FM 55-509-1

Field ManualNo. 55-509-1

HEADQUARTERSDEPARTMENT OF THE ARMY

Washington, DC, 1 September 1994

INTRODUCTION TO MARINE ELECTRICITY

CONTENTS

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PREFACE

This manual is an electrical reference text for the marine engineering field. It provides information forthe 88L10, 88L20, 88L30, 88L40, 881A1, and 881A2 military occupational specialties (MOSs).

This text reinforces good marine electrical practices. A good knowledge of marine electricity helpsmaintain the health and welfare of the crew by promoting the safe operation of the many electrical systems onboard a vessel.

This manual covers marine electrical safety and alternating current (AC) and direct current (DC)fundamentals. It details the vessel distribution system as well as circuit protection and the electrical motor load.This information corresponds with the program of instruction presented to the marine engineering students atFort Eustis.

The marine engineer must understand the entire production, distribution, and user end of the electricalprocess. He will be required to maintain and overhaul all the electrical apparatus for safely operating the vessel.

The proponent of this publication is the US Army Transportation School. Submit changes for improvingthis publication on DA Form 2028 (Recommended Changes to Publications and Blank Forms) directly toCommandant, US Army Transportation School, ATSP-TDL, Fort Eustis, VA 23604-5001.

This publication contains copyrighted material.

Unless this publication states otherwise, masculine nouns and pronouns do not refer exclusively to men.

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CHAPTER 1

SAFETY

INTRODUCTION

Successfully completing everyday activitiesdepends on safe execution. Preparation and conductduring these activities reflects on performance. In noother field is this more significant than in the marinefield.

Safety is an encompassing subject. This textdoes not repeat existing electrical safety practicesoutlined in other references. Instead it emphasizesthose standards necessary to successfully completeArmy watercraft missions.

Current is the measure of shock intensity. Thepassage of even a very small current through a vitalpart of the human body can kill. At about 100 milli-amperes (0.1 ampere), the shock is fatal if it lasts forone second or more. Fatalities have resulted fromvoltages as low as 30 volts.

Conditions on board a vessel add to the chanceof receiving an electrical shock. The body is likely tobe in contact with the metal structure of the vessel.The bodys resistance may be low because ofperspiration or damp clothing. Personnel must beaware that electrical shock hazards exist.

Accidentally placing or dropping a metal tool,ruler, flashlight case, or other conducting articleacross an energized terminal can cause short circuits.The resulting arc and fire, even on relatively low-voltage circuits, may extensively damage equipmentand seriously injure personnel.

Touching one conductor of an ungroundedelectrical system while the body is in contact with thehull of the ship or other metal equipment enclosurescould be fatal.

WARNING

Treat all energized electric circuitsas potential hazards at all times.

DANGER SIGNALS

Be constantly alert for any signs that mightindicate a malfunction of electrical equipment.When any danger signals are noted, report themimmediately to the chief engineer or electrical of-ficer. The following are examples of danger signals:

Fire, smoke, sparks, arcing, or an unusualsound from an electric motor or contactor.

Frayed and damaged cords or plugs.

Receptacles, plugs, and cords that feelwarm to the touch.

Slight shocks felt when handling electricalequipment.

Unusually hot running electric motors andother electrical equipment.

An odor of burning or overheatedinsulation.

Electrical equipment that either fails tooperate or operates irregularly.

Electrical equipment that produces exces-sive vibrations.

CAUTION

Do not operate faulty equipment.Stand clear of any suspectedhazard, and instruct others to dolikewise.

ELECTRIC SHOCK

Electric shock is a jarring, shaking sensation.Usually it feels like receiving a sudden blow. If thevoltage and current are sufficiently high, uncon-sciousness occurs. Electric shock may severely burnthe skin. Muscular spasms may cause the hands to

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clasp the apparatus or wire making it impossible tolet go.

Rescue and Care of Shock Victims

For complete coverage of cardiopulmonaryresuscitation (CPR) and treatment of burn and shockvictims, refer to Ships Medicine Chest and MedicalAid at Sea from the US Department of Health andHuman Services.

The following procedures are recommendedfor the rescue and care of shock victims:

Remove the victim from electrical contactat once, but do not endanger yourself.Touching a shock victim who is still incontact with the energized circuit willmake you another shock victim. Help theshock victim by de-energizing the affectedcircuit. Then use a dry stick, rope, belt,coat, blanket, shirt, or any other noncon-ductor of electricity to drag or push thevictim to safety.

Determine the cardiopulmonary status ofthe casualty. (Start CPR if spontaneousrespiration or circulation is absent.)

Once the person is stabilized, attend otherphysical injuries as they would normally betreated. Lay the victim face up in a proneposition. The feet should be about 12inches higher than the head. Chest or headinjuries require the head to be slightlyelevated. If there is vomiting or if there arefacial injuries that cause bleeding into thethroat, place the victim on his stomach withhis head turned to one side. The headshould be 6 to 12 inches lower than the feet.

Keep the victim warm. The injuredpersons body heat must be conserved.Cover the victim with one or moreblankets, depending on the weather andthe persons exposure to the elements.Avoid artificial means of warming, such ashot water bottles.

Do not give drugs, food and liquids if medi-cal attention will be available within ashort time. If necessary, liquids may beadministered. Use small amounts of

water, tea, or coffee. Never give alcohol,opiates, and other depressant substances.

Send for medical personnel (a doctor, ifavailable) at once, but do not under anycircumstances leave the victim until medi-cal help arrives.

Safety Precautions for Preventing Electric Shock

Observe the following safety precautions whenworking on electrical equipment:

When work must be done in the immediatevicinity of electrical equipment, check withthe senior engineer responsible for main-taining the equipment to avoid any poten-tial hazards. Stand clear of operatingradar and navigational equipment.

Never work alone. Another person couldsave your life if you receive an electricshock.

Work on energized circuits only whenabsolutely necessary. The power sourceshould be tagged out at the nearestsource of electricity for the componentbeing serviced.

Keep covers for all fuse boxes, junctionboxes, switch boxes, and wiring accessoriesclosed. Report any cover that is not closedor that is missing to the senior engineerresponsible for its maintenance. Failure todo so may result in injury to personnel ordamage to equipment if an accidental con-tact is made with exposed live circuits.

Discharge capacitors before working onde-energized equipment. Take specialcare to discharge capacitors properly.Injury or damage to equipment couldresult if improper procedures are used.

When working on energized equipment,stand on a rubber mat to insulate yourselffrom the steel deck.

When working on an energized circuit,wear approved electrical insulating rubbergloves. (The rubber gloves used with NBCsuits are not acceptable.) Cover as much

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of your body as practical with an insulatingmaterial, such as shirt sleeves. This isespecially important when working in awarm space where you may perspire.

If possible, de-energize equipment beforehooking up or removing test equipment.

When working on energized electricalequipment, work with only one hand insidethe equipment. Keep the other hand clearof all conductive materials that may pro-vide a path for current flow.

Wear safety goggles. Sparks coulddamage your eyes. The sulfuric acid con-tained in batteries and the oils in electricalcomponents can cause blindness.

Ensure that all tools are adequately insu-lated when working on energized electricalequipment.

Never work on electrical equipment whilewearing rings, watches, identification tags,or other jewelry.

Never work on electrical equipment whilewearing loose-fitting clothing. Be carefulof loose sleeves and the battle dressuniform (BDU) shirttails.

Ensure all rotating and reciprocating partsof the electric motors are adequatelyprotected by guards.

Remain calm and consider the possibleconsequences before performing anyaction.

DAMAGE AND FIRE

Never enter a flooded compartment that has agenerator actively producing power. Transfer theload and secure the generator before entering.

Secure power to the affected circuits if there isan electrical fire in a compartment. If critical systemsare involved that prevent power from being secured(determined by the chief engineer), extinguish thefire using a nonconducting agent, such as dry chemi-cal, carbon dioxide (C02), or halon.

WARNING

The use of water in any form is notpermitted.

Carbon dioxide is the choice for fightingelectrical fires. It has a nonconductive extinguishingagent and does not damage equipment. However,the ice that forms on the horn of the extinguisher willconduct electricity.

WARNING

Personnel exposed to a high con-centration of C02 will suffocate.

Burning electrical insulation is toxic and cankill in a matter of moments. Use the oxygen breath-ing apparatus (OBA) when fighting electrical fires.For more information, refer to Marine Fire Preven-tion, Firefighting and Fire Safety from the MaritimeAdministration.

PORTABLE AND TEMPORARY ELECTRICALEQUIPMENT

Ensure all electrical extension cords areapproved by either the chief engineer or the electri-cal officer. Never use an extension cord or powerhand tool without it being properly grounded.Regularly inspect all extension cords and portableelectrical equipment. Ground all metal multimetersand test equipment to the hull. Some military metershave a grounding jack for this connection.

WARNING

An ungrounded portable powertool can kill.

