be divided into sub-shells, of which there are four,labelled s, p, d, and f. Like the major shells, the sub-shells are also limited as to the number ofelectrons which they can contain. Thus, the "s" sub-shell is complete when it contains twoelectrons, the "p" sub-shell when it contains 6, the "d" sub-shell when it contains 10, and the "f"sub-shell when it contains 14 electrons.

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- Shells in an atom

ln as much as the K shell can contain no more than two electrons, it must have only one sub-shell, the s sub-shell. The M shell is composed of three sub-shells: s, p, and d. lf the electronsin the s, p, and d sub-shells are added, their total is found to be 18, the exact number requiredto fill the M shell. Notice the electron configuration for copper illustrated in figure 1.4. Thecopper atom contains 29 electrons, which completely fill the first three shells and sub-shells,leaving one electron in the "s" sub-shell of the N shell.

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Figure 1.4 - The copper atom

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ValenceThe number of electrons in the outermost shell determines the valence of an atom. For thisreason, the outer shell of an atom is called the valence shell; and the electrons contained inthis shell are called valence electrons. The valence of an atom determines its ability to gain orIose an electron, which in turn determines the chemical and electrical properties of the atom. Anatom that is lacking only one or two electrons from its outer shell will easily gain electrons tocomplete its shell, but a large amount of energy is required to free any of its electrons. An atomhaving a relatively small number of electrons in its outer shell in comparison to the number ofelectrons required to fill the shell will easily lose these valence electrons. The valence shellalways refers to the outermost shell.

CompoundsPure substances made up more than 1 element which have been joined together by a chemicalreaction therefore the atoms are difficult to separate. The propedies of a compound are differentfrom the atoms that make it up. Splitiing of a compound is called chemical analysis.

Note that a compound:

consists of atoms of two or more different elements bound together,can be broken down into a simpler type of matter (elements) by chemical means (but notby physical means),has properties that are different from its component elements, andalways contains the same ratio of its component atoms.

lonisationWhen the atom loses electrons or gains electrons in this process of electron exchange, it is saidto be ionized. For ionisation to take place, there must be a transfer of energy which iesults in achange in the internal energy of the atom. An atom having more than its noimal amount ofelectrons acquires a negative charge, and is called a negative ion. The atom that gives upsome of its normal electrons is left with less negative charges than positive chargei and iscalled a positive ion. Thus, ionisation is the process by which an atom loses or gains electrons.

Conductors, Semiconductors, and lnsulatorsln this study of electricity and electronics, the association of matter and electricity is important.Since every electronic device is constructed of parts made from ordinary matter, the e{fects o{electricity on matter must be well understood. As a means of accomplishing this, all elements ofwhich matter is made may be placed into one ol three categories: conductors,semiconductors, and insulators, depending on their ability to conduct an electric current.conductors are elements which conduct electricity very readily, insulators have an extremelyhigh resistance to the flow of electricity. All matter between these two extremes may be calledsem iconductors.


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The electron theory states that all matter is composed of atoms and the atoms are composed ofsmaller particles cilled protons, electrons, and neutrons. The electrons orbit the nucleus whichcontains the protons and neutrons. lt is the valence electrons (the electrons in the outer shell)that we are most concerned with in electricity. These are the electrons which are easiest tobreak loose from their parent atom. Normally, conductors have three or less valence electrons;insulators have five or more valence electrohs; and semiconductors usually have four valenceelectrons. The fewer the valence electrons, the better conductor of electricity it will be' Copper,for example, has just one valence electron.

The electrical conductivity of matter is dependent upon the atomic structure of the material fromwhich the conductor is made. ln any solid material, such as copper, the atoms which make upthe molecular structure are bound firmly together. At room temperature, copper will contain aconsiderable amount of heat energy. Since heat energy is one method of removing electronsfrom their orbits, copper will contain many free electrons that can move from atom to atom.When not under the influence of an external force, these electrons move in a haphazardmanner within the conductor. This movement is equal in all directions so that electrons are notlost or gained by any part of the conductor. When controlled by an external force, the electronsmove ginerallyin the same direction. The effect of this movement is felt almost instantly fromone end of the conductor to the other. This electron movement is called an electric current.

some metals are better conductors of electricity than others. silver, copper, gold, andaluminium are materials with many free electrons and make good conductors. Silver is the bestconductor, followed by copper, goid, and aluminium. Copper is used more often than silverbecause of cost. Aluminium is uieO where weight is a maior consideration, such as in high-tension power lines, with long spans between supports Gold is used where oxidation orcorrosion is a consideration ind a good conductivity is required. The ability of a conductor tohandle current also depends upon its physical dimensions. Conductors are usually found in theform of wire, but may be in the form of bars, tubes, or sheets.

Non-conductors have few free electrons. These materials are called insulators. Someexamples of these materials are rubber, plastic, enamel, glass, dry wood, and mica. Just asthere is no perfect conductor, neither is there a perfect insulator.

Some materials are neither good conductors nor good insulators, since their electricalcharacteristics fall between those of conductors and insulators. These in-between materials areclassified as semiconductors. Germanium and silicon are two common semiconductors usedin solid-state devices.

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Element

K M N ) a L MHVdrooen 53 lodine 2 I 1B B 7

2 Helium 2 54 Xenon 2 I 't8 18 B3 Lithium 2 55 Cesium 2 I 18 8 84 Bervllium 2 2 56 Barium 2 B 18 I I 25 Boron 2 3 57 Lanthanum 2 I 18 B I 26 Carbon 2 4 58 Cerium 2 B 1B 19 I 27 Nilroqen 2 5 59 Praseodvmium 2 8 18 20 28 Oxyoen 2 6 60 Neodvmium 2 I 't8 21 I 2s Fluorine 2 7 61 Promethium 2 8 18 22 I 210 Neon 2 B t 62 Samarium 2 I 18 23 I 211 Sodium 2 8 63 Europium 2 B 18 24 I 212 Maqnesium 2 I 2 64 Gadolinium 2 8 '18 25 I 2'13 Aluminium 2 B 3 65 Terbium 2 I 18 26 214 Silicon 2 8 4 Dvsprosium 2 I 18 2t 215 Phosphorus 2 I 5 67 Holmium 2 I 18 2A I 2

