chapter 14 electromagnetism

7/25/2019 Chapter 14 Electromagnetism

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ElectromagnetismElectromagnetism

Topics Covered in Chapter 14

14-1: Ampere-turns of Magnetomotive Force (mmf)

14-2: Field ntensit! (H)

14-": B-H Magneti#ation Curve

14-4: Magnetic $!steresis

14-%: Magnetic Field around an &lectric Current

Chapter

14

2007 The McGraw-Hill Companies, Inc. All rights reserve.


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Topics Covered in Chapter 14Topics Covered in Chapter 14

14-': Magnetic olarit! of a Coil

14-: Motor Action *et+een T+o Magnetic Fields

14-,: nduced Current

14-: .enerating an nduced /oltage 14-10: ela!s

McGraw-Hill 2007 The McGraw-Hill Companies, Inc. All rights reserve.


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14-1: Ampere-turns of14-1: Ampere-turns of

Magnetomotive Force (mmf)Magnetomotive Force (mmf)

The strength of a coils magnetic field is proportional to

the amount of current flo+ing through the coil and the

num*er of turns per given length of coil3 Ampere-turns I5 N mmf

Iis the amount of current flo+ing through Nturns of

+ire3

This formula specifies the amount of magneti#ing forceor magnetic potential (mmf)3


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14-1: Ampere-turns of14-1: Ampere-turns of

Magnetomotive Force (mmf)Magnetomotive Force (mmf)

The 6 a**reviation for

ampere-turn is A 7 t3

The cgs unit of mmf is thegilbert8 a**reviated .*3

Cop!right 9 The Mc.ra+-$ill Companies8 nc3 ermission reuired for reproduction or displa!3

Fig3 14-1: T+o e;amples of eual ampere-

turns for the same mmf3 (a) < is 2 5 % 103

(b) INis 1 5 10 103


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14-: Field !ntensit" (14-: Field !ntensit" (HH))

The length of a coil influences the intensit! of a

magnetic field3 ntensit! is different from mmf3

&uation: H mmf=length

>nits: A7t=m

ampere-turns per meter

His the m?s unit

The cgs unit for His the oersted (@e)8 +hich euals 1

gil*ert per centimeter3 6horter magnetic circuits produce a greater field

intensit!


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Fig3 14-2: elation *et+een ampere-turns of mmf and the resultant field intensit! Hfor different

cores3


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ermea*ilit! ()

Permeabilityis a measure of the a*ilit! to concentrate

magnetic fields3 Materials +ith high permea*ilit! can

concentrate flu;8 and produce large values of flu;densit! Bfor a specified H3

The amount of flu; produced *! Hdepends on the

material in the field3

These factors are reflected in the formulas: B 5 H

= B= H

The unit is teslas per ampere-turn per meter: A 7 t=m

T


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14-#:14-#: B-HB-H Magneti$ation CurveMagneti$ation Curve

The B-Hmagneti#ation

curve sho+s ho+

much flu; densit! B

results from increasing

field intensit! H3 Saturationis the effect

of little change in flu;

densit! +hen the field

intensit! increases3


Fig3 14-": B-Hmagneti#ation curve for

soft iron3


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14-4: Magnetic %"steresis14-4: Magnetic %"steresis

Hysteresis refers to a situation +here the magnetic flu;

lags the increases or decreases in magneti#ing force3

Hysteresis lossis energ! +asted in the form of heat

+hen alternating current reverses rapidl! and moleculardipoles lag the magneti#ing force3

For steel and other hard magnetic materials8 hysteresis

lossesare much higher than in soft magnetic materials

li?e iron3


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14-4: Magnetic %"steresis

$!steresis oop

B1is due to retentivity,

+hich is the flu; densit!

remaining after the

magneti#ing force is

reduced to #ero3


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Demagneti#ation (Also Called Degaussing) To demagneti#e a magnetic material completel!8 the

retentivit! B1must *e reduced to #ero3

The practical +a! to do so is to magneti#e anddemagneti#e the material +ith a decreasing h!steresisloop:

A magnetic field is produced *! alternating current3

The magnetic field and the magnetic material aremoved a+a! from each other8 or the currentamplitude is reduced3

The h!steresis loop then *ecomes smaller andsmaller until it effectivel! collapses3


