(31) electrical fundamentals.pdf

7/27/2019 (31) Electrical Fundamentals.pdf

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2008 Pearson Education, Inc.

Pearson Prentice Hall - Upper Saddle River, NJ 07458

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition

By James D. Halderman



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Prepare for ASE Electrical/Electronic Systems(A6) certification test content area A (GeneralElectrical/Electronic System Diagnosis).

Define electricity. Explain the units of electrical measurement.

Discuss the relationship among volts,amperes, and ohms. Explain how magnetism is used in automotive

applications.

After studying Chapter 31, the reader shouldbe able to:

OBJECTIVES:


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ammeter amperes atom bound electrons

concentric rings conductors conventional theory

coulomb current

electrical potential electricity electrochemistry

electrons electron theory elements electromotiveforce (EMF)

free electrons insulators ion

neutral charge neutrons nucleus

ohmmeter ohms

KEY TERMS:

Continued


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peltier effect photoelectricity piezoelectricity positive temperature coefficient (PTC) potentiometer

protons

resistance resistors rheostat

semiconductors shells static electricity

thermocouple thermoelectricity

valence shell volt voltmeter

watt

KEY TERMS:


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The electrical system is one of the most important

systems in a vehicle today.

Every year, more and more components and

systems use electricity.

Technicians who really know and understand

automotive electrical and electronic systems

will be in great demand.


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ELECTRICITY

Our universe is composed of matter, anything that has mass andoccupies space. All matter is made from slightly over 100

individual components called elements.

The smallest particle that an element can be broken into and stillretain the properties of that element is known as an atom.

Figure 311 In an atom (left),

electrons orbit protons in the

nucleus just as planets orbitthe sun in our solar system

(right).

Continued


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Electricity is the movement of electrons from one atom to another.

The dense center of each atom is called the nucleus. The nucleuscontains protons, which have positive charge, and neutrons,

electrically neutral (no charge).

Electrons surround the nucleus in orbits. Each atom contains an

equal number of electrons and protons.

Because the number of negative-charged electrons is balanced withthe same number of positive-charged protons, an atom has a neutral

charge (no charge).

NOTE: As an example of relative sizes of parts of an atom, consider thatif an atom were magnified so that the nucleus were the size of the period

at the end of this sentence, the whole atom would be bigger than a house.

NOTE: As an example of relative sizes of parts of an atom, consider thatif an atom were magnified so that the nucleus were the size of the period

at the end of this sentence, the whole atom would be bigger than a house.


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Positive and Negative Charges Parts of the atom have different

charges. Orbiting electrons are negatively charged, protons

positively charged. Positive charges are indicated by the plus

sign

(+), and negative charges by the minus

sign ().

Continued

Figure 312 The nucleus of an atom has

a positive () charge and the surroundingelectrons have a negative () charge.

These same + and

signs are

used to identify parts of anelectrical circuit.

Neutrons have no charge at all.

They are neutral.


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In a normal, or balanced, atom, the number of negative particles

equals the number of positive particles. The number of neutronsvaries according to the type of atom.

Continued

Figure 313

This figure shows a balanced atom.


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10Figure 314 Unlike charges attract and like charges repel.

Continued

An ordinary magnet has two ends, or poles. One end is the south

pole, and the other the north pole.

If the opposite poles of the magnets are brought close to each other,

south to north, the magnets will snap together because unlike poles

attract each other. If two magnets are brought close to each other with like poles

together (south to south or north to north), the magnets will pusheach other apart. This is because like poles repel each other.


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Positive and negative charges within an atom are like north and

south poles of a magnet. Charges that are alike will repel each other,

which is why the negative electrons continue to orbit around the

positive protons. They are attracted and held by the opposite chargeof the protons. The electrons keep moving in orbit because they

repel each other. When an atom loses electrons, it becomes unbalanced. It will have

more protons than electrons and will have a positive charge. If itgains more electrons than protons, it will be negatively charged.

When an atom is not balanced, it becomes a charged particle called

an ion. Ions try to regain balance of equal protons and electrons by

exchanging electrons with neighboring atoms. See Figure 315.

This is the flow of electric current orelectricity.Continued


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Figure 315 An unbalanced, positively charged atom (ion) will attract electrons from neighboring

atoms.


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Electron Shells Orbit around the nucleus in definite paths. These

paths form shells, like concentric rings, around the nucleus. Only a

specific number of electrons can orbit within each shell.