Sulphur 2 B 6 68 Erbium 2 8 18 29 I 21/ Chlorine 2 I 7 69 Thulhrm 2 B 18 30 I 2'18 Arqon 2 I 8 70 Ytterbium 2 B 18 31 I 2

Potassium 2 B I 1 71 Lutetium 2 8 18 32 9 220 Calcium 2 I B 2 72 Halnium 2 I 18 32 10 221 Scandium 2 8 I 2 73 Tantalum 2 I 18 32 11 22? Titanium ? 8 10 2 /4 Tunqsten 2 8 18 12 223 Vanadium 2 a 11 2 75 Rhenium 2 I 18 13 224 Chromium 2 B 13 '1 76 C)smium 2 8 18 32 14 2

lvlanOanese 2 8 13 2 77 lridium 2 B 8 15 226 Iron 2 I 't4 2 7A Platinum 2 8 8 16 227 Coball 2 I 15 2 79 Gold 2 8 8 't82A Nickel 2 8 16 2 80 Mercurv 2 I I 32 'tB 229 Copper 2 I '18 1 81 Thallium 2 8 8 18 330 Zinc 2 B 18 2 a2 Lead 2 I B 32 18 431 Gallilrm 2 8 18 3 83 Bismuth 2 I I 32 18 532 Germanium 2 I 18 4 B4 Polonium 2 B 8 32 18 633 Arsenic 2 8 18 5 85 Asatine 2 8 B 32 18 734 Selenium 2 I 1B 86 Radon 2 a 8 32 18 I35 Bromine 2 8 18 7 a7 Francium 2 8 I 32 18 8

Krypton 2 8 18 a 88 Radium 2 B I 32 18 B 23/ Rubidium 2 I 18 8 1 89 Actinium 2 8 '18 32 18 I 238 Strontium 2 8 18 B 2 90 Thorium 2 I 18 32 19 I 23S Yttrium 2 8 18 9 2 91 Proactinium 2 8 18 32 20 I 240 Zirconium 2 I 1B 10 2 92 uranium 2 8 1B 32 21 9 241 Niobium 2 B 18 12 s3 Neptunium 2 8 1B 32 22 9 242 Molybdenum 2 8 18 13 94 Plutonium 2 I 18 32 23 I 243 Technetilrm 2 I 18 14 95 Amerium 2 8 18 32 24 244 Ruthenium 2 8 18 15 96 Curium 2 B I 32 25 9 245 Rhodium 2 B 18 97 Berkelit]m 2 I 8 32 26 I 246 Palladium 2 8 1B 18 0 98 Californium 2 8 I 27 c 247 Silver 2 I 18 '18 9C Einsteinium 2 8 8 32 2A I 248 Cadmium 2 8 18 18 2 100 Fermium 2 a B 32 29 I 249 lndium 2 8 '18 1A 3 101 Mendelevium 2 I 1A 30 250 Tin 2 I '18 18 4 102 Nobelium 2 I 18 32 I 251 AntimonV 2 B 18 18 5 103 2 I 18 32 I 252 Tellurium 2 B 18 18 6


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3.2 Static Electricity and Conduction

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Module 3.2 Static Electricity and Conduction

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Knowledge Levels -

Category A, 81, 82 and C Aircraft MaintenanceLicence

Basic knowledge {or categories A, B1 and 82-are indicated by the allocation o{ knowledge levels indicators ('l' 2 or

3) againsl each applicable "ror""i.

c"i"gorv c applicants must meet either the category 81 or the category 82

basic knowledge levels.The knowledge level indicators are defined as follows:

LEVEL 1A familiarisation wilh the principal elements of the subject'

Objectives:The applicant should be familiar with the basic elements o{ the subiect'The applicant shoutd be Ji" t" gi"" a simple description of the whole subject, using common words andexamples.The applicant should be able to use typical terms'

LEVEL 2A general knowledge of the theoretical and practical aspects of the subiect'An ability to apply that knowledge'

Objectives:The applicant should be able to understand the theoretical fundamentals ol the subject'The applicant shorro o"

"[i" i" giue a generat description ol the subject using, as appropriate, typical

examples.Theapplicantshouldbeabletousemathematicalformulaeinconjunctionwithphysicallawsdescribingthesubject.TheapplicantShouldbeabletoreadandunderstandSketches,drawingsandschematicsdescribingthesubject.Theapplicantshouldbeabletoapplyhisknowledgeinapracticalmannerusingdetai|edprocedures.

LEVEL 3A detailed knowledge oi the theoretical and practical aspects o{ the subject'A capacity to combine and apply th"

""p"rui" elements of knowledge in a logical and comprehensive

manner.Objectives:

TheapplicantshoUldknowthetheoryofthesubjectandinterrelationshipSWithothersUbjects.The applicant sho"ro n" uorL io giue a d"taiteo description of the subject using theoretical lundamenlalsand sPecific examPles.Theapplicantshouldunderstandandbeabletousemathematicalformulaerelatedtothesubject'TheapplicantShourooeaotetoread,understandandpreparesketches,simpledrawingsandschematicsdescribing the subiect. lacturer,sThe applilant sho;ld be able to apply his knowledge in a practical manner usrng manuinstructions.TheapplicantShouldbeabletointerpretresultsfromVarioussourcesandmeasurementsandapplycorrective action where appropriate'

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Table of Contents

Module 3.2 Static Electricity and ConductionlntroductionStatic Electricity

556Nature of Charges

Charged BodiesCoulomb's Law of Charges

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Unit of ChargeElectric FieldsConduction of Electricity in Solids, Liquids and a Vacuum

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Module 3.2 Enabling Obiectives and Certification Statement

Certif ication StatementThese Study Notes comply with the syllabus of EASA Regulation 2O42|2OO3 Annex lll (Part-66)

dix l. and the associated Knowledqe Levels as

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Static Electricitv and ConductionStatic electricity and distribution of electrostatic

Electrostatic laws of attraction andUnits of charqe, Coulomb's LawConduction of electricity in solids, liquids, gasesand a vacuum



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lntroductionElectrostatics (electricity at rest) is a subject with which most persons entering the field ofelectricity and electronics are somewhat familiar. For example, the way a person's hair standson end after a vigorous rubbing is an effect of electrostatics. While pursuing the study ofelectrostatics, you will gain a better understanding of this common occurrence. Of even greatersignificance, the study of electrostatics will provide you with the opportunity to gain importantbackground knowledge and to develop concepts which are essential to the understanding ofelectricity and electronics.

lnterest in the subiect of static electricity can be traced back to the Greeks. Thales of Miletus, aGreek philosopher and mathematician, discovered that when an amber rod is rubbed with fur,the rod has the amazing characteristic of attracting some very light objects such as bits of paperand shavings of wood.

