electromagnetic waves chapter 4. 21.11 introduction: maxwell’s equations electricity and magnetism...
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Electromagnetic WavesElectromagnetic Waves
Chapter 4Chapter 4
21.11 Introduction: 21.11 Introduction: Maxwell’s equationsMaxwell’s equations
Electricity and Electricity and magnetism were magnetism were originally thought to originally thought to be unrelatedbe unrelated
in 1865, James Clerk in 1865, James Clerk Maxwell provided a Maxwell provided a mathematical theory mathematical theory that showed a close that showed a close relationship between relationship between all electric and all electric and magnetic phenomenamagnetic phenomena
Maxwell’s Equations and Hertz’s
Discoveries
the line integral of the magnetic field around any closed path is the sum of μ0 times the net current through that path and μ0ε0 times the rate of change of electric flux through any surface bounded by that path.
Maxwell’s predictionsMaxwell’s predictions Electric field lines originate on positive charges and terminate on negative Electric field lines originate on positive charges and terminate on negative
chargescharges ElectricElectric field is produced by field is produced by chargescharges
Magnetic field lines always form closed loops – they do not begin or end Magnetic field lines always form closed loops – they do not begin or end anywhereanywhere
MagneticMagnetic filed is produced by currents ( filed is produced by currents (moving chargesmoving charges)) A varying magnetic field induces an emf and hence an electric field A varying magnetic field induces an emf and hence an electric field
(Faraday’s Law)(Faraday’s Law) ElectricElectric field is also produced by field is also produced by changing magnetic fieldchanging magnetic field
Magnetic Magnetic field is also produced byfield is also produced by changing electric field. changing electric field.
Question:Question: is there a is there a symmetrysymmetry between electric and magnetic fields, between electric and magnetic fields, i.e. can i.e. can magnetic fieldmagnetic field be produced by be produced by changing electric fieldchanging electric field??????
Maxwell: YES!!!
Maxwell’s PredictionsMaxwell’s Predictions If magnetic field can create electric field and vice versa, there is If magnetic field can create electric field and vice versa, there is
a very interesting phenomenon to be observeda very interesting phenomenon to be observed
Changing electric field produces magnetic field…Changing electric field produces magnetic field… … … which in turn produces changing electric one (but: energy which in turn produces changing electric one (but: energy
conservation)conservation) … … which in turn produces changing magnetic field…which in turn produces changing magnetic field…
Maxwell concluded that visible light and all other Maxwell concluded that visible light and all other electromagnetic waves consist of fluctuating electric and electromagnetic waves consist of fluctuating electric and magnetic fields, with each varying field inducing the othermagnetic fields, with each varying field inducing the other
Maxwell calculated the speed of light to be 3x10Maxwell calculated the speed of light to be 3x1088 m/s m/s
Note: Charges and FieldsNote: Charges and Fields
StationaryStationary charges produce only charges produce only electric fieldselectric fields ChargesCharges in in uniform motionuniform motion (constant velocity) (constant velocity)
produce produce electricelectric and and magnetic fieldsmagnetic fields ChargesCharges that are that are acceleratedaccelerated produce produce electric and electric and
magnetic fields and electromagnetic wavesmagnetic fields and electromagnetic waves
These fields are These fields are in phasein phase At any point, both fields reach their maximum At any point, both fields reach their maximum
value at the same timevalue at the same time
Electromagnetic WavesElectromagnetic WavesEM waves can be produced by an antenna, which is just some kind of wire that is connected to an ac source. The ac source produces oscillating + and - charges which set up electric field (due to the separation of charge) and a magnetic field (due to the current in the wire).
Note that the electric and magnetic fields are perpendicular to each other. This field begins to move away from the antenna and in a little while the ac source has caused the situation to reverse.
Electromagnetic Waves are Electromagnetic Waves are Transverse WavesTransverse Waves
TheThe E E and and BB fields are fields are perpendicularperpendicular to each other to each other
Both fields are perpendicular to Both fields are perpendicular to the direction of motionthe direction of motion Therefore, em waves are Therefore, em waves are
transverse wavestransverse waves
1 LC
Review problem: car radioReview problem: car radio
An An RLC RLC circuit is used to tune a radio to an FM station broadcasting at circuit is used to tune a radio to an FM station broadcasting at 88.9 MHz. The resistance in the circuit is 12.0 Ω and the capacitance is 88.9 MHz. The resistance in the circuit is 12.0 Ω and the capacitance is 1.40 pF. What inductance should be present in the circuit? 1.40 pF. What inductance should be present in the circuit?
An RLC circuit is used to tune a radio to an FM station broadcasting at 88.9 MHz. The resistance in the circuit is 12.0 Ω and the capacitance is 1.40 pF. What inductance should be present in the circuit?
Given:
RLC circuitf0=88.9 HzR = 12.0 C = 1.40 pF
Find:
L=?
The resonance frequency of the circuit should be chosen to match that of the radio station
0 2 20
1 1or
42f L
f CLC
This is sufficient to know for a solution, as we know all of the quantities on the right hand side
6
22 3 12
12.29 10
4 88.9 10 1.40 10L H
Hz F
Properties of EM WavesProperties of EM Waves Electromagnetic waves are transverse wavesElectromagnetic waves are transverse waves Electromagnetic waves travel at the speed of lightElectromagnetic waves travel at the speed of light
Because em waves travel at a speed that is precisely the speed Because em waves travel at a speed that is precisely the speed of light, of light, light is an electromagneticlight is an electromagnetic wave wave
812.99792 10
o o
c m s
The sun is about 1.5x10The sun is about 1.5x101111 m from the earth. How long does it take light to m from the earth. How long does it take light to get here? get here?