REPAIR SAFETY

Before starting any electrical work, secure thepower to the circuit and affix a temporary warningtag to the affected circuit breaker or power source.Check the de-energized circuit with a multimeter. Ifyou must leave the repair and return at a later time,always ensure that the circuit is de-energized beforeresuming work.

Figure 1-1 shows a temporary warning tagavailable through the supply system. Any tag can be

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used as long as it contains the following minimum When you are engaged in electrical repairs onamount of information board a vessel, always work in teams of two or more.

Never start working on an electrical system until theTime and date work is started. chief engineer or electrical officier has been in-

formed. A units operational status reflects theThe person performing the work. vessels operational status and its ability to get under

way. All vessel systems are interrelated. What mayThe affected circuits. appear to be a minor repair may ultimately determine

whether or not the vessel is fully operational.The approval and signature of the chiefengineer or electrical officer. Battery design forces the electrolyte to explode

upwards. Never service batteries without proper eyeThe required position of the affected protection. If battery electrolyte gets in your eyes,switch, breaker, or fuse, such as closed, flush them immediately for 15 minutes and seekopen, or removed. medical attention.

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CHAPTER 2

FUNDAMENTALS OF ELECTRICITY

INTRODUCTION

Electricity is a fundamental entity of nature. Itconsists mainly of negatively and positively chargedparticles commonly found in the atom. Throughman-made influence and natural phenomena, it ispossible to observe how the electron (negativelycharged) and the proton (positively charged) interactmagnetically.

The attraction and repulsion principles of mag-netism are used to make electricity perform work.Magnetic principles determine certain reactions; forexample, the attraction or repulsion of two magneti-cally charged objects. These principles can be usedin a motor to cause motion and to turn a water pump.Electricity, in other words, uses the magnetic proper-ties of subatomic particles to develop magnetic fieldsat a given place and time to perform work.

Taking a magnetically neutral atom and artifi-cially separating the electron from the rest of theatom leaves a positive ion. Exciting this atomthrough mechanical or chemical means prevents theelectron and positive. ion from returning to its naturalstate. Nature seeks equilibrium or a natural balanceand order.

A battery or generator forces all the electronsto one terminal and positive ions to the other ter-minal. As long as the atoms are stimulated, thisimbalance or difference between the terminalsremains. If excitation of the atoms is stopped naturewill cause the negative electrons to return to theirpositive ions through the principles of magnetism.

If excitation of the atoms continues and a com-plete path of conductive material connects the twoterminals (where the negative electrons and positiveions have gathered), a complete circuit is created.Because positive and negative polarities attract, theelectron follows this path from its terminal to thepositive ion terminal seeking equilibrium. In doingso, a magnetic field from the electron is developingin the entire circuit.

An electron is surrounded by a magnetic field.Wherever an electron is present there is also themagnetic field. The more electrons, the greaterthe magnetic field in the circuit. The greater themagnetic field in the circuit, the greater the abilityto attract or repel other magnets or ferrometallicobjects.

Current is measured in amperes and is knownmathematically as a quantity of electrons passing aspecific point in a circuit in a given time period(coulomb per second).

Voltage is the force that allows the electron tobe available to be attracted to the positive ion. Ini-tially, when the electrically neutral atom was excited,a difference in potential was created. This producednegative electrons at one terminal and positive ionsat the other terminal. The greater the difference inpotential, the greater the number of electronsgathered at one terminal and positive ions at theother terminal. The greater this difference, thegreater the potential to do work as the electrons movethroughout the circuit carrying their magnetic field.As long as an imbalance at the terminals resultsfrom the exciting of atoms artificially, there will bea difference in potential, which is another term forvoltage. The greater the difference in potential,the greater the voltage. The greater the negativeand positive attraction, the greater the force toattract electrons back to the positive ions seekingequilibrium.

All the electrons (current) will move throughthe circuit at once, unless impeded or slowed downby some outside force. Wire size or an electrical lightfilament will restrict or resist the flow of the electronsreturning to the positive ions. Everything thatprevents or resists the maximum flow of electrons intheir natural desire to seek out their positive ions iscalled resistance. If there is no resistance, a shortcircuit, which is a very dangerous condition, exists.

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MATTER

Matter is anything that occupies space. Examplesof matter are air, water, automobiles, clothing, andeven our own bodies. Matter can be found in any oneof three states: solid, liquid, and gaseous.

Subatomic particles are the building blocks ofall matter. Even though these particles cannot bemeasured by the usual mechanical tools, they arenonetheless matter. Over 99 percent of the matter inthe universe is subatomic material called plasma.Plasma exists throughout the universe as interstellargases and stars. Plasma is a kind of subatomicparticle soup. Plasma exists on earth only in smallquantities. It is seen in the form of the AuroraBorealis, inside neon lamps, lightning bolts, andelectricity. Plasma is a collection of positive andnegative charges, about equal in number or densityand forming a neutral charge (distribution) of matter.Plasma is considered the fourth state of matter.

Elements and Compounds

An element is a substance that cannot bereduced to a simpler substance by chemical means.It is composed of only one type of atom. Someexamples are iron, gold, silver, copper, and oxygen.Now more than 100 elements are known. All sub-stances are composed of one or more of theseelements.

When two or more elements are chemicallycombined, the resulting substance is a compound. Acompound is a chemical combination of elementsthat can be separated by chemical but not by physicalmeans. Examples of common compounds are water(hydrogen and oxygen) and table salt (sodium andchlorine). A mixture is a combination of elementsand/or compounds, not chemically combined, thatcan be separated by physical means. Examples ofmixtures are air, which is made up of nitrogen,oxygen, carbon dioxide, and small amounts of severalrare gases, and sea water, which consists chiefly ofsalts and water.

Atoms and Molecules

An atom is the smallest particle of an elementthat retains the characteristics of that element. Theatoms of one element differ from the atoms of allother elements. Since more than 100 elements are

known, there must be more than 100 different atoms,or a different atom for each element. Just asthousands of words can be made by combining theproper letters of the alphabet, so thousands of dif-ferent materials can be made by chemically combin-ing the proper atoms.

Any particle that is a chemical combination oftwo or more atoms is a molecule. In a compound, themolecule is the smallest particle that has all the char-acteristics of that compound. Water, for example, isa compound made up of two atoms of hydrogen andone atom of oxygen. It maybe chemically or electri-cally divided into its separate atoms, but it cannot bedivided by physical means.

The electrons, protons, and neutrons of oneelement are identical to those of any other element.However, the number and arrangement of electronsand protons within the atom are different for eachelement.

The electron is a small negative charge ofelectricity. The proton has a positive chargeequal and opposite to the electron. Scientistshave measured the mass and size of the electronand proton and found the mass of the proton isapproximately 1,837 times that of the electron. In thenucleus is a neutral particle called the neutron. Aneutron has a mass approximately equal to that of aproton, but with no electrical charge. According toa popular theory, the electrons, protons, andneutrons of the atoms are arranged like a miniaturesolar system. The protons and neutrons form theheavy nucleus with a positive charge around whichthe very light electrons revolve.

Figure 2-1 is a theoretical representation of onehydrogen and one helium atom. Each has a relativelysimple structure. The hydrogen atom has only oneproton in the nucleus with one electron rotatingaround it. The helium atom has a nucleus made upof two protons and two neutrons, with two electronsrotating outside the nucleus. Elements are classifiednumerically according to the complexity of theiratoms. The number of protons in the atoms nucleusdetermines its atomic number.

Individually, an atom contains an equal numberof protons and electrons. An atom of hydrogen,which contains one proton and one electron, has anatomic number of 1. Helium, with two protons andtwo electrons, has an atomic number of 2. The

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complexity of atomic structure increases with thenumber of protons and electrons.

ENERGY LEVELS

Since an electron in an atom has both mass andmotion, it contains two types of energy. By virtue ofits motion, the electron contains kinetic energy. Dueto its position, it also contains potential energy. Thetotal energy contained by an electron (kinetic pluspotential) is the factor that determines the radius ofthe electron orbit. To keep this orbit, an electronmust neither gain nor lose energy.

Light is a form of energy, but the physicalform in which this energy exists is not known. Oneaccepted theory proposes the existence of light astiny packets of energy called photons. Photons cancontain various quantities of energy. The amountdepends upon the color of the light involved. If aphoton of sufficient energy collides with an orbitalelectron, the electron absorbs the photons energy(Figure 2-2). The electron, which now has a greaterthan normal amount of energy, will jump to a neworbit farther from the nucleus. The first new orbit towhich the electron can jump has a radius four timesthe radius of the original orbit. Had the electronreceived a greater amount of energy, the next pos-sible orbit to which it could jump would have a radiusnine times the original. Thus, each orbit representsone of a large number of energy levels that theelectron may attain. However, the electron cannotjump to just any orbit. The electron will remain in itslowest orbit until a sufficient amount of energy isavailable, at which time the electron will accept theenergy and jump to one of a series of permissible

orbits. An electron cannot exist in the space betweenenergy levels. This indicates that the electron will notaccept a photon of energy unless it contains enoughenergy to elevate itself to one of the higher energylevels. Heat energy and collisions with other par-ticles can also cause the electron to jump orbits.