About 1600, William Gilbert, an English scientist, made a study ol other substances which hadbeen found to possess qualities of attraction similar to amber. Among these were glass, whenrubbed with silk, and ebonite, when rubbed with fur. Gilbert classified all the substances whichpossessed properties similar to those of amber as electrics, a word of Greek origin meaningambe r.

Because of Gilbert's work with electrics, a substance such as amber or glass when given avigorous rubbing was recognized as being electrified, or charged with electricity.ln the year 1733, Charles Dufay, a French scientist, made an impodant discovery aboutelectrif ication. He found that when a glass was rubbed with fur, both the glass rod and the furbecame electri{ied. This realization came when he systematically placed the glass rod and thefur near other electrified substances and found that certain subslances which were attracted tothe glass rod were repelled by the fur, and vice versa. From experiments such as this, heconcluded that there must be two exactly opposite kinds of electricity.

Benjamin Franklin, American statesman, inventor, and philosopher, is credited with first usingthe terms positive and negative to describe the two opposite kinds of electricity. The chargeproduced on a glass rod when it is rubbed with silk, Franklin labelled positive. He attached iheterm negative to the charge produced on the silk. Those bodies which were not electrified orcharged, he called neutral.

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Static Electricityln a natural or neutral state, each atom in a body of matter will have the proper number ofelectrons in orbit around it. Consequently, the whole body of matter composed of the neutralatoms will also be electrically neutral. ln this state, it is said to have a "zero charge." Electronswill neither leave nor enter the neutrally charged body should it come in contact with otherneutral bodies. lf, however, any number of electrons is removed from the atoms of a body ofmatter, there will remain more protons than electrons and the whole body of matter will becomeelectrically positive. Should the positively charged body come in contact with another bodyhaving a normal charge, or having a negative (too many electrons) charge, an electric currentwill flow between them. Electrons will leave the more negative body and enter the positive boo\This electron flow will continue until both bodies have equal charges. When two bodies ofmatter have unequal charges and are near one another, an electric force is exerted betweenthem because of their unequal charges. However, since they are not in contact, their chargescannot equalize. The existence of such an electric force, where current cannot flow, is referredto as static electricity. ("Static" in this instance means "not moving.") lt is also referred to as anelectrostatic force.

One of the easiest ways to create a static charge is by friction. When two pieces of matter arerubbed together, electrons can be "wiped off" one material onto the other. lf the materials usedare good conductors, it is quite difficult to obtain a detectable charge on either, since equalizingcurrents can flow easily between the conducting materials. These currents equalize the chargesalmost as fast as they are created. A static charge is more easily created between non-conducting materials. When a hard rubber rod is rubbed with fur, the rod will accumulateelectrons given up by the fur, as shown in figure 2.1. Since both materials are poor conductors.very little equalizing current can flow, and an electrostatic charge builds up. When the chargebecomes great enough, current will flow regardless of the poor conductivity of the materials.These currents will cause visible sparks and produce a crackling sound.

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Nature of ChargesWhen in a natural or neutral state, an atom has an equal number of electrons and protons.Because of this balance, the net negative charge of the electrons in orbit is exactly balanced bythe net positive charge ol the protons in the nucleus, making the atom electrically neutral.

An atom becomes a positive ion whenever it loses an electron, and has an overall positivecharge. Conversely, whenever an atom acquires an extra electron, it becomes a negative ionand has a negative charge.

Due to normal molecular activity, there are always ions present in any material. lf the number ofpositive ions and negative ions is equal, the material is electrically neutral. When the number ofpositive ions exceeds the number of negative ions, the material is positively charged. Thematerial is negatively charged whenever the negative ions outnumber the positive ions.Since ions are actually atoms without their normal number of electrons, it is the excess or thelack of electrons in a substance that determines its charge. ln most solids, the transfer ofcharges is by movement of electrons rather than ions. The transfer of charges by ions willbecome more significant when we consider electrical activity in liquids and gases. At this time,we will discuss electrical behaviour in terms ol electron movement.

Charged BodiesOne of the fundamental laws of electricity is that like charges repel each other and unlikecharges attract each other. A positive charge and negative charge, being unlike, tend to movetoward each other. ln the atom, the negative electrons are drawn toward the positive protons inthe nucleus. This attractive force is balanced by the electron's centrifugal force caused by itsrotation about the nucleus. As a result, the electrons remain in orbit and are not drawn into thenucleus. Electrons repel each other because of their like negative charges, and protons repeleach other because of their like positive charges.

The law ol charged bodies may be demonstrated by a simple experiment. Two pith (paper pulp)balls are suspended near one another by threads, as shown in figure 2.2.

Figure 2.2 - Repulsion and attraction of charged bodies

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lf a hard rubber rod is rubbed with fur to give it a negative charge and is then.held against the..,ighi-h""Jbiri in parl (A), y'" i"J *iir givE off u n"gi1u" charge to the.ball. The right-hand ballwill have a negative

"6urg" *itt'i""pe-ct to the leftlhand ball. When released, the two balls will

be drawn together, u" .nounn in tig ui,re 2.2 (A\. They will touch and remain in contact until theleft-hand ball gains a portion'oi tn"e ;d"ti"; charge of the right-hand ball, at.which time they willswing aparl as shown in tigure 2.2 Q\: ll a positive or a negitive charge is placed on both balls$igule 2-2 (B)), the balls will repel each other'

Coulomb's Law of ChargesThe relationship between attracting or repelling-charged bodies was first discovered and

written

uUouin' u French scientisi nur"iCnurt"s A.boulomb. Coulomb's Law states that

charged bodies attract or repel each other.with a force that is directly proportional to thepr"Ji"iot ttt"ir individual c-harges, and is inversely proportional to the square of thedistance between them.