Continued

Figure 316

The hydrogen atom is the simplest atom, with

only one proton, one neutron, and one electron.

More complex elements contain higher numbers

of protons, neutrons, and electrons.

If there are too many electrons

for the first and closest shell to the nucleus, others will orbit in additional shells until allelectrons have an orbit within

a shell. There can be as many as seven shells around a singlenucleus.


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Free and Bound Electrons The outermost electron shell or ring,

called the valence shell, is the most important to our study of

electricity.

The number of electrons in this shell determines the valence of the

atom and indicates its capacity to combine with other atoms. If the valence ring of an atom has three or fewer electrons in it, the

ring has room for more. The electrons are held very loosely, and it iseasy for a drifting electron to join the ring and push another electronaway. These loosely held electrons are called free electrons.

When a valence ring has five or more electrons, it is fairly full. The

electrons are held tightly, and it is hard for a drifting electron to

push its way into the ring. These tightly held electrons are called

bound electrons. See Figures 317 and 318.Continued


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15Figure 317 As the number of electrons increases, they occupyincreasing energy levels that are further from the center of the atom.

Figure 318 Electrons in the outer orbit,

or shell, can often be drawn away from

the atom and become free electrons.

The movement of these drifting electrons is called current. Electric

current is controlled, directed movement of electrons from atom to

atom within a conductor.

Continued


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Conductors Materials with fewer than four electrons in their

atoms outer orbit are Conductors. Copper is excellent as a

conductor because it has only one electron in its outer orbit. Thisorbit is far enough away from the nucleus of the atom that the pull

or force holding the outermost electron in orbit is relatively weak.

Figure 319 A conductor is any element thathas one to three electrons in its outer orbit.Continued


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Copper is the conductor most

used in vehicles because the

price of copper is reasonable

compared to the relative cost

of other conductors with

similar properties.

Continued

Figure 3110 Copper is an excellent conductor of electricity because it has just one electron in i ts

outer orbi t, making i t easy to be knocked out of its orbi t and flow to other nearby atoms. This

causes electron flow, which is the definition of electricity.


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Pure water is an insulator; however, if anything is in the water, such assalt or dirt, then the water becomes conductive. Because it is difficult to

keep water from becoming contaminated, water is usually thought of as

being capable of conducting electricity, especially high voltage such as

from household 110-volt or 220-volt outlets.

Pure water is an insulator; however, if anything is in the water, such assalt or dirt, then the water becomes conductive. Because it is difficult to

keep water from becoming contaminated, water is usually thought of as

being capable of conducting electricity, especially high voltage such as

from household 110-volt or 220-volt outlets.

Is Water a Conductor?

Insulators The protons and neutrons in the nucleus are held

together very tightly. Normally the nucleus does not change.

Some outer electrons are held very loosely, and can move from

one atom to another. Some materials hold their electrons verytightly; electrons do not move through them very well.

These materials are called insulators.Continued


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Insulators are materials with more than four electrons in their

atoms outer orbit. Because they have more than four electrons, it

becomes easier for these materials to acquire (gain) electrons than

to release electrons.

Continued

Figure 3111 Insulators are elementswith f ive to eight electrons in the outer

orbit.

Insulators include plastics, wood,glass, rubber, ceramics (spark plugs),

and varnish for covering (insulating)

copper wires in alternators andstarters.


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Semiconductors

Materials with exactly four

electrons in their outer orbit

are neither conductors norinsulators; they are called

semiconductor materials.

Continued

Figure 3112 Semiconductor elements

contain exactly four electrons in the

outer orbit.


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How Electrons Move Through a Conductor If an outside source

of power, such as a battery, is connected to the ends of a conductor,

a positive charge (lack of electrons) is placed on one end of theconductor and a negative charge is placed on the opposite end

of the conductor.The negative charge will repel the free electrons from the atoms

of

the conductor, whereas the positive charge on the opposite end of

the conductor will attract electrons. As a result of this attraction ofopposite charges and repulsion of like charges, electrons will flow

through the conductor.

ContinuedFigure 3113 Current electric ity is the movement of electrons through a conductor .


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Figure 3114 Conventional theory states that current flows through

a circuit from posit ive (+) to negative (-). Automotive electric ity usesthe conventional theory in all electrical diagrams and schematics.

Conventional Theory versus Electron Theory It was once

thought that electricity had only one charge and moved from

positive to negative. This theory of the flow of electricity through aconductor is called the conventional theory of current flow.

This book uses theconventional theory

unless stated otherwise.