Once the electron is elevated to an energy levelhigher than the lowest possible energy level, the atomis in an excited state. The electron remains in thisexcited condition for only a fraction of a secondbefore it radiates the excess energy and returns to alower energy orbit.

To illustrate this principle, assume that a nor-mal electron has just received a photon of energysufficient to raise it from the first to the third energylevel. In a short period of time, the electron mayjump back to the first level and emit a new photonidentical to the one it received. Another alternativewould be for the electron to return to the lower levelin two jumps: from the third to the second, and thenfrom the second to the first. In this case, the electronwould emit two photons, one for each jump. Each ofthese photons would have less energy than theoriginal photon which excited the electron.

This principle is used in the fluorescent lightwhere ultraviolet light photons, invisible to thehuman eye, bombard a phosphor coating on theinside of a glass tube. When the phosphor electronsreturn to their normal orbits, they emit photons oflight that are visible. By using the proper chemicalsfor the phosphor coating, any color of light, includingwhite, may be obtained.

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These basic principles apply equally to theatoms of more complex elements. In atoms contain-ing two or more electrons, the electrons interact witheach other and the exact path of any one electron isvery difficult to predict. However, each electron liesin a specific energy band and the orbits are con-sidered as an average of the electrons position.

SHELLS AND SUBSHELLS

The difference between the chemical activityand stability of atoms depends on the number andposition of the electrons within the atom. In general,the electrons reside in groups of orbits called shells.These shells are elliptically shaped and are assumedto be located at fixed intervals. Thus, the shells arearranged in steps that correspond to freed energylevels. The shells and the number of electrons re-quired to fill them maybe predicted by the employ-ment of Paulis exclusion principle. This principlespecifies that each shell will contain a maximum of 2nsquared electrons, where n corresponds to the shellnumber starting with the one closest to the nucleus.By this principle the second shell, for example,would contain 2(22) or 8 electrons when full.

In addition to being numbered, the shells aregiven letter designations (Figure 2-3). Starting withthe shell closest to the nucleus and progressing out-ward, the shells are labeled K, L, M, N, O, P, and Q,respectively. The shells are considered full or com-plete when they contain the following quantities ofelectrons: 2 in the K shell, 8 in the L shell, 18 in theM shell, and so on, in accordance with the exclusionprinciple. Each of these shells is a major shell andcan be divided into four subshells, labeled s, p, d, andf. Like the major shells, the subshells are limited asto the number of electrons they can contain. Thus,the s subshell is complete when it contains

2 electrons, the p subshell when it contains 6, the dsubshell when it contains 10, and the f subshell whenit contains 14 electrons.

Since the K shell can contain no more than twoelectrons, it must have only one subshell, the s sub-shell. The M shell has three subshells: s, p, and d.Adding together the electrons in the s, p, and dsubshells equals 18, the exact number required to fillthe M shell. Figure 2-4 shows the electron configura-tion for copper. The copper atom contains 29electrons, which completely fill the first three shellsand subshells, leaving one electron in thes subshellof the N shell.

VALENCE

The number of electrons in the outermost shelldetermines the valence of an atom. The outer shellof an atom is called the valence shell and its electronsare called valence electrons. The valence of an atom

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determines its ability to gain or lose an electron,which in turn determines the chemical and electricalproperties of the atom. An atom lacking only one ortwo electrons from its outer shell will easily gainelectrons to complete its shell. However, a largeamount of energy is required to free any of itselectrons. An atom with a relatively small number ofelectrons in its shell compared to the number ofelectrons required to fill the shell will easily lose thesevalence electrons.

IONIZATION

For an atom to lose or gain an electron, it mustbe ionized. For ionization to take place, the internalenergy of the atom must be changed by a transfer ofenergy. An atom with more than its normal amountof electrons acquires a negative charge and is calleda negative ion. The atom that gives up some of itsnormal electrons is left with less negative chargesthan positive charges and is called a positive ion.Thus, ionization is the process by which an atom losesor gains electrons.

CONDUCTORS, SEMICONDUCTORS, ANDINSULATORS

Since every electrical device is constructedof parts made from ordinary matter, the effectsof electricity on matter must be well understood.Depending on their ability to conduct an electriccurrent, all elements of matter fit into one ofthree categories: conductors, semiconductors,and insulators. Conductors are elements that trans-fer electrons very readily. Insulators have an ex-tremely high resistance to the flow of electrons. Allmaterial between these two extremes is referred to asa semiconductor.

The electron theory states that all matter iscomposed of atoms and the atoms are composed ofsmaller particles called protons, electrons, andneutrons. The electrons orbit the nucleus, whichcontains the protons and neutrons. Electricity ismost concerned with the valence electrons. Theseelectrons break loose from their parent atom theeasiest. Normally, conductors have no more thanthree valence electrons; insulators have five or more,and semiconductors have four.

The electrical conductivity of matter dependson the atomic structure of the material from which

the conductor is made. In any solid material, such ascopper, the atoms that make up the molecular struc-ture are bound firmly together. At room tempera-ture, copper contains a large amount of heat energy.Since heat energy is one method of removingelectrons from their orbits, copper contains manyfree electrons that can move from atom to atom.When not under the influence of an external force,these electrons move in a haphazard manner withinthe conductor. This movement is equal in all direc-tions so that electrons are not lost or gained by anypart of the conductor. When controlled by an exter-nal force, the electrons move generally in the samedirection. The effect of this movement is felt almostinstantly from one end of the conductor to the other.This electron movement is called an electric current.

Some metals are better conductors ofelectricity than others. Silver, copper, gold, andaluminum exchange valence electrons readily andmake good conductors. Silver is the best conductor,followed by copper, gold, and aluminum. Copper isused more often than silver because of cost.Aluminum is used where weight is a major considera-tion, such as in high-tension power lines with longspans between supports. Gold is used where oxida-tion or corrosion is a consideration and good conduc-tivity is required. The ability of a conductor to handlecurrent also depends on its physical dimensions.Conductors are usually found in the form of wire, butmay be bars, tubes, or sheets.

Nonconductors fail to exchange valenceelectrons because their outer shells are com-pleted with tightly bound valence electrons oftheir own. These materials are called insulators.Some examples of these materials are rubber, plas-tic, enamel, glass, dry wood, and mica. Just as thereis no perfect conductor, neither is there a perfectinsulator.

Some materials are neither good conductorsnor good insulators, since their electrical charac-teristics fall between those of conductors andinsulators. These in-between materials are semicon-ductors. Germanium and silicon are two commonsemiconductors used in solid-state devices.

ELECTROSTATICS

Electrostatics is electricity at rest. An exampleof an effect of electrostatics is the way a persons hairstands on end after a vigorous rubbing. Studying

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electrostatics provides important backgroundknowledge for developing concepts essential to under-standing electricity and electronics.

When an amber rod is rubbed with fur, the rodattracts some very light objects such as bits of paperand shavings of wood. Other substances possessqualities of attraction similar to amber. Among theseare glass, when rubbed with silk, and ebonite, whenrubbed with fur. All the substances with propertiessimilar to those of amber are called electrics, a wordof Greek origin meaning amber. A substance such asamber or glass when given a vigorous rubbing iselectrified or charged with electricity.

When a glass rod is rubbed with fur, both theglass rod and the fur become electrified. Certainsubstances attracted to the glass rod are repelled bythe fur and vice versa. There are two opposite kindsof electricity positive and negative. The chargeproduced on a glass rod when it is rubbed with silk ispositive. The charge produced on the silk is negative.Those bodies that are not electrified or charged areneutral.

STATIC ELECTRICITY

In a natural or neutral state, each atom in abody of matter has the proper number of electrons inorbit around it. Thus, the whole body of mattercomposed of the neutral atoms is also electricallyneutral. In this state, it has zero net charge.Electrons will neither leave nor enter the neutrallycharged body if it comes in contact with other neutralbodies. If, however, any electrons are removed fromthe atoms of a body of matter, more protons thanelectrons will remain and the whole body of matterwill become electrically positive. If the positivelycharged body comes in contact with a body having anormal charge or a negative (too many electrons)charge, an electric current will flow between them.Electrons will leave the more negative body and enterthe positive body. This electron flow will continueuntil both bodies have equal charges. When twobodies of matter with unequal charges are nearone another, an electric force is exerted betweenthem. However, since they are not in contact,their charges cannot equalize. Such an electricforce, where current cannot flow, is called staticelectricity. (Static in this instance means notmoving.) It is also referred to as an electrostaticforce.