The amount of attracting or repelling force which acts between two electrically charged bodiesin free space depends on i*oif'tgi - (.1) their charges and (2) the distance between them'

Unit of GhargeThe process of electrons arriving or leaving_is exactly what happens when certain combinationsof materials are rubbed togeth"i electrons"from the atoms of one material are forced by therubbing to leave their resp"ective atoms and transfer over to the atoms of the other material' ln

other words, etectrons ";;;i;; if'"

;tfriO' hypothesized by Beniamin Franklin. The operationaldefinition of a coulomb as ihe unit of electrical charge (in terms of force generated betweenpoint

"hutg"") was found to be equal to an excess or deficiency of about.

6,280,000,000,000,000,000 etectrons. or, stated in reverse terms, one electron has a charge ofa6out 0.00000000000000000016 coulombs" Being that one electron is the smallest knowncarrier of electric charge, this last figure of chargelor the electron is defined as lhe elementarycharge.

1 coulomb = 6,280,000,000,000,000,000 electrons

Electric FieldsThe space between and around charged bodies in which their influence is felt is called anelectric field of force. lt can exist inlir, glass, paper, or a vacuum. electrostatic fields anddielectric fields are other names used to refer to this region of force.Fields of force spread out in the space surrounding their point o{ origin and, in general'diminish in proportion to the square of the distance from their source'

The field about a charged body is generally represented by lines which are.referred to aselectrostatic lines of t"i"". irt"si lines ire imaginary and are used merely to represent lhedirection and strength ot irr" ti"ro. To avoid confuJion, ihe lines of force exerted by a positive

"frurg" ur" always lhown leaving the charge, and for a negative 9!a1Oe tnlV are shown

entering. Figure 2.3 illustrates th6 use of lines to represenfthe lield about charged bodies'

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{B)Figure 2.3 - Electrostatic lines of force

Figure 2.3 (A) represents the repulsion of like-charged bodies and their associated fields. Part(B) represents the attraction of unlike-charged bodies and their associated fields.

Conduction of Electricity in Solids, Liquids and a Vacuum

SolidsElectric current is the movement of valence electrons. Gonduction is the name of this process.It is more fully described in Chapter 1 of this Module. Generally, only metals conduct electricity.Some conduct better than others.

The exception to this is graphite (one ol the forms of the element Carbon). Carbon is a non-metal which exhibits some electrical conductivity.

LiquidsThe only liquid elements which conduct are the liquid metals. At room temperature liquidmercury is a conductor. Other metals continue to conduct electricity when they are melted.Non-metals such as water, alcohol, ethanoic acid, propanone, hexane and so on, are all nonconductors of electricity.

However, it is possible to make some non-conducting liquids conduct electricity, by a processcalled ionization. lonized substances are called ionic substances.

lonic substances are made of charged particles - positive and negative ions. ln the solid statethey are held very firmly in place in a lattice structure. ln the solid state the ions cannot moveabout at all. When the ionic solid is melted, the bonds holding the ions in place in the lattice arebroken. The ions can then move around f reely.

When an electric current is applied to an ionic melt the electricity is carried by the ions that arenow able to move. ln an ionic melt the electric current is a flow of ions.

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Taking water as an example. Remember firstly, that water is considered to be a non-conductorof electricity. lt can allow some electricity through it if a high voltage is applied to it. This is dueto the presence of a minute concentration of H* and OH- ions in the water. However, electronscannot f low through water.

Covalent substances do not conduct at all in solution.

lonic substances are able to conduct electricity when they are dissolved in water.

The reason lies again in the fact that ionic substances are made of charged particles - ions.When the ionic solid is dissolved in water the ionic lattice breaks up and the ions become free tomove around in the water. When you pass electricity through the ionic solution, the ions areable to carry the electric current because of their ability to move freely. A solution conducts bymeans of lreely moving ions.

An electrolyte is a liquid which can carry an electric current through it. lonic solutions and ionicmelts are all electrolytes.

Electrolysis describes the process which takes place when an ionic solution or melt haselectricity passed through it.

GasesA gas in its normal state is one of the best insulators known. However, in a similar way asliquid, it can be forced to conduct electricity by ionisation of the gas molecules. lonisation of thegas molecules can be effected by extremely high voltages. For example, lightning, is electriccurrent flowing through an ionised path through air due to the huge electrical potential differencebetween the storm cloud and the ground.

ln air, and other ordinary gases, the dominant sourceof electrical conduction is via a relatively small numberof mobile ions produced by radioactive gases,ultraviolet light, or cosmic rays. Since the electricalconductivity is extremely low, gases are dielectrics orinsulators. However, once the applied electric fieldapproaches the breakdown value, f ree electronsbecome sufficiently accelerated by the electric field tocreate additional free electrons by colliding, andionizing, neutral gas atoms or molecules in a processcalled avalanche breakdown. The breakdown processforms a plasma that contains a significant number ofmobile electrons and positive ions, causing it tobehave as an electrical conductor. ln the process, itforms a light emitting conductive path, such as a spark,arc or lightning.

Figure 2.4 -

Lightning is electriccurrent flowing through an ionized

plasma of its own making

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Plasma is the state of matter where some of the electrons in a gas are stripped or "ionized"'rcm their molecules or atoms. A plasma can be formed by high temperature, or by application:f a high electric or alternating magnetic field as noted above. Due to their lower mass, the? ectrons in a plasma accelerate more quickly in response to an electric field than the heavier:csitive ions, and hence carry the bulk of the current.

Vacu u m: s a common belief that electricity cannot flow through a vacuum. This is however incorrect.lemember that a conductor is "something through which electricity can flow," rather than'scmething which contains movable electricity." A vacuum offers no blockage to moving:r'arges. Should electrons be injected into a vacuum, the electrons will flow uninhibited and-rretarded. As such, a vacuum is an ideal conductor.-";s fact is taken advantage of in many situations, from televisions to vacuum valves. Avacuum arc can arise when the sudaces of metal electrodes in contact with a good vacuum3egin to emit electrons either through heating (thermionic emission) or via an electric field thats sufficient to cause {ield emission. Once initiated, a vacuum arc can persist since the freed3a(icles gain kinetic energy from the electric field, heating the metal surfaces through highsceed particle collisions. This process can create an incandescent cathode spot which frees-cre particles, thereby sustaining the arc. At sufficiently high currents an incandescent anodesoot may also be formed.