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Demagneti#ation (Also Called Degaussing)

This method of demagneti#ation is called degaussing3

Applications of degaussing include:

Metal electrodes in a color picture tu*e &rasing the recorded signal on magnetic tape3


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14-&: Magnetic Field around an14-&: Magnetic Field around an

Electric CurrentElectric Current

6traight Conductor

A straight conductorcan *e a short *ut continuous

length of conducting +ire +ith no *ends3

A magnetic field is produced *! the flo+ of currentthrough a straight conductor3

The magnetic field around a straight conductor is

circular and perpendicular to the a;is of the conductor3

The polarit! of the circular field is countercloc?+ise+hen vie+ed along the conductor in the direction of

electron flo+3

These reuirements appl! to an! charge in motion3


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14-&: Magnetic Field around an14-&: Magnetic Field around an

Electric CurrentElectric Current

Cloc?+ise and Countercloc?+ise Fields

The left-handrule for conductors:

.rasp the conductor +ith !our left hand so the thum*

points in the direction of electron flo+3 Eour fingers+ill encircle the conductor in the same direction as

the circular magnetic field lines3


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14-': Magnetic olarit" of a14-': Magnetic olarit" of a

CoilCoil Bending a straight conductor into the form of a loop

produces t+o effects:

The magnetic field lines are more dense inside the

loop3All the lines inside the loop aid in the same direction3

This ma?es the loop field the same as a *ar magnet8

+ith opposite poles at opposite faces of the loop3


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CoilCoil

Magnetic olarit!

A coil of +ire conductor +ith more than one turn is

called a solenoid3

To determine magnetic polarit! for a solenoid8 grasp theelectromagnet +ith the left hand3 hen the fingers of

the left hand curl around the turns of an electromagnet

in the direction of electron flo+8 the thum* points to the

north pole3 The magnetic polarit! depends on the direction of

current flo+ and the direction of +inding3 The current is

determined *! the connections to the voltage source:

flo+ runs from negative to positive3


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CoilCoil


Fig3 14-10: eft-hand rule for north pole of a coil +ith currentI3 The Iis electron flo+3


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14-: Motor Action *et+een T+o14-: Motor Action *et+een T+o

Magnetic FieldsMagnetic Fields

Motor actionis the result of t+o magnetic fields

interacting +ith one another3

The fields can attract or repel3

Motion is produced from a stronger field to+ard a

+ea?er field3


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Current in a conductor has its o+n magnetic field3

hen placed in the magnetic field of a separate source8

the t+o can produce motor action3

The conductor must *e perpendicular to the field3 t

must also *e in the same plane3


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Fig3 14-1": Motor action of current in a straight conductor +hen it is in an e;ternal magnetic

field3 The HIis the circular field of the current The HMindicates field lines *et+een the north and

south poles of the e;ternal magnet3


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Torque is the effect of a force producing rotation3

Torue is produced +hen opposing magnetic fields in a

loop produce an up+ard force on one side of the loopand do+n+ard force on the other3

Torue is the *asis of all electric motors3

Torue is proportional to current8 so the amount of

rotation indicates ho+ much current flo+s through the

coil3


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14-,: !nduced Current14-,: !nduced Current

.enerator Action

hen a moving conductor cuts across flu; lines8 a

voltage is induced3

The amount of induced voltage is proportional to: The conductor velocit!

The amount of flu;

The num*er of turns of +ire

The polarit! of induced voltage is determined *! en#s

la+ (discussed in section 14-)3


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14-,: !nduced Current14-,: !nduced Current

en#s la+ states that the direction of an induced

current must *e such that its o+n magnetic field +ill

oppose the action that produced the induced current3


Fig3 14-1%: nduced current produced *! magnetic flu; cutting across turns of +ire in a coil3

Direction of Ihere is for electron flo+3

The directionof the induced currentis

determined by the left-hand rule for electron

flow. If the fingers coil around the direction of

electron shown, under and over the winding, the

thumb will oint to the left for the north ole.