One of the easiest ways to create a static chargeis by friction. When two pieces of matter are rubbedtogether, electrons can be wiped off one materialonto the other. If both materials are good conduc-tors, it is hard to obtain a detectable charge on eithersince equalizing currents can flow easily between theconducting materials. These currents equalize thecharges almost as fast as they are created. A staticcharge is more easily created between nonconduct-ing materials. When a hard rubber rod is rubbed withfur, the rod will accumulate electrons given up by thefur (Figure 2-5). Since both materials are poor con-ductors, very little equalizing current can flow and anelectrostatic charge builds up. When the charge be-comes great enough, current will flow regardless ofthe poor conductivity of the materials. These cur-rents cause visible sparks and produce a cracklingsound.

NATURE OF CHARGES

When in a natural or neutral state, an atom hasan equal number of electrons and protons. Becauseof this balance, the net negative charge of theelectrons in orbit is exactly balanced by the net posi-tive charge of the protons in the nucleus, making theatom electrically neutral.

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An atom becomes a positive ion whenever itloses an electron and has an overall positive charge.Conversely, whenever an atom acquires an extraelectron, it becomes a negative ion and has a negativecharge.

Due to normal molecular activity, ions are al-ways present in any material. If the number of posi-tive ions and negative ions is equal, the material iselectrically neutral. When the number of positiveions exceeds the number of negative ions, thematerial is positively charged. The material is nega-tively charged whenever the negative ions outnumberthe positive ions.

Since ions are actually atoms without their nor-mal number of electrons, the excess or lack ofelectrons in a substance determines its charge. Inmost solids, the transfer of charges is by movementof electrons rather than ions. The transfer of chargesby ions is more significant when considering theelectrical activity in liquids and gases.

CHARGED BODIES

A fundamental law of electricity is that likecharges repel each other and unlike charges attracteach other. A positive charge and negative charge,being unlike, tend to move toward each other. In theatom, the negative electrons are drawn toward thepositive protons in the nucleus. This attractive forceis balanced by the electrons centrifugal force causedby its rotation about the nucleus. As a result, theelectrons remain in orbit and are not drawn into thenucleus. Electrons repel each other because of theirlike negative charges. Protons repel each otherbecause of their like positive charges.

A simple experiment demonstrates the law ofcharged bodies. Suspend two pith (paper pulp) ballsnear one another by threads (Figure 2-6). Rub a hardrubber rod with fur to give it a negative charge. Thenhold it against the right-handball (view A). The rodwill give off a negative charge to the ball. The right-hand ball has a negative charge with respect to theleft-hand ball. Release the two balls. They will bedrawn together (view A). They Will touch and remainin contact until the left-hand ball gains a portion ofthe negative charge of the right-handball. Then theywill swing apart. If a positive or a negative charge isplaced on both balls (views B and C), the balls willrepel each other.

COULOMBS LAW OF CHARGES

A French scientist named Charles Coulombfirst discovered the relationship between attractingor repelling charged bodies. Coulombs Law statesthat charged bodies attract or repel each other witha force that is directly proportional to the productof their individual charges and is inversely propor-tional to the square of the distance between them.The strength of the attracting or repelling forcebetween two electrically charged bodies in freespace depends on two things: their charges and thedistance between them.

ELECTRIC FIELDS

The space between and around charged bodiesin which their influence is felt is an electric field offorce. It can exist in air, glass, paper, or a vacuum.Electrostatic fields and dielectric fields are othernames for this region of force.

Fields of force spread out in the space sur-rounding their point of origin. They generallydiminish in proportion to the square of the distancefrom their source.

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The field about a charged body is normallyrepresented by lines called electrostatic lines offorce. These imaginary lines represent the directionand strength of the field. To avoid confusion, thelines of force exerted by a positive charge are alwaysshown leaving the charge. For a negative charge theyare shown entering. Figure 2-7 shows these lines torepresent the field about charged bodies. View Ashows the repulsion of like-charged bodies and theirassociated fields. View B shows the attraction ofunlike-charged bodies and their associated fields.

MAGNETISM

To understand the principles of electricity, it isnecessary to study magnetism and the effects of mag-netism on electrical equipment. Magnetism andelectricity are so closely related that the study ofeither subject would be incomplete without at least abasic knowledge of the other.

Much of todays modern electrical andelectronic equipment could not function withoutmagnetism. Modern computers, tape recorders, andvideo reproduction equipment use magnetized tape.High-fidelity speakers use magnets to convertamplifier outputs into audible sound. Electricalmotors use magnets to convert electrical energy intomechanical motion. Generators use magnets to con-vert mechanical motion into electrical energy.

Magnetic Materials

Magnetism is generally defined as that propertyof material which enables it to attract pieces of iron.

A material possessing this property is a magnet. Theword originated with the ancient Greeks who foundstones with this characteristic. Materials that areattracted by a magnet, such as iron, steel, nickel, andcobalt, can become magnetized. These are calledmagnetic materials. Materials, such as paper,wood, glass, or tin, which are not attracted by mag-nets, are nonmagnetic. Nonmagnetic materials can-not become magnetized.

The most important materials connected withelectricity and electronics are the ferromagneticmaterials. Ferromagnetic materials are relativelyeasy to magnetize. They include iron, steel, cobalt,and the alloys Alnico and Permalloy. (An alloy ismade by combining two or more elements, one ofwhich must be a metal.) These new alloys can be verystrongly magnetized. They can obtain a magneticstrength great enough to lift 500 times their ownweight.

Natural Magnets

Magnetic stones such as those found by theancient Greeks are natural magnets. These stonescan attract small pieces of iron in a manner similar tothe magnets common today. However, the magneticproperties attributed to the stones are products ofnature and not the result of the efforts of man. TheGreeks called these substances magnetite.

The Chinese are said to have been aware ofsome of the effects of magnetism as early as 2600 B.C.They observed that stones similar to magnetite, whenfreely suspended, had a tendency to assume a nearlynorth and south direction. Because of the directionalquality of these stones, they are referred to as lode-stones or leading stones.

Natural magnets, found in the United States,Norway, and Sweden, no longer have any practicaluse. It is now possible to easily produce more power-ful magnets.

Artificial Magnets

Magnets produced from magnetic materialsare called artificial magnets. They can be made in avariety of shapes and sizes and are used extensivelyin electrical apparatus. Artificial magnets aregenerally made from special iron or steel alloys whichare usually magnetized electrically. The material tobe magnetized is inserted into a coil of insulated wire.

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A heavy flow of electrons is produced by stroking amagnetic material with magnetite or with anotherartificial magnet. The forces causing magnetizationare represented by magnetic lines of force, verysimilar in nature to electrostatic lines of force.

Artificial magnets are usually classified as per-manent or temporary, depending on their ability toretain their magnetic properties after the magnetiz-ing force has been removed. Magnets made fromsubstances, such as hardened steel and certain alloyswhich retain a great deal of their magnetism, arecalled permanent magnets. These materials are rela-tively difficult to magnetize because of the oppositionoffered to the magnetic lines of force as the lines offorce try to distribute themselves throughout thematerial. The opposition is called reluctance. Allpermanent magnets are produced from materialshaving a high reluctance.

A material with a low reluctance, such as softiron or annealed silicon steel, is relatively easy tomagnetize. However, it retains only a small part ofits magnetism once the magnetizing force is removed.Materials that easily lose most of their magneticstrength are called temporary magnets. The amountof magnetism that remains in a temporary magnet isreferred to as its residual magnetism. The ability ofa material to retain an amount of residual magnetismis called the retentivity of the material.

The difference between a permanent and tem-porary magnet is indicated in terms of reluctance. Apermanent magnet has a high reluctance, and a tem-porary magnet has a low reluctance. Magnets arealso described in terms of the permeability of theirmaterials or the ease with which magnetic lines offorce distribute themselves throughout the material.A permanent magnet, produced from a material witha high reluctance, has a low permeability. A tem-porary magnet, produced from a material with a lowreluctance, has a high permeability.

Magnetic Poles

The magnetic force surrounding a magnet isnot uniform. There is a great concentration of forceat each end of the magnet and a very weak force atthe center. To prove this fact, dip a magnet into irontilings (Figure 2-8). Many filings will cling to the endsof the magnet, while very few adhere to the center.The two ends, which are the regions of concentratedlines of force, are called the poles of the magnet.

Magnets have two magnetic poles, and both poleshave equal magnetic strength.

Law of Magnetic Poles. To demonstrate thelaw of magnetic poles, suspend a bar magnet freelyon a string (Figure 2-9). It will align itself in a northand south direction. Repeat this experiment. Thesame pole of the magnet will always swing toward thenorth geographical pole of the earth. Therefore, itis called the north-seeking pole or simply the northpole. The other pole of the magnet is the south-seeking pole or the south pole.

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A practical use of the directional characteristicof the magnet is the compass. The compass has afreely rotating magnetized needle indicator thatpoints toward the North Pole. The poles of asuspended magnet always move to a definite position.This indicates opposite magnetic polarity exists.