11
8
distance 1.5 10500 sec 8.3 min
time 3 10
mt
m s
QuestionQuestion
Properties of EM Waves, 2Properties of EM Waves, 2
The ratio of the electric field to the magnetic The ratio of the electric field to the magnetic field is equal to the speed of lightfield is equal to the speed of light
Electromagnetic waves carry energy as they Electromagnetic waves carry energy as they travel through space, and this energy can be travel through space, and this energy can be transferred to objects placed in their pathtransferred to objects placed in their path
B
Ec
Properties of EM Waves, 3Properties of EM Waves, 3
Energy carried by em waves is Energy carried by em waves is shared equally by the electric and shared equally by the electric and magnetic fieldsmagnetic fields
o
2max
o
2max
o
maxmax
2
Bc
c2
E
2
BE
areaunitperpowerAverage
Properties of EM Waves, Properties of EM Waves, finalfinal
Electromagnetic waves transport Electromagnetic waves transport linear momentum as well as energylinear momentum as well as energy For complete absorption of energy U, For complete absorption of energy U,
p=U/cp=U/c For complete reflection of energy U, For complete reflection of energy U,
p=(2U)/cp=(2U)/c Radiation pressures can be Radiation pressures can be
determined experimentallydetermined experimentally
The Spectrum of EM WavesThe Spectrum of EM Waves Forms of electromagnetic waves exist that Forms of electromagnetic waves exist that
are distinguished by their frequencies and are distinguished by their frequencies and wavelengthswavelengths
c = ƒλc = ƒλ
Wavelengths for visible light range from Wavelengths for visible light range from 400 nm to 700 nm400 nm to 700 nm
There is no sharp division between one There is no sharp division between one kind of em wave and the nextkind of em wave and the next
The EMThe EMSpectrumSpectrum
Note the overlap Note the overlap between types of between types of waveswaves
Visible light is a Visible light is a small portion of small portion of the spectrumthe spectrum
Types are Types are distinguished by distinguished by frequency or frequency or wavelengthwavelength
Notes on The EM SpectrumNotes on The EM Spectrum
Radio WavesRadio Waves Used in radio and television Used in radio and television
communication systemscommunication systems MicrowavesMicrowaves
Wavelengths from about 1 mm to 30 cmWavelengths from about 1 mm to 30 cm Well suited for radar systemsWell suited for radar systems Microwave ovens are an applicationMicrowave ovens are an application
Notes on the EM Spectrum, Notes on the EM Spectrum, 22
Infrared wavesInfrared waves Incorrectly called “heat waves”Incorrectly called “heat waves” Produced by hot objects and Produced by hot objects and
moleculesmolecules Readily absorbed by most materialsReadily absorbed by most materials
Visible lightVisible light Part of the spectrum detected by the Part of the spectrum detected by the
human eyehuman eye Most sensitive at about 560 nm Most sensitive at about 560 nm
(yellow-green)(yellow-green)
Notes on the EM Spectrum, Notes on the EM Spectrum, 33
Ultraviolet lightUltraviolet light Covers about 400 nm to 0.6 nmCovers about 400 nm to 0.6 nm Sun is an important source of uv lightSun is an important source of uv light Most uv light from the sun is absorbed in the Most uv light from the sun is absorbed in the
stratosphere by ozonestratosphere by ozone X-raysX-rays
Most common source is acceleration of high-Most common source is acceleration of high-energy electrons striking a metal targetenergy electrons striking a metal target
Used as a diagnostic tool in medicineUsed as a diagnostic tool in medicine
Notes on the EM Spectrum, Notes on the EM Spectrum, finalfinal
Gamma raysGamma rays Emitted by radioactive nucleiEmitted by radioactive nuclei Highly penetrating and cause serious Highly penetrating and cause serious
damage when absorbed by living tissuedamage when absorbed by living tissue Looking at objects in different Looking at objects in different
portions of the spectrum can produce portions of the spectrum can produce different informationdifferent information
Example: talking to a Example: talking to a submarinesubmarine
The U.S. Navy has long proposed the construction of extremely low-The U.S. Navy has long proposed the construction of extremely low-frequency (ELF) communications systems; such waves could penetrate frequency (ELF) communications systems; such waves could penetrate the oceans to reach distant submarines. Calculate the length of a quarter-the oceans to reach distant submarines. Calculate the length of a quarter-wavelength antenna for a transmitter generating ELF waves of frequency wavelength antenna for a transmitter generating ELF waves of frequency 75 Hz. How practical is this?75 Hz. How practical is this?
The U.S. Navy has long proposed the construction of extremely low-frequency The U.S. Navy has long proposed the construction of extremely low-frequency (ELF) communications systems; such waves could penetrate the oceans to reach (ELF) communications systems; such waves could penetrate the oceans to reach distant submarines. Calculate the length of a quarter-wavelength antenna for a distant submarines. Calculate the length of a quarter-wavelength antenna for a transmitter generating ELF waves of frequency 75 Hz. How practical is this?transmitter generating ELF waves of frequency 75 Hz. How practical is this?
Given:
¼ wavelengthantennaf0=75 Hz
Find:
L=?
First determine the wavelength, a forth of which will give us the length of an antenna
863.00 10
4.00 10 400075
v m sm km
f Hz
The required length of antenna is then a quarter of this
40001000
4 4
kmL km
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