= ectrical discharge in vacuum is important for certain types of vacuum tubes and for high. o lage vacuum switches.


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3.3 Electrical Terminology

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Copyright Noticeo copyright. All worldwide rights reserved. No part of this publication may be reproduced,stored in a retrieval system or transmitted in any form by any other means whatsoever: i.e-photocopy, electronic, mechanical recording or otherwise without the prior written permission ofTotal Training Support Ltd.

Knowledge Levels -

Category A, 81 , 82 and C Aircraft MaintenanceLicence

Basic knowledge for categories A, Bl and 82 are indicated by the allocation o{ knowledge levels indicators (1, 2 or3) against eacti applicable subject. Category C applicants must meet either lhe category 81 or the category 82basic knowledge levels.The knowledge level indicators are defined as lollows:

LEVEL 1A familiarisation with the principal elements of the subjecl.

Objectives:The applicant should be lamiliar with the basic elements of the subject.The applicant should be able to give a simple description of the whole subject, using common words andexamples.The applicant should be able to use typical terms.

LEVEL 2A general knowledge o{ the theoretical and practical aspects of the subiect.An ability to apply thal knowledge.

Objectives:The applicant should be able to understand the theoretical fundamentals of the subject.The applicant should be able to give a generat description ol the subject using, as appropriate, typicalexamples.The applicant should be able to use mathematical formulae in conjunction with physical laws describing thesubject.The applicant should be able to read and understand sketches, drawings and schematics describing thesubject.The applicant should be able to apply his knowledge in a practical manner using detailed procedures.

LEVEL 3A detailed knowledge of the theoretical and practical aspects of the subjecl.A capacity to combine and apply the separate elements of knowledge in a logical and comprehensivemanner.

Objectives:The applicant should know the theory of the subject and interrelationships with other subjects.The applicanl should be able to give a detailed description of the subject using theoretical {undamentalsand specif ic examples.The applicant should understand and be able to use mathematical formulae related to the subject.The applicant should be able to read, understand and prepare sketches, simple drawings and schemat :sdescribing the sublect.The applicant should be able to apply his knowledge in a practical manner using manufacturersinstructions.The applicant should be able to interpret results Jrom various sources and measurements and applycorrective action where appropriate.

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Module 3.3 Enabling Obiectives and Certification Statement

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The following terms, their units and factorsaflecting them: potential difference,electromotive force, voltage, current,resistance, conductance, charge, conventional



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Electrical Energyln the field of physical science, work must be defined as the product of force anddisplacement. That is, the lorce applied to move an object and the distance the object ismoved are the factors of work performed.

It is imporlant to notice that no work is accomplished unless the force applied causes a changein the position of a stationary object, or a change in the velocity of a moving object. A workermay tire by pushing against a heavy wooden crate, but unless the crate moves, no work will beaccomplished.

Energyln our study of energy and work, we must define energy as the ability to do work. ln order toperform any kind of work, energy must be expended (converted from one form to anotheO.Energy supplies the required force, or power, whenever any work is accomplished.One form of energy is that which is contained by an object in motion. When a hammer is set inmotion in the direction of a nail, it possesses energy of motion. As the hammer strikes the nail,the energy of motion is conveded into work as the nail is driven into the wood. The distance thenail is driven into the wood depends on the velocity of the hammer at the time it strikes the nail.Energy contained by an object due to its motion is called kinetic energy. Assume that thehammer is suspended by a string in a position one meter above a nail. As a result ofgravitational attraction, the hammer will experience a force pulling it downward. lf the string issuddenly cut, the force of gravity will pull the hammer downward against the nail, driving it intothe wood. While the hammer is suspended above the nail it has abitity to do work because of itselevated position in the earth's gravitational field. Since energy is the ability to do work, thehammer contains energy.

Energy contained by an object due to its position is called potential energy. The amount ofpotential energy available is equal to the product of the force required to elevate the hammerand the height to which it is elevated.

Another example of potential energy is that contained in a tightly coiled spring. The amount ofenergy released when the spring unwinds depends on the amount of force required to wind thespring initially.

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Electrical ChargesFrom the previous study of electrostatics, you learned that a field of force exists in the

space

surrounding any electrical "fr"tg".

ih" "length

of the field is directly dependent on the force of

the charge.

Thechargeofoneelectronmightbeusedasa'unitofelectricalcharge,sincechargesare.i""[Jn"v ol"pracement of ele"ctrons; but the charge of one electron is so small that it isimpractical to use.

Thepracticalunitadoptedformeasuringchargesisthecoulomb,namedafterthescientistCharles Coulomb. On"

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"qui to th6 charge of 6,280,000,000,000,000,000 (six

q"i"tifri"" tto hundred uni "ighty

quadrillion) or 6'28 x 1018 electrons'

when a charge of one coulomb exists between two bodies, one unit of electrical potential

energy exists, which is caffeJ tfie Jitf"r"n"" of potential beiween the two bodies' This is referredto aJelectromotive force, or voltage, and the unit of measure is the volt'

Electrical charges are created by the displacement of electrons, so that there exists an excessof electrons at one point, anJa leticiendy at another point .Consequentlyl,

-?llSe must

always have either a n"guiiu" ;i positive'polarlty A. b.-ogy *ith 1,9L"-::t^ of electrons iscon"iO"r"O to be negative, *f,"r"L. a body witlL a.deficiency of electrons is positive.A difference of potential

""n u*i.t between two points, or bodies, only if they have different

t'rr"ig;". i;"itrerwords, there is no difference in potential between two bodies if both have adeficiency of electrons to tr"

"ut" o"gree. lf, however, one body is deficient of 6 coulombs

(representing 6 volts), unO tf'" otft"t is-def icient by 1 2 coulombs (representing 12 volts), there isi iitt"r""""i,t poteniiat oi o uorti. 11" body with ihe greater deficiency is positive with respectto the other.

