Chapter 8 Ppt.maxwell.important

7/27/2019 Chapter 8 Ppt.maxwell.important

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ElectromagnetismChapter 8


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Summary of Important Equations tounderstand

for the HW:1. Vo No

--- = ---

Vi

Ni

2. v = c = f

3. max =0.0029/T


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Magnetism and The Magnetic Field

Understanding introduction to magnetism (10

mins)

Standard Deviants on Earth's magnetic field (10mins)

Earth's geographic north precesses and magnetic

north also moves around Transparency 1: Fig. 8.6 on p. 280


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Electricity and Magnetism

Moving electric charges (currents) produce magnetic fields (Right-HandRule)

Examples: solenoids, electrons in orbit around nucleus, protons andelectrons spinning around, etc.

When electron domains align (say, with external H), ferromagnetbecomes magnetic

Magnetic Field exerts force on a current carrying wire (that'sperpendicular)

Electricity and Magnetism are both different manifestations of thesame thing -- charge!

Magnetic fields used to trap plasmas and in particle accelerators

A moving magnet produces a circularelectric field in the space aroundit

Coil of wire in motion will have current induced in it --Electromagnetic Induction

This is the principle behind AC generators

Coil of wire is rotated in a magnetic field and produces an electriccurrent


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Electromagnetism

Changing Electric Field (or moving

charges/current) induces a

magnetic field Changing Magnetic Field induces

an electric Field

Changing can mean direction orstrength


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Transformers (more than meets the eye):

Steps up ordown AC Voltages

Two coils close to each other

AC in the inputcoil induces an oscillating magnetic fieldthrough both coils

This changingmagnetic field produces an AC current in theoutputcoil

DC current would produce a steadymagnetic field in theinputcoil and would not induce a current in the output coil

Each loop of the output coil has same induced voltage

Therefore, more loops (in output coil) == more output

voltage (and vis versa) Ratio ofnumber of turns in the coils determines ratio of input

and output voltages Vo No--- = ---

Vi Ni


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In Class Exercise #1: A transformer is required to take a 120-V inputvoltage to a

600-V outputvoltage. If the inputcoil has 200 turns thenhow many turns should the outputcoil have?

Known Unknown

Vi = 120V

Ni = 200turns

No = ?turns

Vo/Vi = No/Ni

Vo = 600V


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Electromagnetic Waves Introduction Imagine a charge is pushed forward and backward

someplace (oscillates) What does the Electric Field look like? Pushed forward and

backward (increases then decreases)

Since we know E extends out to infinity, an oscillation increasesthen decreases this whole field (remember, field drops off inmagnitude the farther out it is since E = F/Q)

But we know changingelectric fields induce magnetic fields

But this induced magnetic field also increases anddecreases (also oscillating since it's induced by theoscillating electric field)

And we know changingmagnetic fields induce electric fields Thus, an endless "loop" is established -- this combination of

oscillating electric and magnetic fields is a transverse wavecalled an electromagnetic wave


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EM Waves (contd.)

Transverse because both fields oscillateperpendiculartodirection of propagation

Electric Field wave and Magnetic Field wave cannotexistseparately

Travel at the speed of light (so-called because it was firstmeasured for visible light), c = 3 x 108m/s

c stands for celeritas, which is Latin for swift

velocity = v = c = frequency * wavelength = f

Amplitude is the maximum value of the electric field and isproportional to the strength of the wave

Standard Deviants on Electromagnetism and light, spectra,etc.


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In Class Exercise #2: What is the wavelength, , of an EM wave broadcast by

the radio station 95.5 FM?

velocity = c = * f

Known Unknown

f = 95.5MHz = ?m

c = 3 x 108m/s


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BlackBody Radiation

Temperature affects amount and types of radiation emitted

Everyobject emits EM radiation because of the thermal

motion of its atoms

Blackbody:perfectabsorberand emitter of radiant energy Foreach Temperature, T, the distribution of radiant heat

emission is characterized by a curve with a characteristic

peak at a certain wavelength,

The size and shape of the radiation curve changes with theobject's temperature

Thepeakalso changes with temperature: max = 0.0029m-K/T


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Mainly IR emitted All objects emit manytypes of radiation; the amount

of each increases with temperature IR can be emitted or reflected, just like all light, but

IR light is the peak wavelength emitted by allobjects with a Temp between about 9 K and 700 K

(see here and problem 14) Sample IR photographs of objects emitting, or

reflecting, IR radiation (courtesy ofhttp://www.holly-cam.com/):

http://holly.mine.nu:8080/holly/irfairyreaching.jpg

http://holly.mine.nu:8080/holly/iralmondchurchnew.jpg

http://holly.mine.nu:8080/holly/irstatuenew.jpg
http://emma.la.asu.edu/MARS_SURVEYOR/MGSTES/TIR_description.htmlhttp://www.holly-cam.com/http://www.holly-cam.com/http://holly.mine.nu:8080/holly/irfairyreaching.jpghttp://holly.mine.nu:8080/holly/iralmondchurchnew.jpghttp://holly.mine.nu:8080/holly/iralmondchurchnew.jpghttp://holly.mine.nu:8080/holly/irstatuenew.jpghttp://holly.mine.nu:8080/holly/irstatuenew.jpghttp://holly.mine.nu:8080/holly/iralmondchurchnew.jpghttp://holly.mine.nu:8080/holly/iralmondchurchnew.jpghttp://holly.mine.nu:8080/holly/irfairyreaching.jpghttp://www.holly-cam.com/http://www.holly-cam.com/http://www.holly-cam.com/http://emma.la.asu.edu/MARS_SURVEYOR/MGSTES/TIR_description.html


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In Class Exercise #3: Assuming that the human body is a blackbody with a

temperature of 310 K, at what wavelength, , does it radiatethe most energy?

max = 0.0029m-K/T

Known Unknown

T = 300K peak= ?m

M ll' E ti i I t l


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Maxwell's Equations in IntegralForm (very optional)

Note: the integrals should be closedintegrals o E dS = q says that charges (q) produce

electric (E) fields

B dS= 0 says there are no such things asmagnetic charges/monopoles

B dl= o(o dE/dt + i) says magnetic fieldsare produced both by currents (i) and by changingelectric fields

E dl = -dB/dt says electric (E) fields areproduced by changingmagnetic fields


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Differential Form (Optional)

In differential form (see here and here for more):

E = o= 4 (in cgs)

B = 0

B =ooE t +o

J

=1

cE t +4

cJ (incgs)

E = - B t = - 1cB t (in cgs)
http://courses.ece.uiuc.edu/ece229/lectrs2/lecture20.htmlhttp://scienceworld.wolfram.com/physics/MaxwellEquations.htmlhttp://scienceworld.wolfram.com/physics/MaxwellEquations.htmlhttp://courses.ece.uiuc.edu/ece229/lectrs2/lecture20.html

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Chapter 8 Ppt.maxwell.important