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082-Engineering-Mathematics-Anthony-Croft-Robert-Davison-Martin-Hargreaves-James-Flint-Edisi-5-2017

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9.8 Phasors 343

Therefore,

Ṽ S

=ṼR +ṼC +ṼL

Ĩ

=ĨR +ĨjωL +

jωC

(

=Ĩ R +jωL+ 1 )

jωC

Therefore the impedance of the circuit is Z = R + jωL + 1 . We can calculate the

jωC

frequency forwhich the impedance of the circuithas minimum magnitude:

Now

Z=R+jωL+ 1

jωC

=R+jωL− j

(

=R+j

|Z| =

R 2 +

ωC

ωL − 1

ωC

)

(

ωL − 1 ) 2

ωC

and so, as ω varies |Z| isaminimum when

ωL − 1

ωC = 0

ω 2 = 1

LC

1

ω =

LC

This minimum value is |Z| = R. Examining Figure 9.15 it is clear that the minimum

impedance occurs whenṼL andṼC have the same magnitude, in which caseṼS has no

imaginary component. The frequency at which this occurs is known as the resonant

frequencyof the circuit.

Engineeringapplication9.1

ThePoyntingvector

An electromagnetic wave freely travelling in space has electric and magnetic field

componentswhichoscillateatrightanglestoeachotherandtothedirectionofpropagation.

This type of wave is known as a transverse wave. Figure 9.16 illustrates

anelectromagneticwavetravellinginfreespace.ThePoyntingvectorisusedtodescribetheenergyfluxassociatedwithanelectromagneticwave.IthasunitsofWm

−2

andisapowerperunitarea,thatisapowerdensity.Ifthetotalpowerassociatedwith

anelectromagneticwavefrontisrequiredthenthePoyntingvectorcanbeintegrated

over anareaofinterest.

ThePoynting vector, S, forelectricfield,E, and magnetic field, H,is definedas

S=E×H

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