The law of electricity regarding the attractionand repulsion of charged bodies may also be appliedto magnetism if the pole is considered as a charge.The north pole of a magnet will always be attractedto the south pole of another magnet and will show arepulsion to another north pole. The law of magneticpoles is that like poles repel and unlike poles attract.

The Earths Magnetic Poles. The fact that acompass needle always aligns itself in a particulardirection, regardless of its location on earth, indi-cates that the earth is a huge natural magnet. Thedistribution of the magnetic force about the earth isthe same as that which might be produced by a giantbar magnet running through the center of the earth(Figure 2-10). The magnetic axis of the earth is about15 degrees from its geographical axis, thereby locat-ing the magnetic poles some distance from thegeographical poles. The ability of the north pole ofthe compass needle to point toward the north

magnets lined up so that the north pole of eachmolecule points in one direction and the south polefaces the opposite direction. A material with itsmolecules thus aligned will then have one effectivenorth pole and one effective south pole. Figure 2-11illustrates Webers Theory. When a steel bar isstroked several times in the same direction by amagnet, the magnetic force from the north pole of themagnet causes the molecules to align themselves.

geographical pole is due to the presence of the mag-netic pole nearby. This magnetic pole of the earth ispopularly considered the magnetic north pole. How-ever, it actually must have the polarity of magnetssouth pole since it attracts the north pole of a com-pass needle. The reason for this conflict in terminol-ogy can be traced to the early users of the compass.Because they did not know that opposite magneticpoles attract, they called the end of the compassneedle that pointed toward the north geographicalpole the north pole of a compass needle. However,the north pole of a compass needle (a small barmagnet) can be attracted only by an unlike magneticpole, a pole with the same magnetic polarity as thesouth pole of a magnet.

Theories of Magnetism

Webers Theory. A popular theory of mag-netism considers the molecular alignment of thematerial. This is known as Webers Theory. Thistheory assumes that all magnetic substances are com-posed of tiny molecular magnets. Any magnetizedmaterial has the magnetic forces of its molecularmagnets, thereby eliminating any magnetic effect. Amagnetized material will have most of its molecular

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Domain Theory. A more modern theory ofmagnetism is based on the electron spin principle.All matter is composed of vast quantities of atoms,each atom containing one or more orbital electron.The electrons are considered to orbit in various shellsand subshells depending on their distance from thenucleus. The structure of the atom has previouslybeen compared to the solar system. The electronsorbiting the nucleus correspond to the planets orbit-ing the sun. Along with its orbital motion about thesun, each planet also revolves on its axis. It is believedthat the electron also revolves on its axis as it orbitsthe nucleus of an atom.

An electron has a magnetic field about it alongwith an electric field. The number of electrons spin-ning in each direction determines the effectiveness ofthe magnetic field of an atom. If an atom has equalnumbers of electrons spinning in opposite directions,the magnetic fields surrounding the electrons cancelone another and the atom is unmagnetized. How-ever, if more electrons spin in one direction thananother, the atom is magnetized. An atom with anatomic number of 26, such as iron, has 26 protons inthe nucleus and 26 revolving electrons orbiting itsnucleus. If 13 electrons are spinning in a clockwisedirection and 13 electrons are spinning in acounterclockwise direction, the opposing magneticfields will be neutralized. When more than 13electrons spin in either direction, the atom is mag-netized. Figure 2-12 shows an example of a mag-netized atom of iron.

Magnetic Fields

The space surrounding a magnet where mag-netic forces act is the magnetic field. Magnetic forceshave a pattern of directional force observed by per-forming an experiment with iron filings. Place apiece of glass over a bar magnet. Then sprinkle ironfilings on the surface of the glass. The magnetizingforce of the magnet will be felt through the glass, andeach iron filing becomes a temporary magnet. Tapthe glass gently. The iron particles will align them-selves with the magnetic field surrounding the mag-net just as the compass needle did previously. Thefilings form a definite pattern, which is a visible rep-resentation of the forces comprising the magneticfield. The arrangements of iron filings in Figure 2-13indicate that the magnetic field is very strong at thepoles and weakens as the distance from the polesincreases. They also show that the magnetic fieldextends from one pole to the other in a loop aroundthe magnet.

Lines of Force

To further describe and work with magneticphenomena, lines are used to represent the force exist-ing in the area surrounding a magnet (Figure 2-14).These magnetic lines of force are imaginarylines used to illustrate and describe the pattern ofthe magnetic field. The magnetic lines of force areassumed to emanate from the north pole of a magnet,pass through the surrounding space, and enter the

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south pole. They then travel inside the magnet fromthe south pole to the north pole, thus completing aclosed loop.

When two magnetic poles are brought closetogether, the mutual attraction or repulsion of thepoles produces a more complicated pattern than thatof a single magnet. These magnetic lines of force canbe plotted by placing a compass at various pointsthroughout the magnetic field, or they can be roughlyillustrated using iron filings as before. Figure 2-15shows a diagram of magnetic poles placed closetogether.

Although magnetic lines of force are imagi-nary a simplified version of many magneticphenomena can be explained by assuming they havecertain real properties. The lines of force are similarto rubber bands which stretch outward when a forceis exerted on them and contract when the force isremoved. Characteristics of magnetic lines of forceare as follows:

Magnetic lines of force are continuous andwill always form closed loops.

Magnetic lines of force will never cross oneanother.

Parallel magnetic lines of force traveling inthe same direction repel one another.Parallel magnetic lines of force traveling inopposite directions extend to unite witheach other and form single lines travelingin a direction determined by the magneticpoles creating the limes of force.

Magnetic lines of force tend to shortenthemselves. Therefore, the magnetic linesof force existing between two unlike polescause the poles to be pulled together.

Magnetic lines of force pass throughall materials, both magnetic andnonmagnetic.

Magnetic lines of force always enter orleave a magnetic material at right angles tothe surface.

Magnetic Effects

Magnetic Flux. The total number of magneticlines of force leaving or entering the pole of a magnetis called magnetic flux. The number of flux lines perunit area is called flux density.

Field Intensity. The intensity of a magneticfield is directly related to the magnetic force exertedby the field.

Attraction/Repulsion. The intensity of attrac-tion or repulsion between magnetic poles may bedescribed by a law almost identical to Coulombs Lawof Charged Bodies. The force between two poles isdirectly proportional to the product of the polestrengths and inversely proportional to the square ofthe distance between the poles.

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Magnetic Induction

All substances that are attracted by a magnetcan become magnetized. The fact that a materialis attracted by a magnet indicates the material mustitself be a magnet at the time of attraction. Know-ing about magnetic fields and magnetic lines offorce simplifies the understanding of how amaterial becomes magnetized when brought neara magnet. As an iron nail is brought close to a barmagnet (Figure 2-16), some flux lines emanatingfrom the north pole of the magnet pass through theiron nail in completing their magnetic path. Sincemagnetic lines of force travel inside a magnet fromthe south pole to the north pole, the nail will bemagnetized so its south pole will be adjacent to thenorth pole of the bar magnet.

If another nail is brought in contact with theend of the first nail, it is magnetized by induction.This process can be repeated until the strength of themagnetic flux weakens as distance from the bar mag-net increases. However, as soon as the first iron nailis pulled away from the bar magnet, all the nails willfall. Each nail had become a temporary magnet, butonce the magnetizing force was removed, the nailsdomains once again assumed a random distribution.

Magnetic induction always produces a polepolarity on the material being magnetized oppositethat of the adjacent pole of the magnetizing force. Itis sometimes possible to bring a weak north pole of a

magnet near a strong magnet north pole and noteattraction between the poles. The weak magnet,when placed within the magnetic field of the strongmagnet, has its magnetic polarity reversed by the fieldof the stronger magnet. Therefore, it is attracted tothe opposite pole. For this reason, keep a very weakmagnet, such as a compass needle, away from a verystrong magnet.

Magnetism can be induced in a magneticmaterial by several means. The magnetic materialmay be placed in the magnetic field, brought intocontact with a magnet, or stroked by a magnet. Strok-ing and contact both indicate actual contact with thematerial but are considered in magnetic studies asmagnetizing by induction.

Magnetic Shielding

Magnetic flux has no known insulator. If anonmagnetic material is placed in a magnetic field,there is no appreciable change in flux. That is,the flux penetrates the nonmagnetic material.For example, a glass plate placed between the polesof a horseshoe magnet will have no appreciable effecton the field, although glass itself is a good insulatorin an electric circuit. If a magnetic material such assoft iron is placed in a magnetic field, the flux may beredirected to take advantage of the greater per-meability of the magnetic material (Figure 2-17).Permeability is the quality of a substance that deter-mines the ease with which it can be magnetized.

Stray magnetic fields can influence the sensi-tive mechanisms of electric instruments and meterscausing errors in their readings. Instrumentmechanisms cannot be insulated against magneticflux. Therefore, the flux must be directed around theinstrument by placing a soft-iron case, called a mag-netic screen or magnetic shield, about the instru-ment. Because the flux is established more readilythrough the iron (even though the path is larger) thanthrough the air inside the case, the instrument iseffectively shielded. Figure 2-18 shows a soft ironmagnetic shield around a watch.