Discovery of the electron and its negative charge led to the electron

theory, which states there is electron flow from negative to positive.


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Amperes The ampere is the unit used to measure current flow.

When 6.28 billion billion electrons (a coulomb) move past a certain

point in 1 second, this represents 1 ampere of current.

The ampere is the electrical unit for amount of electron flow just as

gallons per minute is the unit used to measure water flow.The ampere was named for the French electrician Andr

Marie

Ampre (17751836).

Continued

Figure 3115

One ampere is the movement of 1 coulomb (6.28 billion billion electrons) past a point in 1 second.


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Conventional abbreviations and measurement for amperes are

summarized as follows:

1.

The ampere is the unit of measurement for the amount of

current flow.

2.

Acceptable abbreviations foramperes areA and amps.

3.

The capital letterI, forintensity, is used in mathematical

calculations to represent amperes.

4. Amperes are measured by an ammeter (notampmeter).Figure 3116

An ammeter is installed in the path of

the electrons simi lar to a water meter

used to measure the flow of water in

gallons per minute. the ammeter

displays current flow in amperes.

Continued


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Volts The volt is the unit of measurement for electrical pressure.

Named for Alessandro Volta (17451827), an Italian physicist.

The comparable unit using water as an example would be pounds

per square inch (psi). It is possible to have very high pressures

(volts) and low water flow (amperes). It is also possible to have highwater flow (amperes) and low pressure (volts).

Voltage is also called electrical potential, because if there isvoltage present in a conductor, there is a potential (possibility) for

current flow. Voltage does notflow through conductors, but voltage

does cause current (in amperes) to flow through conductors.

Figure 3117

Voltage is the electrical pressure

that causes the electrons to flow

through a conductor.


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The conventional abbreviations and measurement for voltage are:

Continued

1. Volt is the measurement for amount of electrical pressure.2.

Another term for voltage is Electromotive force, (EMF).

3.

The letterVis the generally accepted abbreviation forvolts.

4. The symbol used in calculations is E, for electromotive force.5. Volts are measured with a voltmeter.

Figure 3118

This digital multimeter set to read DC

volts is being used to test the voltageof a vehicle battery. Most mult imeters

can also measure resistance (ohms)

and current flow (amperes).


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Ohms Resistance to the flow of current through a conductor is

measured in units called ohms, named after the German physicist

Georg Simon Ohm (17871854). The resistance to the flow of free electrons through a conductor

results from the countless collisions the electrons cause within theatoms of the conductor.

Continued

Figure 3119 Resistance to the flow of electrons through a conductor is measured in ohms.


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Conventional abbreviations and measurement for resistance are:

Continued

1.

The ohm is the unit of measurement for electrical resistance.

2.

The symbol for ohms is

(Greek capital letter omega), the

last letter of the Greek alphabet.

3.

The symbol used in calculations isR, forresistance.

4.

Ohms are measured with an ohmmeter.


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Figure 3120 A display at the Henry Ford Museum in Dearborn, Michigan, that includes a hand-

cranked generator and a series of l ight bulbs. This f igure shows a young man attempting to light

as many bulbs as possible. The crank gets harder to turn as more bulbs l ight because it requires

more power to produce the necessary watts of electrici ty.

The symbol for power is P. Electrical power is calculated as

amperes times volts: P (power) =I(amperes)

E (volts)

Watts A watt is the electrical

unit forpower, the capacity to

do work. Named for Scottishinventor, James Watt (1736

1819).


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SOURCES OF ELECTRICIY

There are several sources of electrical energy, but only a few ofthem are used in automotive electrical systems.

Continued

Friction When certain different materials are rubbed together, the

friction causes electrons to be transferred from one to the other.

Both materials become electrically charged.

These charges are not in motion but stay on the surface where theywere deposited. Because the charges are stationary, or static, this

type of voltage is called static electricity.

Vehicle tires rolling on pavement often create static electricity thatinterferes with radio reception.


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Heat When pieces of two metals are joined together at both ends

and one junction is heated, current passes through the metals. Only

millionths of an ampere, but enough to use in a temperature- measuring device called a thermocouple.

Continued

Figure 3121 Electron flow is produced by heating the connection of two different metals.

Some engine temperature

sensors operate in this

manner.

This form of voltage iscalled thermoelectricity.


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In 1823, German physicist Thomas Johann

Seebeck discovered that

voltage was developed in a loop containing two dissimilar metals,

provided the junctions were maintained at different temperatures. A decade later, French scientist Jean Charles

Peltier found electrons

moving through a solid can carry heat from one side of the materialto the other side. This effect is called the

Peltier effect.

