electromagnetism inel 4152 ch 9 sandra cruz-pol, ph. d. ece uprm mayag ü ez, pr

ElectromagnetismElectromagnetismINEL 4152 CH 9INEL 4152 CH 9

Sandra Cruz-Pol, Ph. D.Sandra Cruz-Pol, Ph. D.ECE UPRMECE UPRM

MayagMayagüüez, PRez, PR

In summaryIn summary Stationary ChargesStationary Charges

Steady currentsSteady currents

Time-varying Time-varying currentscurrents

Electrostatic fieldsElectrostatic fields

Magnetostatic fieldsMagnetostatic fields

Electromagnetic Electromagnetic (waves!)(waves!)

Cruz-Pol, Electromagnetics Cruz-Pol, Electromagnetics UPRMUPRM

OutlineOutline Faraday’s Law & Origin of emagFaraday’s Law & Origin of emag Maxwell Equations explain Maxwell Equations explain waveswaves Phasors Phasors and Time Harmonic fieldsand Time Harmonic fields

Maxwell eqs for time-harmonic fieldsMaxwell eqs for time-harmonic fields

Faraday’s LawFaraday’s Law9.29.2


Electricity => MagnetismElectricity => Magnetism In 1820 Oersted discovered that a steady In 1820 Oersted discovered that a steady

current produces a magnetic field while current produces a magnetic field while teaching a physics class. teaching a physics class.

This is what Oersted This is what Oersted discovered accidentally:discovered accidentally:


Would magnetism would Would magnetism would produce electricity?produce electricity?

Eleven years later, Eleven years later, and at the same time, and at the same time, (Mike) Faraday in (Mike) Faraday in London & (Joe) Henry London & (Joe) Henry in New York in New York discovered that a discovered that a time-varying time-varying magnetic magnetic field would produce field would produce an electric current! an electric current!

dtdNVemf


Electromagnetics was born!Electromagnetics was born! This is Faraday’s Law -This is Faraday’s Law -

the principle of motors, the principle of motors, hydro-electric generators hydro-electric generators and transformers and transformers operation.operation.

*Mention some examples of em waves

Faraday’s LawFaraday’s Law For For NN=1 and =1 and BB=0=0


dtdNVemf

Transformer & Motional Transformer & Motional EMFEMF

9.39.3

Three ways B can vary by Three ways B can vary by having…having…

1.1. A stationary loop in a t-varying B fieldA stationary loop in a t-varying B field2.2. A t-varying loop area in a static B fieldA t-varying loop area in a static B field3.3. A t-varying loop area in a t-varying B fieldA t-varying loop area in a t-varying B field


1. Stationary loop in 1. Stationary loop in a time-varying B fielda time-varying B field


2. Time-varying loop area 2. Time-varying loop area in a static B fieldin a static B field


3. A t-varying loop area in 3. A t-varying loop area in a t-varying B fielda t-varying B field


Transformer ExampleTransformer Example


Displacement Current, Displacement Current, JJdd9.49.4


Maxwell noticed something Maxwell noticed something was missing…was missing…

And added And added JJdd, the , the displacement currentdisplacement current

IIdSJdlH encSL

1

02

SL

dSJdlHI

S2

S1

L

IdtdQdSD

dtddSJdlH

SSd

L

22

At low frequencies J>>Jd, but at radio frequencies both terms are comparable in magnitude.

Maxwell’s Equation Maxwell’s Equation in Final Formin Final Form

9.49.4


Summary of TermsSummary of Terms E E = electric field intensity [V/m]= electric field intensity [V/m] DD = electric field density = electric field density HH = magnetic field intensity, [A/m] = magnetic field intensity, [A/m] B B = magnetic field density, [Teslas]= magnetic field density, [Teslas] J J = current density [A/m= current density [A/m22]]


Maxwell Equations Maxwell Equations in General Form in General Form

Differential formDifferential form Integral FormIntegral FormGaussGauss’’ss Law Law for for EE field.field.

GaussGauss’’ss Law Law for for HH field. Nonexistence field. Nonexistence of monopole of monopole FaradayFaraday’’ss LawLaw

AmpereAmpere’’ss Circuit Circuit LawLaw

vD

0 B

tBE

tDJH

v

vs

dvdSD

0s

dSB

sL

dSBt

dlE

sL

dStDJdlH


MaxwellMaxwell’’s Eqs.s Eqs. Also the equation of continuityAlso the equation of continuity

Maxwell addedMaxwell added the term to Ampere the term to Ampere’’s s Law so that it not only works for Law so that it not only works for staticstatic conditions but also for conditions but also for time-varyingtime-varying situations. situations. This added term is called the This added term is called the displacement displacement

current densitycurrent density, while , while JJ is the conduction is the conduction current.current.

tJ v

tD

Relations & B.C.Relations & B.C.


