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2011년 11월 4일 제8장 Magnetism Interactions between Electricity and Magnetism Principles of Electromagnetism Applications to Sound Reproduction Electromagnetic Waves Blackbody Radiation EM Waves and the Earth’s Atmosphere Lodestone of Magnesia Compass Like poles repel, unlike poles attract S N N S Fish, frogs, turtles, birds, newts, whales 자기부상 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) Almost all devices everyday we use Electricity and Magnetism interact with each other ELECTROMAGNETISM 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 A moving electric charge produces a magnetic field in the space around it An electric current produces a magnetic field around it Solenoid A magnetic field exerts a force on a moving electric charge Thus magnetic field exerts a force on a current-carrying wire F qv B Lorentz Force 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 An electric current or a changing electric field induces a magnetic field A changing magnetic field induces an electric field + + + 𝑉𝑜 𝑉𝑖 = 𝑁𝑜 𝑁𝑖 120V input and 600V output 800 turns input coil, output coil turns? 𝑉𝑜 𝑁o = 𝑉𝑖 𝑁𝑖 𝑁𝑜 = 4000turns Sound to AC by dynamic microphone AC to sound by speaker CD or DVD Electromagnetic Wave(EM Wave) ◦ A transverse wave consisting of a combination of oscillating electric and magnetic fields 𝑐 = 299,792,458 𝑚/𝑠 Radio waves, microwaves, infrared radiation(IR), visible light, ultraviolet radiation(UV), x-rays, gamma rays 100 MHz -> wavelength? 𝑐 = 𝑓𝜆 𝜆 =3𝑚 𝑢 = 1 2 𝜀𝐸 2 + 1 𝐵2 2 𝜇 100 Hz ~ 1GHz ELF(extremely low frequency) VHF(very high frequency) UHF(ultra high frequency) AM, FM, TV 1 GHz ~ 1 THz 0.3 m ~ 0.3 mm Wi-Fi, cell phones, microwave oven(2.45 GHz, 122mm), radar - + 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 4.5-4.0 750 THz ~ 1 EHz(1018 𝐻𝑧) Suntans and sunburns, sterilize Ozone layer protect us 10 PHz ~ 100 EHz (1016 𝐻𝑧 ~ 1020 𝐻𝑧) 10 nm ~ 0.01 nm Used for medical image It may harmful by ionization (allowed dose) More than 30 EHz 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 Sun’s temperature is 6000K, 𝜆𝑚𝑎𝑥 ? 𝜆𝑚𝑎𝑥 0.0029 = = 4.8 × 10−7 𝑚 = 480𝑛𝑚 𝑇 Ozone Layer : 20km ~ 40 km 50km ~ 90 km Compton Gamma-Ray Observatory(1991) Chandra X-Ray Observatory(1999) Spitzer Space Telescope(IR, 2003) Gravitational Wave Observatory(future, ??)