Magnetic Shapes

Because of their many uses, magnets are foundin various shapes and sizes. However, they usuallycome under one of three general classifications: bar,ring, or horseshoe magnets.

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The bar magnet is most often used in schoolsand laboratories for studying the properties andeffects of magnetism. The bar magnet helpeddemonstrate magnetic effects in Figure 2-14.

The ring magnet is used for computer memorycores. A common application for a temporary ringmagnet is the shielding of electrical instruments.

The horseshoe magnet is most frequently usedin electrical and electronic equipment. A horseshoemagnet is similar to a bar magnet but is bent in theshape of a horseshoe. The horseshoe magnet is mag-netically stronger than a bar magnet of the same sizeand material because the magnetic poles are closertogether. The magnetic strength from one pole to theother is greatly increased because the magnetic fieldis concentrated in a smaller area. Electrical measur-ing devices often use horseshoe magnets.

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Care of Magnets

A piece of steel that has been magnetized canlose much of its magnetism by improper handling. Ifit is jarred or heated, its domains will be misaligned,and it loses some of its effective magnetism. If thispiece of steel formed the horseshoe magnet of ameter, the meter would no longer operate or wouldgive inaccurate readings. Therefore, be careful whenhandling instruments containing magnets. Severejarring or subjecting the instrument to high tempera-tures will damage the device.

A magnet may also become weakened fromloss of flux. When storing magnets, always try toavoid excess leakage of magnetic flux. Always storea horseshoe magnet with a keeper, a soft iron barused to join the magnetic poles. By storing the mag-net with a keeper, the magnetic flux continuouslycirculates through the magnet and does not leak offinto space.

When storing bar magnets, follow the sameprinciple. Always store bar magnets in pairs with anorth pole and a south pole placed together. Thisprovides a complete path for the magnetic fluxwithout any flux leakage.

ENERGY AND WORK

In the field of physical science, work is definedas the product of force and displacement. That is,the force applied to move an object and the distancethe object is moved are the factors of work per-formed. No work is accomplished unless the forceapplied causes a change in position of a stationaryobject or a change in the velocity of a moving object.For example, a worker may tire by pushing against aheavy wooden crate, but unless the crate moves, nowork will be accomplished.

In the study of energy and work, energy isdefined as the ability to do work. To perform anykind of work, energy must be expended (convertedfrom one form to another). Energy supplies therequired force or power whenever any work isaccomplished.

One form of energy is that contained by anobject in motion. When a hammer is set in motion inthe direction of a nail, it possesses energy of motion.As the hammer strikes the nail, the energy of motionis converted into work as the nail is driven into the

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wood. The distance the nail is driven into the wooddepends on the velocity of the hammer at the time itstrikes the nail. Energy contained in an object due toits motion is called kinetic energy.

If a hammer is suspended one meter above anail by a string, gravity will pull the hammerdownward. If the string is suddenly cut, the force ofgravity will pull the hammer down against the nail,driving it into the wood. While the hammer issuspended above the nail, it has the ability to do workbecause of its elevated position in the earths gravita-tional field. Since energy is the ability to do work, thehammer contains energy.

Energy contained in an object because of itsposition is called potential energy. The amount ofpotential energy available equals the product of theforce required to elevate the hammer and the heightto which it is elevated.

Another example of potential energy is thatcontained in a tightly coiled spring. The amount ofenergy released when the spring unwinds depends onthe amount of force required to wind the springinitially.

ELECTRICAL CHARGES

The study of electrostatics shows that a field offorce exists in the space surrounding any electricalcharge. The strength of the field depends directly onthe force of the charge.

The charge of one electron might be used asa unit of electrical charge since displacingelectrons creates charges. However, the charge ofone electron is so small that it is impractical to use.The practical unit adopted for measuring chargesis the coulomb, named after the scientist CharlesCoulomb. A coulomb equals the charge6,242,000,000,000,000,000 (six quintillion, twohundred forty-two quadrillion or 6.242 times 10 to the18th power) electrons.

When a charge of 1 coulomb exists between twobodies, one unit of electrical potential energy exists.This difference in potential between the two bodiesis called electromotive force (EMF) or voltage. Theunit of measure is the volt.

Electrical charges are created by the displace-ment of electrons, so that there is an excess of

electrons at one point and a deficiency at anotherpoint. Therefore, a charge must always have either anegative or positive polarity. A body with an excessof electrons is negative; a body with a deficiency ofelectrons is positive.

A difference in potential can exist between twopoints or bodies only if they have different charges.In other words, there is no difference in potentialbetween two bodies if both have a deficiency ofelectrons to the same degree. If, however, one bodyis deficient by 6 coulombs (6 volts) and the other isdeficient by 12 coulombs (12 volts), the difference inpotential is 6 volts. The body with the greaterdeficiency is positive with respect to the other.

In most electrical circuits only the differencein potential between two points is important. Theabsolute potentials of the points are of little concern.Often it is convenient to use one standard referencefor all of the various potentials throughout a piece ofequipment. For this reason, the potentials at variouspoints in a circuit are generally measured withrespect to the metal chassis on which all parts of thecircuit are mounted. The chassis is considered to beat zero potential and all other potentials are eitherpositive or negative with respect to the chassis. Whenused as the reference point, the chassis is said to beat ground potential.

Sometimes rather large values of voltage maybe encountered and the volt becomes too small a unitfor convenience. In this situation, the kilovolt (kV),meaning 1,000 volts, is used. For example, 20,000volts would be written as 20 kV. Sometimes the voltmay be too large a unit when dealing with very smallvoltages. For this purpose, the millivolt (mV),meaning one-thousandth of a volt, and themicrovolt (uV), meaning one-millionth of a volt,are used. For example, 0.001 volt would be writtenas 1 mV, and 0.000025 volt would be written as 25 uV.

When a difference in potential exists betweentwo charged bodies connected by a conductor,electrons will flow along the conductor. This flow isfrom the negatively charged body to the positivelycharged body until the two charges are equalized andthe potential difference no longer exists.

Figure 2-19 shows an analogy of this action inthe two water tanks connected by a pipe and valve.At first, the valve is closed and all the water is in tankA. Thus, the water pressure across the valve is at

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maximum. When the valve is opened, the water flowsthrough the pipe from A to B until the water levelbecomes the same in both tanks. The water thenstops flowing in the pipe because there is no longer adifference in water pressure between the two tanks.

Electron movement through an electric circuitis directly proportional to the difference in potentialor EMF across the circuit, just as the flow of waterthrough the pipe in Figure 2-19 is directly propor-tional to the difference in water level in the two tanks.

A fundamental law of electricity is that theelectron flow is directly proportional to the appliedvoltage. If the voltage is increased, the flow is in-creased. If the voltage is decreased, the flow isdecreased.

VOLTAGE PRODUCTION

It has been demonstrated that a charge can beproduced by rubbing a rubber rod with fur. Becauseof the friction involved, the rod acquires electronsfrom the fur, making it negative. The fur becomespositive due to the loss of electrons. These quantitiesof charge constitute a difference in potential betweenthe rod and the fur. The electrons that make up thisdifference in potential are capable of doing work if adischarge is allowed to occur.

To be a practical source of voltage, the poten-tial difference must not be allowed to dissipate. Itmust be maintained continuously. As one electronleaves the concentration of negative charge, anothermust be immediately provided to take its place or thecharge will eventually diminish to the point where nofurther work can be accomplished. A voltage source,

therefore, is a device that can supply and maintainvoltage while an electrical apparatus is connected toits terminals. The internal action of the source is suchthat electrons are continuously removed from oneterminal to keep it positive and simultaneously sup-plied to the second terminal to keep it negative.

Presently, six methods for producing a voltageor electromotive force are known. Some are morewidely used than others, and some are used mostlyfor specific applications. The six known methods ofproducing a voltage are

Friction. Voltage is produced by rubbingcertain materials together.

Pressure (piezoelectricity). Voltage isproduced by squeezing crystals of certainsubstances.

Heat (thermoelectricity). Voltage isproduced by heating the joint (junction)where two unlike metals are joined.

Light (photoelectricity). Voltage isproduced by light striking photosensitive(light sensitive) substances.

Chemical action. Voltage is produced bychemical reaction in a battery cell.

Magnetism. Voltage is produced in a con-ductor when the conductor moves througha magnetic field, or a magnetic field movesthrough the conductor so that the mag-netic lines of force of the field are cut.

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Voltage Produced by Friction

The first method discovered for creating a volt-age was generation by friction. The development ofcharges by rubbing a rod with fur is a prime exampleof the way friction generates voltage. Because of thenature of the materials producing this voltage, itcannot be conveniently used or maintained. There-fore, this method has very little practical use.