Time Varying Time Varying PotentialsPotentials

9.69.6

We had definedWe had defined Electric Scalar & Magnetic Vector potentials:Electric Scalar & Magnetic Vector potentials:

Related to B as:Related to B as: To find out what happens for time-varying fieldsTo find out what happens for time-varying fieldsSubstitute into Faraday’s law:Substitute into Faraday’s law:


Electric & Magnetic potentials:Electric & Magnetic potentials: If we take the divergence of If we take the divergence of EE::

We have:We have:

Taking the curl of: & add Ampere’sTaking the curl of: & add Ampere’swe getwe get


Electric & Magnetic potentials:Electric & Magnetic potentials: If we apply this If we apply this vector identityvector identity

We end up with We end up with


Electric & Magnetic potentials:Electric & Magnetic potentials: We use the We use the Lorentz condition:Lorentz condition:

To get:To get:

and: and:


Which are both wave equations.

Time Harmonic Time Harmonic FieldsFields

Phasors ReviewPhasors Review

9.79.7

Time Harmonic FieldsTime Harmonic Fields DefinitionDefinition: is a field that varies periodically : is a field that varies periodically

with time.with time. Ex. SinusoidEx. Sinusoid

Let’s review Phasors!Let’s review Phasors!



Phasors & complex #Phasors & complex #’’ssWorking with Working with harmonic fieldsharmonic fields is easier, but is easier, but

requires knowledge of requires knowledge of phasorphasor, let, let’’s review s review complex numberscomplex numbers and and phasorsphasors


COMPLEX NUMBERS:COMPLEX NUMBERS: Given a complex number Given a complex number zz

wherewhere

sincos jrrrrejyxz j

magnitude theis || 22 yxzr

angle theis tan 1

xy


Review:Review: Addition, Addition, Subtraction, Subtraction, Multiplication, Multiplication, Division, Division, Square Root, Square Root, Complex ConjugateComplex Conjugate


For a Time-varying phaseFor a Time-varying phase

Real and imaginary parts are:Real and imaginary parts are:


PHASORSPHASORS For a sinusoidal current For a sinusoidal current equals the real part of equals the real part of tjj

o eeI

joeI

tje

sI

The complex term which results from The complex term which results from dropping the time factor dropping the time factor is called the is called the phasor current, denoted by (phasor current, denoted by (s comes from sinusoidal)


Advantages of Advantages of phasorsphasors TimeTime derivativederivative in time is equivalent to in time is equivalent to

multiplying its phasor by multiplying its phasor by jj

TimeTime integralintegral is equivalent to dividing by is equivalent to dividing by the same term.the same term.

sAjtA

jA

tA s


How to How to changechange back from back from Phasor to tPhasor to time domainime domain

The phasor is The phasor is 1.1.multiplied by the time factor, multiplied by the time factor, e e jjtt, , 2.2.and taken the real part.and taken the real part.

}Re{ tjseAA

Time Harmonic Time Harmonic FieldsFields

9.79.7


Time-Harmonic fields Time-Harmonic fields (sines and cosines)(sines and cosines)

The wave equation can be derived from The wave equation can be derived from Maxwell equations, indicating that the Maxwell equations, indicating that the changes in the fields behave as a wave, changes in the fields behave as a wave, called an called an electromagneticelectromagnetic wave or field. wave or field.

Since any periodic wave can be Since any periodic wave can be represented represented as a sumas a sum of sines and cosines of sines and cosines (using Fourier), then we can deal only with (using Fourier), then we can deal only with harmonic fields to simplify the equations.harmonic fields to simplify the equations.


tDJH

tBE

0 B

vD

Maxwell Equations Maxwell Equations for Harmonic fields for Harmonic fields

(phasors)(phasors)Differential form* Differential form*

GaussGauss’’ss Law for E field. Law for E field.

GaussGauss’’ss Law for H field. Law for H field. No monopoleNo monopole

FaradayFaraday’’ss Law Law

AmpereAmpere’’ss Circuit Law Circuit Law

vE

0 H

HjE

* (substituting and )ED BH

Ex. Given E, find HEx. Given E, find H EE = E = Eoo cos( cos(t-t-z) z) aaxx


Ex. 9.23Ex. 9.23 In free space, In free space,

Find Find k, Jk, Jdd and and H using phasors and H using phasors and Maxwells eqs. Recall:Maxwells eqs. Recall:


mVkztE /)10cos(50 8

electromagnetism inel 4152 ch 9 sandra cruz-pol, ph. d. ece uprm mayag ü ez, pr

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