ln most electrical circuits only the difference of potential between two points is of importanceunJ tn" absolute potentials of the points are of iittle concern. Very often it is-convenient to useone standard reference to|. uff of the various potentials throughout a piece.of equipment' For thisreason, the potentials ut uuiiors points in a circuit are generally measured with respect to themetal chassis on which all parts of the circuit are mounled. The chassis is considered to be at

zeio potential and alt oher'pot"nii"it ut" either positive or negative with respe,ct to the chassis'When used as the reference point, the chassis is said to be at ground potential'

occasionally,ratherlargevaluesofVoltagemaybeencountered,inwhichcasetheVoltbecomestoosmallaunitforconvenience-.lna-situationofthisnature,thekilovolt(kV)'meaning1,000 volts, is frequently used. As an example, 20,000 volts would be written as 20 kV' ln othercases, the volt may Oe ioolurg" a unit, as when dealing with very. small voltages. For thisp"ip"." tn" rnittivdn lmvf r""unrg o;"-tho.us1ngil of I volt, and the microvolt (pV), meaningone-millionth of a volt, url, ,""0. F6r example, 0'001 volt would be written as 1 mV' and0.000025 volt would be written as 25 UV'

when a difference in potential exists between two charged bodies that are connected by aconductor, electrons *irr flow arong the conductor. This-flow is from the negatively cha.rged

body

to the positively cfrargeO btdy, ,niit tn" two charges are equalized and the potential differenceno longer exists.


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An analogy of this action is shown in the two water tanks connected by a pipe and valve infigure 3.1. At first the valve is closed and all the water is in tank A. Thus, the water pressureacross the valve is at maximum. When the valve is opened, the water flows through the pipef rom A to B until the water level becomes the same in both tanks. The water then stops flowingin the pipe, because there is no longer a difference in water pressure between the two tanks.

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Figure 3.1 - An analogy of potential difference

Electron movement through an electric circuit is directly proportional to the difference inpotential or electromotive force (EMF), across the circuit, just as the flow of water through thepipe in figure 3.1 is directly proportional to the difference in water level in the two tanks.A fundamental law of electricity is that the electron flow is directly proportional to theapplied voltage. lf the voltage is increased, the flow is increased. lf the voltage is decreased,the flow is decreased.

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Electric Current

Electron f lowIt has been proven that electrons (negative charges) move through a conductor in response toan electric field. Electron current flow will be used throughout this explanation. Electroncurrent is defined as the directed flow of electrons. The direction of electron movement is from aregion of negative potential to a region of positive potential. Therefore electron flow can be saidtoilow from -negative to positive. Tie direction of current flow in a material is determined by thepolarity of the apPlied voltage.

Conventional Current Flowln the UK and Europe, conventional current flow is said to be from positive to negative potential

- the opposite way to the actual flow of electrons.

Conventional current was defined early in thehistory of electrical science as a flow of positivecharge. ln solid metals, like wires, the positivecharge carriers are immobile, and only thenegatively charged electrons f low. Because theelectron carries negative charge, the electroncurrent is in the direction opposite to that ofconventional (or electric) current.

Electric charge moves from the positive side ofthe power source to the negative.

ln other conductive materials, the electric current Figure 3'2 - Conventional currentis due to the flow of charged parlicles in both flow directiondirections at the same time. Electric currents inelectrolytes are flows of electrically charged atoms (ions), which exist in both positive andnegative varieties. For example, an elecirochemical cell may be constructed. with salt water (asolution of sodium chloride) on one side of a membrane and pure water on the other. Themembrane lets the positive sodium ions pass, but not the negative chloride ions, so a netcurrent results. Electric currents in plasma are flows of electrons as well as positive andnegative ions. ln ice and in certain solid electrolytes, flowing protons constitute the electriccuirent. To simplify this situation, the original definition oi conventional current still stands.

There are also materials where the electric current is due to the flow of electrons and yet it isconceptually easier to think of the current as due to the llow of positive "holes" (the spots thatshould have an electron to make the conductor neutral). This is the case in a p-typesemiconductor.

These EASA Part-66 Module 3 notes will use conventional current noiation throughout, unlessoiherwise stated, and then only for specif ic reasons'

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Random DriftAll materials are composed of atoms, each of which is capable of being ionised. lf some form ofenergy, such as heat, is applied to a material, some electrons acquire sufficient energy to moveto a higher energy level. As a result, some electrons are freed from their parent atom's whichthen becomes ions. Other forms of energy, particularly light or an electric field, will causeionisation to occur.

The number of free electrons resulting from ionisation is dependent upon the quantity of energyapplied to a material, as well as the atomic structure of the material. At room temperature somematerials, classified as conductors, have an abundance of free electrons. Under a similarcondition, materials classif ied as insulators have relatively few f ree electrons.

ln a study of electric current, conductors are of major concern. Conductors are made up ofatoms that contain loosely bound electrons in their outer orbits. Due to the effects of increasedenergy, these outermost electrons frequently break away from their atoms and freely driftthroughout the material. The free electrons, also called mobile electrons, take a path that is notpredictable and drift about the material in a haphazard manner. Consequently such a movementis termed random drift.

It is imporlant to emphasize that the random drift of electrons occurs in all materials. The degreeof random drift is greater in a conductor than in an insulator.

Directed DriftAssociated with every charged body there is an electrostatic field. Bodies that are charged alikerepel one another and bodies with unlike charges attract each other. An electron will be affectedby an electrostatic field in exactly the same manner as any negatively charged body. lt isrepelled by a negative charge and attracted by a positive charge. lf a conductor has a differencein potential impressed across it, as shown in figure 3.3, a direction is imparled to the randomdrift. This causes the mobile electrons to be repelled away from the negative terminal andattracted toward the positive terminal. This constitutes a general migration of electrons from oneend of the conductor to the other. The directed migration of mobile electrons due to the potentialdifference is called directed drift.