While searching for ways to produce largeramounts of voltage with more practical nature,machines were developed that transferred chargesfrom one terminal to another by rotating glass discsor moving belts. The most notable of these machinesis the Van de Graaff generator. It is used today toproduce potentials in the order of millions of volts fornuclear research. As these machines have little valueoutside the field of research, their theory of operationwill not be described here.

Voltage Produced by Pressure

One specialized method of generating an EMFuses the characteristics of certain ionic crystals suchas quartz, Rochelle salts, and tourmaline. Thesecrystals can generate a voltage whenever stresses areapplied to their surfaces. Thus, if a crystal of quartzis squeezed, charges of opposite polarity appear ontwo opposite surfaces of the crystal. If the force isreversed and the crystal is stretched, charges againappear but are of the opposite polarity from thoseproduced by squeezing. If a crystal of this type isvibrated, it produces a voltage of reversing polaritybetween two of its sides. Quartz or similar crystalscan thus be used to convert mechanical energy intoelectrical energy. Figure 2-20 shows thisphenomenon, called the piezoelectric effect. Someof the common devices that use piezoelectric crystalsare microphones, phonograph cartridges, and oscil-lators used in radio transmitters, radio receivers, andsonar equipment. This method of generating anEMF is not suitable for applications having largevoltage or power requirements. But it is widely usedin sound and communications systems where smallsignal voltages can be effectively used.

Crystals of this type also possess another inter-esting property, the converse piezoelectric effect.They can convert electrical energy into mechanicalenergy. A voltage impressed across the proper sur-faces of the crystal will cause it to expand or contractits surfaces in response to the voltage applied.

(A)(B)(C)(D)

Noncrystallized Structure.Crystallized Structure.Compression of a Crystal.Decompression of a Crystal.

Voltage Produced by Heat

When a length of metal, such as copper, isheated at one end, valence electrons tend to moveaway from the hot end toward the cooler end. Thisis true of most metals. However, in some metals suchas iron, the opposite takes place and electrons tendto move toward the hot end. Figure 2-21 illustratesthese characteristics. The negative charges(electrons) are moving through the copper awayfrom the heat and through the iron toward the heat.They cross from the iron to the copper through thecurrent meter to the iron at the cold junction. Thisdevice is called a thermocouple.

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Thermocouples have a greater power capacitythan crystals, but it is still very small compared tosome other sources. The thermoelectric voltage in athermocouple depends mainly on the difference intemperature between the hot and cold junctions.They are therefore widely used to measure tempera-ture and are used in heat-sensing devices in auto-matic temperature control equipment.Thermocouples generally can be subjected to muchgreater temperatures then ordinary thermometers,such as mercury or alcohol types.

Voltage Produced by Light

When light strikes the surface of a substance, itmay dislodge electrons from their orbits around thesurface atoms of the substance. This occurs becauselight has energy, the same as any moving force. Somesubstances, mostly metallic ones, are far more sensi-tive to light than others. That is, more electrons aredislodged and emitted from the surface of a highlysensitive metal, with a given amount of light, than areemitted from a less sensitive substance. Upon losingelectrons, the photosensitive (light-sensitive) metalbecomes positively charged, and an electric force iscreated. Voltage produced in this manner is calledphotoelectric voltage.

The photosensitive materials most commonlyused to produce a photoelectric voltage are variouscompounds of silver oxide or copper oxide. A com-plete device which operates with photoelectric volt-age is a photoelectric cell. Many different sizes andtypes of photoelectric cells are in use, and each servesthe special purpose for which it is designed. Nearlyall, however, have some of the basic features of thephotoelectric cells in Figure 2-22.

The cell in view A has a curved, light-sensitivesurface focused on the central anode. When light

from the direction shown strikes the sensitive surface,it emits electrons toward the anode. The moreintense the light, the greater the number of electronsemitted. When a wire is connected between thefilament and the back, or dark side of the cell, theaccumulated electrons will flow to the dark side.These electrons will eventually pass through themetal of the reflector and replace the electrons leav-ing the light-sensitive surface. Thus, light energy isconverted to a flow of electrons, and a usable currentis developed.

The cell in view B is constructed in layers. Abase plate of pure copper is coated with light-sensitive copper oxide. An extremely the semi-transparent layer of metal is placed over the copperoxide. This additional layer serves two purposes:

It permits the penetration of light to thecopper oxide.

It collects the electrons emitted by thecopper oxide.

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An externally connected wire completes theelectron path, the same as in the reflector-type cell.The photocells voltage is used as needed by connect-ing the external wires to some other device, whichamplifies (enlarges) it to a usable level.

The power capacity of a photocell is very small.However, it reacts to light-intensity variations in anextremely short time. This characteristic makes thephotocell very useful in detecting or accurately con-trolling many operations. For instance, thephotoelectric cell, or some form of the photoelectricprinciple, is used in television cameras, automaticmanufacturing process controls, door openers, andburglar alarms.

Voltage Produced by Chemical Action

Voltage maybe produced chemically when cer-tain substances are exposed to chemical action. Iftwo dissimilar substances, usually metals or metallicmaterials, are immersed in a solution that producesa greater chemical action on one substance than onthe other, a difference in potential exists between thetwo. If a conductor is then connected between them,electrons flow through the conductor to equalize thecharge. This arrangement is called a primary cell.The two metallic pieces are electrodes, and the solu-tion is the electrolyte. The voltaic cell in Figure 2-23is a simple example of a primary cell. The differencein potential results from the fact that material fromone or both of the electrodes goes into the electrolyte.In the process, ions form near the electrodes. Due tothe electric field associated with the charged ions, theelectrodes acquire charges. The amount of dif-ference in potential between the electrodes dependsmainly on the metals used.

The two types of primary cells are the wet celland the dry cell. In a wet cell, the electrolyte is aliquid. A cell with a liquid electrolyte must remain inan upright position and is not readily transportable.An automotive battery is an example of this type ofcell. The dry cell is more commonly used than thewet cell. The dry cell is not actually dry, but it con-tains an electrolyte mixed with other materials toform a paste. Flashlights and portable radios arecommonly powered by dry cells.

Batteries are formed when several cells areconnected together to increase electrical output.

Voltage Produced by Magnetism

Magnets or magnetic devices are used forthousands of different jobs. One of the most usefuland widely employed applications of magnets is toproduce vast quantities of electric power from mechani-cal sources. A number of different sources mayprovide the mechanical power, such as gasolineor diesel engines and water or steam turbines.However, the final conversion of these source

energies to electricity is done by generators usingthe principle of electromagnetic induction. Thereare many types and sizes of these generators. Thefundamental operating principle of all electro-magnetic induction generators is discussed below.

Three fundamental conditions must existbefore a voltage can be produced by magnetism:

There must be a conductor in which thevoltage will be produced.

There must be a magnetic field in theconductors vicinity.

There must be relative motion between thefield and conductor. The conductor mustbe moved so it cuts across the magneticlines of force, or the field must be movedso the conductor cuts the lines of force.

When a conductor or conductors move acrossa magnetic field and cut the lines of force, electronswithin the conductor are propelled in one directionor another. This creates an electric force or voltage.

Figure 2-24 shows the three conditions neededto create an induced voltage. There is a magneticfield between the poles of the C-shaped magnet. The

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copper wire is the conductor. The wire is movedback and forth across the magnetic field for relativemotion.

In view A, the conductor moves toward thefront of the page and the electrons move from left toright. The movement of the electrons occurs because

of the magnetically induced EMF acting on theelectrons in the copper. The right-hand end be-comes negative and the left-hand end positive. Theconductor is stopped in view B, and motion iseliminated (one of the three required conditions).Since there is no longer an induced EMF, there is nolonger any difference in potential between the twoends of the wire. In view C, the conductor is movingaway from the front of the page. An induced EMF isagain created. However, the reversal of motion hascaused a reversal of direction in the induced EMF.

If a path for electron flow is provided betweenthe ends of the conductor, electrons will leave thenegative end and flow to the positive end. View Dshows this condition. Electron flow will continue aslong as the EMF exists. Note that the induced EMFin Figure 2-24 could also have been created by hold-ing the conductor stationary and moving the mag-netic field back and forth.

ELECTRIC CURRENT

Electrons move through a conductor inresponse to a magnetic field. Electron current is thedirected flow of electrons. The direction of electronmovement is from a region of negative potential to aregion of positive potential. Therefore, electron cur-rent flow in a material is determined by the polarityof the applied voltage.

Random Drift

All materials are composed of atoms, eachcapable of being ionized. If some form of energy,such as heat, is applied to a material, some electronsacquire enough energy to move to a higher energylevel. As a result, some electrons are freed from theirparent atoms, which then become ions. Other formsof energy, particularly light or an electric field, willalso cause ionization.

The number of free electrons resulting fromionization depends on the quantity of energy appliedto a material and the atomic structure of the material.At room temperature, some materials, classified asconductors, have an abundance of free electrons.Under a similar condition, materials classified asinsulators exchange relatively few free electrons.

