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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.html7/27/2019 Chapter 8 Ppt.maxwell.important
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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