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Figure 3.3 - Directed drift

The directed movement of the electrons occurs at a relatively low velocity (rate of motion in aparticular direction). The effect of this directed movement, however, is felt almost,instantaneously, as explained by the use of figure 3.3. As a difference in potential is impressedacross the con'ductor, ihe positive terminal of the battery attracts electrons from point A. Point Anow has a deficiency of electrons. As a result, electrons are attracted from point B to point A'point B has now developed an electron deficiency, therefore, it will attract electrons. This sameeffect occurs throughoui the conductor and repeats itself from points D to C At the same instanitfte positive battery"terminal attracted electrons f rom point A, the negative terminal repelledelectrons toward foint D. These electrons are attracted to point D as it gives up electrons topoint C. This process is continuous for as long as a difference of potential exists across theconductor. Though an individual electron mov:es quite slowly through-the conductor, the effect o"a directed drift oJcurs almost instantaneously. As an electron moves into the conductor at pointD, an eleciron is leaving at point A. This action takes place at approximately the speed a light(186,000 Miles Per Second).

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Figure 3.4 - Effect of directed drift.

Magnitude of Current FlowElectric current has been defined as the directed movement of electrons. Directed drift,therefore, is current and the terms can be used interchangeably. The expression directed drift isparticularly helpful in differentiating between the random and directed motion of electrons.However, current flow is the terminology most commonly used in indicating a directedmovement of electrons.

The magnitude of current flow is directly related to the amount of energy that passes through aconductor as a result of the drift action. An increase in the number of energy carriers (the mobileelectrons) or an increase in the energy of the existing mobile electrons would provide anincrease in current flow. When an electric potential is impressed across a conductor, there is anincrease in the velocity of the mobile electrons causing an increase in the energy of the carriers.There is also the generation of an increased number of electrons providing added carriers ofenergy. The additional number of free electrons is relatively small, hence the magnitude ofcurrent flow is primarily dependent on the velocity of the existing mobile electrons.

The magnitude of current flow is affected by the difference of potential in the following manner.lnitially, mobile electrons are given additional energy because of the repelling and attractingelectrostatic field. lf the potential difference is increased, the electric field will be stronger, theamount of energy imparted to a mobile electron will be greater, and the current will beincreased. lf the potential difference is decreased, the strength of the field is reduced, theenergy supplied to the electron is diminished, and the current is decreased.

Measurement of CurrentThe magnitude of current is measured in amperes. A current of one ampere is said to flowwhen one coulomb of charge passes a point in one second. Remember, one coulomb is equalto the charge of 6.28 x 101b electrons.



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Frequently, the ampere is much too large a unit for measuring current. Therefore, themilliampere (mA), one-thousandth of an ampere, or the microampere (pA), one-millionth of anampere, is used. The device used to measure current is called an ammeter and will bediscussed in detail in a later module.

A current of 1 Amp is flowing when a quantity of 1 Goulomb of charge flows for 'l second'

The current I in amperes can be calculated with the following equation:

Where:

a is the electric charge in coulombs (ampere seconds)

t is the time in seconds

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Electrical ResistanceIt is known that the directed movement of electrons constitutes a current flow. lt is also knownthat the electrons do not move freely through a conductor's crystalline structure. Some materialsoffer little opposition to current flow, while others greatly oppose current flow. This opposition tocurrent flow is known as resistance (R), and the unit of measure is the ohm. The standard ofmeasure for one ohm is the resistance provided at zero degrees Celsius by a column ofmercury having a cross-sectional area of one square millimetre and a length ol 106.3centimetres.

A conductor has one ohm of resistance when an applied potential of one volt produces acurrent of one ampere. The symbol used to represent the ohm is the Greek letter omega().Resistance, although an electrical property, is determined by the physical structure of amaterial. The resistance of a material is governed by many of the same factors that controlcurrent flow. Therefore, in a subsequent discussion, the factors that affect current flow will beused to assist in the explanation of the factors affecting resistance.

ConductanceElectricity is a study that is frequently explained in terms of opposites. The term that is theopposite of resistance is conductance. Conductance is the ability of a material to passelectrons. The lactors that affect the magnitude of resistance are exactly the same forconductance, but they affect conductance in the opposite manner. Therefore, conductance isdirectly proportional to area, and inversely proportional to the length of the material. Thetemperature of the material is definitely a factor, but assuming a constant temperature, theconductance of a material can be calculated.

The unit of conductance is the mho (G), which is ohm spelled backwards. Recently the termmho has been redesignated siemens (S). Whereas the symbol used to represent resistance(R) is the Greek letter omega ( ), the symbol used to represent conductance (G) is (S). Therelationship that exists between resistance (R) and conductance (G) or (S) is a reciprocal one. Areciprocal of a number is 'one' divided by that number. ln terms of resistance and conductance:

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Electrical Laws

Faraday's LawFaraday's law of induction states that the induced electromotive torce in a closed loop of wire isdirectlyproportionaltothetimerateofchangeofmagneticfluxthroughtheloop.

Ohm's LawAn electrical circuit, the current passing through a conductor between two points is directlypioportional to the potential differenceli.e. uoltage drop or voltage) across the two points, andinversely proportional to the resistance between them'

Kirchhoff 's Laws

current Law -At any point in an electrical circuit where charge density is- not changing intime, the sum of currents flowing towards that point is equal to the sum of currentsflowing awaY f rom that Point.

Voltage Law -The directed sum of the electrical potential differences around any closedcircuit must be zero.

Lens's LawThe induced current in a loop is in the direction that creates a magnetic field that opposes thechange in magnetic flux through the area enclosed by the loop. That is, the induced currenttendJto keeplhe original magnetic flux through the field f rom changing'


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3.4 Generation of Electricity

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Copyright NoticeO Copyright. All worldwide rights reserved. No part of this publication may be reproduced,stored in a retrieval system or transmitted in any form by any other means whatsoever: i.e.photocopy, electronic, mechanical recording or otherwise without the prior written permission ofTotal Training Support Ltd.

Knowledge Levels -

Category A, 81, 82 and C Aircraft MaintenanceLicence

Basic knowledge for categories A, 81 and 82 are indicated by the allocation ol knowledge levels indicators (1, 2 or3) against each applicable subject. Category C applicants must meet either the category B1 or the category 82basic knowledge levels.The knowledge level indicators are defined as follows:

LEVEL 1A familiarisation with the principal elements ol the subject.

Objectives:The applicant should be familiar with the basic elements of the subject.The applicant should be able to give a simple description of the whole subject, using common words andexamples.The applicant should be able to use typical terms.

LEVEL 2A general knowledge ol the theoretical and praclical aspects of the subject.An ability to apply that knowledge.