In a study of electric current, conductors are ofmajor concern. Conductors consist of atoms withloosely bound electrons in their outer orbits. Due to

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the effects of increased energy, these outermostelectrons frequently break away from their atoms andfreely drift throughout the material. The freeelectrons take an unpredictable path and drift hap-hazardly about the material. This movement is calledrandom drift. Random drift of electrons occurs in allmaterials. The degree of random drift is greater in aconductor than in an insulator.

Directed Drift

Associated with every charged body is anelectrostatic field. Bodies with like charges repel oneanother, and bodies with unlike charges attract eachother. An electron is affectedly an electrostatic fieldin the same manner as any negatively charged body.It is repelled by a negative charge and attracted by apositive charge. If a conductor has a difference inpotential impressed across it, a direction is impartedto the random drift (Figure 2-25). This causes thefree electrons to be repelled away from the negativeterminal and attracted toward the positive terminal.This constitutes a general migration of electronsfrom one end of the conductor to the other. Thedirected migration of free electrons due to the poten-tial difference is called directed drift.

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difference in potential is impressed across the con-ductor, the positive terminal of the battery attractselectrons from point A. Point A now has a deficiencyof electrons. As a result, electrons are attracted frompoint B to point A. Point B now has an electrondeficiency therefore, it will attract electrons. Thissame effect occurs throughout the conductor andrepeats itself from points D to C. At the same instantthe positive battery terminal attracts electrons frompoint A, the negative terminal repels electronstoward point D. These electrons are attracted topoint D as it gives up electrons to point C. Thisprocess continues for as long as a difference in poten-tial exists across the conductor. Though an in-dividual electron moves quite slowly through theconductor, the effect of a directed drift occurs almostinstantly. As an electron moves into the conductorat point D, an electron is leaving at point A. Thisaction takes place at approximately the speed of light.

The directed movement of the electrons occursat a relatively low velocity (rate of motion in a particu-lar direction). The effect of this directed movement,however, is almost instantaneous (Figure 2-26). As a

Magnitude of Current F1ow

Electric current is the directed movement ofelectrons. Directed drift, therefore, is current, andthe terms can be used interchangeably. The termdirected drift helps distinguish the random anddirected motion of electrons. However, currentflow is the term most commonly used to indicate adirected movement of electrons.

The magnitude of current flow is directlyrelated to the amount of energy that passes througha conductor as a result of the drift action. An

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increase in the number of energy carriers (movingfree electrons) or an increase in the energy of theexisting valence electrons increases the current flow.When an electric potential is impressed across aconductor, the velocity of the free electronsincreases, causing an increase in the energy of thecarriers. An increased number of electrons is alsogenerated, providing added carriers of energy. Theadditional number of free electrons is relativelysmall. Thus, the magnitude of current flow dependsmainly on the velocity of the existing movingelectrons.

The difference in potential affects the mag-nitude of current flow. Initially, free electrons aregiven additional energy because of the repelling andattracting electrostatic field. If the difference inpotential (voltage) is increased, the electric field willbe stronger, the amount of energy imparted to avalence electron will be greater, and the current willbe increased. If the potential difference isdecreased, the strength of the field is reduced, theenergy supplied to the electron is diminished, and thecurrent is decreased.

Measurement of Current

The magnitude of current is measured inamperes. A current of 1 ampere is said to flow when1 coulomb of charge passes a point in one second (1coulomb equals the charge of 6.242 times 10 to the18th power electrons). Often the ampere is much toolarge a unit for measuring current. Therefore, themilliampere (mA), one-thousandth of an ampere, orthe microampere (uA), one-millionth of an ampere,is used. The device that measures current is calledan ammeter.

ELECTRICAL RESISTANCE

The directed movement of electrons con-stitutes a current flow. Electrons do not move freelythrough a conductors crystalline structure. Somematerials offer little opposition to current flow, whileother materials greatly oppose current flow. Thisopposition to current flow is resistance (R), and theunit of measure is the ohm. The greater the resis-tance in the circuit, the smaller the current will befrom the power supply. Resistance is essential in acircuit. If all the resistance in a circuit waseliminated, a short circuit would result. If notprevented, this maximum current flow will damage

the electrical system. The standard of measure for 1ohm is the resistance provided at 0 degrees Celsiusby a column of mercury having a cross-sectional areaof 1 square millimeter and a length of 106.3 cen-timeters. A conductor has 1 ohm of resistance whenan applied potential of 1 volt produces a current of 1ampere. The symbol used to represent the ohm is theGreek letter omega (W).

Resistance, although an electrical property, isdetermined by the physical structure of a material.Many of the same factors that control current flowgovern the resistance of a material. Therefore, thefactors that affect current flow will help explain thefactors affecting resistance.

The magnitude of resistance is determined inpart by the number of free electrons available withinthe material. Since a decrease in the number offree electrons will decrease the current flow, theopposition to current flow (resistance) is greater ina material with fewer free electrons. Thus, theresistance of a material is determined by the numberof free electrons available in a material. The condi-tions that limit current flow also affect resistance.The type of material, physical dimensions, andtemperature affect the resistance of a conductor.

Effect of Type of Material

Depending on their atomic structure, differentmaterials have different quantities of free electrons.Therefore, the various conductors used in electricalapplications have different values of resistance.

Consider a simple metallic substance. Mostmetals are crystalline in structure and consist ofatoms that are tightly bound in the lattice network.The atoms of such elements are so close togetherthat the electrons in the outer shell of the atom areassociated with one atom as much as with its neighbor(Figure2-27 view A). As a result, the force of attach-ment of an outer electron with an individual atom ispractically zero. Depending on the metal, at leastone electron, sometimes two, and, in a few cases,three electrons per atom exist in this state. In sucha case, a relatively small amount of additionalelectron energy would free the outer electronsfrom the attraction of the nucleus. At normal roomtemperature, materials of thpe type have many freeelectrons and are good conductors. Good conduc-tors have a low resistance.

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If the atoms of a material are farther apart, theelectrons in the outer shells will not be equallyattached to several atoms as they orbit the nucleus(view B). They are attracted to the nucleus of theparent atom only. Therefore, a greater amount ofenergy is required to free any of these electrons.Materials of this type are poor conductors and havea high resistance.

Silver, gold, and aluminum are good conduc-tors. Therefore, materials composed of their atomswould have a low resistance. The element copper isthe conductor most widely used throughout electricalapplications. Silver has a lower resistance than cop-per, but its cost limits usage to circuits where a highconductivity is demanded. Aluminum, which is muchlighter than copper, is used as a conductor whenweight is a major factor.

Effect of Physical Dimensions

Cross-sectional Area. Cross-sectional areagreatly affects the magnitude of resistance. If thecross-sectional area of a conductor is increased, agreater quantity of electrons are available to movethrough the conductor. Therefore, a larger currentwill flow for a given amount of applied voltage. Anincrease in current indicates that when the cross-sec-tional area of a conductor is increased, the resistancemust have decreased. If the cross-sectional area of aconductor is decreased, the number of availableelectrons decreases and, for a given applied voltage,the current through the conductor decreases. Adecrease in current flow indicates that when thecross-sectional area of a conductor is decreased, theresistance must have increased. Thus, the resistance

of a conductor is inversely proportional to its cross-sectional area.

Conductor Diameter. The diameter of con-ductors used in electronics is often only a fraction ofan inch. Therefore, the diameter is expressed in mils(thousandths of an inch). It is also standard practiceto assign the unit circular mil to the cross-sectionalarea of the conductor. The circular mil is found bysquaring the diameter, when the diameter is ex-pressed in mils. Thus, if the diameter is 35 mils (0.035inch, the circular mil area equals 352 or 1,225 cir-cular mils. Figure 2-28 shows a comparison betweena square mil and circular mil.

Conductor Length. The length of a conductoris also a factor that determines the resistance of aconductor. If the length of a conductor is increased,the amount of energy given up increases. As freeelectrons move from atom to atom, some energy isgiven off as heat. The longer a conductor is, the moreenergy is lost to heat. The additional energy losssubtracts from the energy being transferred throughthe conductor, resulting in a decrease in current flowfor a given applied voltage. A decrease in currentflow indicates an increase in resistance, since voltagewas held constant. Therefore, if the length of a con-ductor is increased the resistance increases. Theresistance of a conductor is directly proportional toits length.

Effect of Temperature

Temperature changes affect the resistance ofmaterials in different ways. In some materials, anincrease in temperature causes an increase in resis-tance. In others, an increase in temperature causesa decrease in resistance. The amount of change of

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resistance per unit change in temperature is thetemperature coefficient. If for an increase intemperature the resistance of a material increases, ithas a positive temperature coefficient. A materialwhose resistance decreases with an increase intemperature has a negative temperature coefficient.Most conductors used in electronic applicationshave a positive temperature coefficient. However,carbon, a frequently used material, is a substancewith a negative temperature coefficient. Severalmaterials, such as the alloys constantan and man-