A Peltier effect device is often used in portable coolers to keep food

items cool if the current flows in one direction and to keep items

warm if the current flows in reverse.

Continued


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Light In 1839, Edmond Becquerel noticed that shining a beam of

sunlight over two different liquids developed electric current.

When certain metals are exposed to light, some light energy is

transferred to free electrons of the metal. This excess energy breaks

electrons loose from the metal. They can be collected and made toflow in a conductor.

Photoelectricity is

widely used in light-

measuring devicessuch as photographic

exposure meters andautomatic headlamp

dimmers.

ContinuedFigure 3122 Electron flow is produced by light str iking a light-sensitive material.


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Pressure The first demonstration of a connection between

macroscopic piezoelectric phenomena and crystallographic structure

was published in 1880 by Pierre and Jacques Curie. Whensubjected to pressure, certain crystals, such as quartz, develop

a

potential difference, or voltage, on the crystal faces. This current is

used in phonograph pickups, crystal microphones, underwaterhydrophones, and certain stethoscopes.

Figure 3123 Electron flow is producedby pressure on certain crystals.

The voltage created is called piezoelectricity.

Some automobile engine control

sensors, such as the knock sensor(KS), use piezoelectricity to create voltage or to vary resistance and control a computer input signal.

Continued


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Chemistry Two different materials (usually metals) placed in a

conducting and reactive chemical solution create a difference in

potential, or voltage, between them. This principle is calledelectrochemistry and is the basis of the automotive battery.Conductors and Resistance All conductors have some resistanceto current flow. Several principles of conductors and their resistance

include the following:

If the conductor length is doubled, its resistance doublesThis is why battery cables are designed as short as possible.

If the conductor diameter is increased, resistance is reduced

This is the reason starter motor cables are larger in diameterthan other wiring in the vehicle.

See Chapter 7 for further details on wiring sizes. Continued


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As the temperature increases, the resistance of the

conductor also increases This is the reason for installing heat

shields on some starter motors. The shield helps protect theconductors (copper wiring inside the starter) from excessive

engine heat and reduces resistance of starter circuits. Because a

conductor increases in resistance with increased temperature,the conductor is called a positive temperature coefficient

(PTC)

resistor.

Materials used in the conductor have an impact on itsresistance Silver has the lowest resistance of any conductor,

but is expensive. Copper is the next lowest in resistance and it

is reasonably priced. See the following chart for a comparison of materials.

Continued


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CONDUCTION CHART

1. Silver

2. Copper3. Gold

4. Aluminum

5. Tungsten

6. Zinc

7. Brass (copper and zinc)

8. Platinum9. Iron

10. Nickel

11. Tin

12. Steel

13. Lead

Starting with the best

See this chart on Page 323 of your textbook.

Continued


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38Figure 3124 This figure shows a resistor color code interpretation.

Resistors Resistance is opposition to current flow. Resistors

represent an electrical load, or resistance, to current flow.

Continued


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Most electrical and electronic devices use resistors of specific

values

to limit and control the flow of current.

Resistors can be made from carbon or from other materials that

restrict the flow of electricity and are available in various sizes and

resistance values. Most resistors have a series of painted color bands around them.

These color bands are coded to indicate the degree of resistance.

Continued

Figure 3125 This figure

shows a typical carbonresistor.


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Variable Resistors Two types of mechanically operated variable

resistors are used in automotive applications. A potentiometer is a

three-terminal variable resistor where the majority of the currentflow travels through the resistance of the unit and a wiper contact

provides a variable voltage output.

Continued

Potentiometers are most commonly used as throttle position (TP)

sensors on computer-equipped engines.

Figure 3126 A three-wire variableresistor is called a potentiometer.


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41Figure 3127 A two-wire variable resistor is called a rheostat.

Another type of mechanically operated variable resistor is the

rheostat. A rheostat is a two-terminal unit in which all of the

current flows through the movable arm. A rheostat is commonlyused for a dash light dimmer control.


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SUMMARY

1. Electricity is movement of electrons from one atom to another.2.

Automotive electricity uses the conventional theory of current

flow (electricity flows from positive to negative).

3.

The ampere is the measure of the amount of current flow.

4.

Voltage is the unit of electrical pressure.

5. The ohm is the unit of electrical resistance.6.

Sources of electricity include: friction, heat, light, pressure,

and chemical.


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