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14-: .enerating an14-: .enerating an

!nduced /oltage!nduced /oltage

The motion of flu; across a conductor in an open circuit

forces free electrons to move3

6ince the ends are open8 electrons accumulate at them8

creating a potential difference3 The potential difference is an electromotive force

!emf",generated *! the +or? of cutting across the flu;3

nduced emf increases +ith the num*er of turns in a

coil3 The polarit! of the induced voltage follo+s from the

direction of induced current3


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#araday$sa+ of nduced /oltage

The amount of voltage induced *! flu; cutting the turns

of a coil is determined *! three factors:

The amount of flu; More voltage is generated *! a stronger magnet3

The num*er of turns ncreasing the turns generates more voltage3

The time rate of cutting3 ess voltage is generated +hen the conductor moves slo+l!3

&ither the flu; or the conductor can move3


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Farada!s a+ of nduced /oltage

The amount of induced voltage can *e calculated *!

Farada!s la+:

N num*er of turns

d/dt ho+ fast the flu; cuts across the conductor3

vind Nd(+e*ers)

dt(seconds)


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Fig3 14-1: /oltage induced across coil cut *! magnetic flu;3 (a) Motion of flu; generating voltage

across coil3 (b) nduced voltage acts in series +ith coil3 (c) nduced voltage is a source that can

produce current in an e;ternal load resistor RLconnected across the coil3


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Consider a magnetic flu; cutting a

conductor that is not in a closed

circuit8 as sho+n in Figure 14-1'3The motionof flu; across the

conductor forces free electrons to

move3The potential difference is an

electromotive force (emf) generated

and onl! present +hile the motion of

flu; is cutting across the conductor3



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Fig3 14-1,: .raphs of induced voltage produced *! magnetic flu; changes in a coil3 (a) inear

increase of flu; 3 (b) Constant rate of change for d/dtat 2 *=s3 (c) Constant induced voltage

of '00 /8 for a coil +ith "00 turns3


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Farada!s a+ of nduced /oltage

The amount of induced voltage is calculated *!

Farada!s la+:

The induced voltage is directl! proportional to the

num*er of turns times d/dt.

To generate more voltage:

ncrease the num*er of turns

ncrease the flu;

Decrease the time

vind=N

d(Wb)

dt(seconds)


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olarit! of nduced /oltage

The polarit! of induced voltage is determined *! en#s

la+3

An induced voltage has the polarit! that opposes thechange causing the induction3

A*solute polarit! depends upon three points:

hether the flu; is increasing or decreasingG

The method of +indingG

hich end of the coil is the reference3


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14-10: ela"s14-10: ela"s

A relayis an electromechanical device that operates on

the *asis of electromagnetic induction3

t uses an electromagnet to open or close one or more

sets of contacts3 ela!s8 li?e s+itches8 have poles and thro+s3

ela!s can s+itch or control high po+er loads +ith a

lo+ amount of input po+er3

n remote-control applications8 rela!s can control highpo+er loads long distances a+a! more efficientl! than

can mechanical s+itches3


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14-10: ela"s14-10: ela"s

The s+itching contacts of a rela! ma! *e:


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14-10: ela"s14-10: ela"s

An 6DT rela! has *oth


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14-10: ela"s14-10: ela"s

ela! 6pecifications:

The follo+ing are a rela!s most important ratings:

ic?up voltage The minimum amount of rela! coil voltagenecessar! to energi#e or operate the rela!3

ic?up current The minimum amount of rela! coil currentnecessar! to energi#e or operate the rela!3

$olding current The minimum amount of current reuired to ?eep

a rela! energi#ed or operating (less than thepic?up current)3

Dropout voltage The ma;imum rela! coil voltage at +hich the rela!is no longer energi#ed3


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14-10: ela"s14-10: ela"s

ela! 6pecifications:

mportant ratings8 cont3

Contact voltage

rating

The ma;imum voltage the rela! contacts are

capa*le of s+itching safel!3

Contact currentrating

The ma;imum current the rela! contacts arecapa*le of s+itching safel!3

Contact voltage drop The voltage drop across the closed contacts of arela! +hen operating3

nsulation resistance The resistance measured across the rela!contacts in the open position3


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14-10: ela"s14-10: ela"s

The pic?up current is greater than the hold current *ecause of the air gap3

ess current is no+

reuired to

overcome the spring

and hold the rela!closed3

There is continuit! *et+een the main contact and the

chapter 14 electromagnetism

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