Download Electromagnetic Waves

2011년 11월 4일
제8장
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Magnetism
Interactions between Electricity and
Magnetism
Principles of Electromagnetism
Applications to Sound Reproduction
Electromagnetic Waves
Blackbody Radiation
EM Waves and the Earth’s Atmosphere
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Lodestone
of
Magnesia
Compass
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Like poles repel, unlike poles attract
S
N
N
S
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Fish, frogs, turtles, birds, newts, whales
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자기부상
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Two kind of pole and two kind of charges
Like poles(charges) repels, unlike
poles(charges) attract
But no separated magnetic pole (no magnetic
monopole)
Seems to be independent (only for static)
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Almost all devices everyday we use
Electricity and Magnetism
interact with each other
ELECTROMAGNETISM
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Charge create electric field
Electric field cause a force on a charge
Magnet create magnetic field
Magnetic field cause a force on a magnet
𝑭
𝑩
𝑭
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A moving electric charge produces a
magnetic field in the space around it
An electric current produces a magnetic field
around it
Solenoid
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A magnetic field exerts a force on a moving
electric charge
Thus magnetic field exerts a force on a
current-carrying wire
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F  qv  B
Lorentz Force
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A moving magnet produces an electric field in
the space around it
A coil of wire moving through a magnetic
field has a current induced in it
Faradya’s Law
Animation
Electricity and magnetism
is not independent
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An electric current or a changing electric field
induces a magnetic field
A changing magnetic field induces an electric
field
+
+
+
𝑉𝑜
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𝑉𝑖
=
𝑁𝑜
𝑁𝑖
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120V input and 600V output
800 turns input coil, output coil turns?
𝑉𝑜 𝑁o
=
𝑉𝑖 𝑁𝑖
𝑁𝑜 = 4000turns
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Sound to AC by dynamic microphone
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AC to sound by speaker
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CD or DVD
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Electromagnetic Wave(EM Wave)
◦ A transverse wave consisting of a combination of
oscillating electric and magnetic fields
𝑐 = 299,792,458 𝑚/𝑠
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Radio waves, microwaves, infrared
radiation(IR), visible light, ultraviolet
radiation(UV), x-rays, gamma rays
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100 MHz -> wavelength?
𝑐 = 𝑓𝜆
𝜆 =3𝑚
𝑢
=
1
2
𝜀𝐸
2
+
1 𝐵2
2 𝜇
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100 Hz ~ 1GHz
ELF(extremely low frequency)
VHF(very high frequency)
UHF(ultra high frequency)
AM, FM, TV
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1 GHz ~ 1 THz
0.3 m ~ 0.3 mm
Wi-Fi, cell phones, microwave oven(2.45 GHz,
122mm), radar
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+
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1 THz ~ 400 THz
0.3 mm ~ 0.75 𝜇𝑚
Heat radiation, IR commander
Color
Frequency range
(× 𝟏𝟎𝟎 𝑻𝑯𝒛)
Wavelength range
(× 𝟎. 𝟏𝝁𝒎)
Red
4.0-4.8
7.5-6.3
Orange
4.8-5.1
6.3-5.9
Yellow
5.1-5.4
5.9-5.6
Green
5.4-6.1
5.6-4.9
Blue
6.1-6.7
4.9-4.5
Violet
6.7-7.5
4.5-4.0
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750 THz ~ 1 EHz(1018 𝐻𝑧)
Suntans and sunburns, sterilize
Ozone layer protect us
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10 PHz ~ 100 EHz (1016 𝐻𝑧 ~ 1020 𝐻𝑧)
10 nm ~ 0.01 nm
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Used for medical image
It may harmful by ionization (allowed dose)
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More than 30 EHz
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The amount of each type of radiation emitted
increases with temperature
The total amount of energy per unit time per
unit area increases with temperature
𝑃~𝑇 4
At higher temperature, more of the power is
emitted at successively shorter wavelengths
of electromagnetic radiation
0.0029
𝜆𝑚𝑎𝑥 =
𝑚
𝑇
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Sun’s temperature is 6000K, 𝜆𝑚𝑎𝑥 ?
𝜆𝑚𝑎𝑥
0.0029
=
= 4.8 × 10−7 𝑚 = 480𝑛𝑚
𝑇
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Ozone Layer : 20km ~ 40 km
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50km ~ 90 km
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Compton Gamma-Ray Observatory(1991)
Chandra X-Ray Observatory(1999)
Spitzer Space Telescope(IR, 2003)
Gravitational Wave Observatory(future, ??)