Objectives:The applicanl should be able to understand the theoretical fundamentals of the subject.The applicant should be able to give a general description of the subject using, as appropriate, typicalexamples.The applicant should be able to use mathematical formulae in conjunction with physical laws describing thesu bject.The applicant should be able to read and understand skelches, drawings and schematics describing thesubject_The applicant should be able to apply his knowledge in a practical manner using detailed procedures.

LEVEL 3A detailed knowledge of ihe theoretical and practical aspects of the subject.A capacity to combine and apply the separate elements of knowledge in a logical and comprehensivemanner.

Objectives:The applicant should know the theory of the subject and interrelationships with other subjects.The applicant should be able to give a detailed description of the subject using theoretical lundamentalsand specific examples.The applicant should understand and be able to use mathematical lormulae related to the subject.The applicant should be able to read, understand and prepare sketches, simple drawings and schematicsdescribing the subiect.The applicant should be able lo apply his knowledge in a practical manner using manufacturer'sinstructions.The applicant should be able to interpret results lrom various sources and measurements and applycorreclive action where appropriale.

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Contents

Module 3.4 Generation of ElectricityHow Voltage is ProducedVoltage Produced by FrictionVoltage Produced by PressureVoltage Produced by HeatVoltage Produced by LightVoltage Produced by Chemical Action

10Voltage Produced by Magnetism


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How Voltage is ProducedIt has been demonstrated that a charge can be produced by rubbing a rubber rod with fur.Because of the friction involved, the rod acquires electrons lrom the fur, making it negative; thefur becomes positive due to the loss of electrons. These quantities of charge c6nstitute adifference of potential between the rod and the fur. The electrons which mike up this differenceof potential are capable of doing work if a discharge is allowed to occur.

To be a practical source of voltage, the potential difference must not be allowed to dissipate, butmust be maintained continuously. As one electron leaves the concentration of negative charge,another must be immediately provided to take its place or the charge will eventually diminish-tothe point where no further work can be accomplished. A voltage source, therefore, is a devicewhich is capable of supplying and maintaining voltage while some type of electrical apparatus isconnected to its terminals. The internal action of the source is such that electrons arecontinuously removed from one terminal, keeping it positive, and simultaneously supplied to thesecond terminal which maintains a negative charge.

Presently, there are six known methods for producing a voltage or electromotive force (EMF).some of these methods are more widely used than olhers, and some are used mosfly iorspecific applications. Following is a list of the six known methods of producing a voltage.

Friction - Voltage produced by rubbing certain materials together.Pressure (piezoelectricity) - Voltage produced by squeezing crystals of cerlainsubstancesHeat (thermoelectricity) - voltage produced by heating the joint (iunction) where twounlike metals are joined.Light (photoelectricity) - Voltage produced by light striking photosensitive (light sensitive)substances.chemical Action - voltage produced by chemicar reaction in a battery cell.Magnetism - voltage produced in a conductor when the conductor moves through amagnetic field, or a magnetic field moves through the conductor in such a manner as tocut the magnetic lines of force of the field.

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Voltage Produced bY FrictionThe first method discovered for creating a voltage was that of generation by friction Theoeueropment of charges oy ruoning u r5d *ith fJr is a prime example.of tl." *3y in which a

""fi"gli" g"""rated"by friction. Beiause of the nature of the materials with which this voltage is

generated, it cannot o" "onuuni"ntrv

used or maintained. For this reason, very little practical use

tas been iound for voltages generated by this method'

ln the search for methods to produce a voltage of a larger amplitude.and of a more practicalnature, machines were OevltlpeO in which c-hatges wele transferred from one terminal to,""if'dt Uy r"un. of rotating das" 0i""" ot movlng belts. The most notable of these machinesis the Van de Graaff gen"r"ioi. lt is used today to produce potentials in lhe..order of millions ofvolts for nuclear research. n" tn""" machines have little vaiue outside the field of research' theirtheory of operation will not be described here'

Voltage Produced bY Pressureone specialized method of generating an EMF utilizes the characteristics of certain ionic

"'G;ir ;r;t"s quartz, noJfrette salti, and tourmaline. These crystals have the remarkable

ability to generate a voltage whenever stresses are.applied to their surface: T1'"' if a crystal o{quu,i, i" iqr"ezeO, charg'es'oi opposite polarity will appear on two opposite surfaces of thecrystal. lf the force is reversed und thu crystal ii stretihed, charges will again appear, but will beoi'tf'" ofpo"it" polarity from those produced by squeezing. lf a crystal of thislype is given avibratory motion, it wiff produce i ublt"g" of reversing pol-rity between two of its sides' Quartzor similir crystals can thus be used to convert mechanical energy into electrical energy'

This phenomenon, called the piezoelectric elfect, is shown in figure 4.1 ' some of the commondevices that make use of fiezoelectric crystals are microphones, phonograph.cartridges, andoscillators used in radio transmitters, radio receivers, and sonar equipment. This method ofgenerating an EMF is not suitable foi applications having large voltage or power requirements,6ut is wid6ly used in sound and communications systemi where small signal voltages can beeffectively used.

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crystals of this type also possess another interesting property, the "converse piezoelectriceffect." That is, they have the ability to convert electrical energy into mechanical energy. Avoltage impressed across the proper surfaces of the crystal will cause it to expand or contractits surfaces in response to the voltage applied.

Voltage Produced by HeatWhen a length of metal, such as copper, is heated at one end, electrons tend to move awayfrom the hot end toward the cooler end. This is true of most metals. However, in some metals,such as iron, the opposite takes place and electrons tend to move toward the hot end. Thesecharacteristics are illustrated in figure 4.2.The negative charges (electrons) are moving throughthe copper away from the heat and through the iron toward the heat. They cross from the iron tothe copper through the current meter to the iron at the cold junction. This device is generallyre{erred to as a thermocouple

Figure 4.2 - Voltage produced by heat.

Thermocouples have somewhat greater power capacities than crystals, but their capacity is stillvery small if compared to some other sources. The thermoelectric voltage in a thermocoupledepends mainly on the difference in temperature between the hot and c-old junctions.consequently, they are widely used to measure temperature, and as heat-se

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