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Electromagnetic Waves

Published in: Physics
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  • Shashank S

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The subject of this note is about Electromagnetic Waves for 11th standard.

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    ELECT 0 AG TIC ES
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    Basics Electrostatics — charges at rest Electrodynamics charges in motion Magnetostatics — charges moving with constant velocity Electromagnetism — accelerating charges
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    What are electromagnetic waves ? Electromagnetic waves are periodic changes in electric and magnetic fields which propagate through space and can propagate through any medium. James Maxwell predicted their existence in 1870's Hertz proved that they exist in 1888
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    Hertz experiment sphere 1 Induction coil sphere 2 spark gap loop of wire
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    FIBRE TUBE PR M ARV IRON CORE SECON CONDENSER TO BATTERY
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    Characteristics of EM waves Time varying electric and magnetic fields such that they are perpendicular to each other and also to the direction of propagation. They can travel through any medium They are produced by accelerating charges the formula is c= 1 ax 108 They travel with the speed of light c = m/s
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    Transverse nature of EM waves direction of propagation
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    The electric and magnetic fields are perpendicular to each other and also they are perpendicular to the direction of propagation They vary sinusoidally with x, hence they are mathematically given as, Ey = Eo sin (kx - wt) BZ = Bo sin (kx - wt) Where k = propagation constant Propagation is along E x B
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    Electromagnetic spectrum The orderly distribution of EM waves according to their wavelengths (or frequency) in different groups is known as EM spectrum.
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    Energy increases Short wavelength 103nm 106 nm 10- nm Long wavelength 10-5 nm Gamma rays 1024Hz 1022 Hz High frequency 1 nm X rays 1020Hz 1018 Hz 7 x 1014Hz Ultraviolet 1016 Hz Infrared 1012 Hz Microwaves 1010Hz 108 Hz 4X 1014Hz 1m 106 Hz 103m Radio waves 104 1-Iz 102 Hz Low frequency Visible light
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    y-rays Highly penetrating can penetrate through lead and iron Affect photographic plates Produce fluorescence in ZnS 10 12 m to 10 10 m wavelength Used for preservation of food Used in radiotherapy for treatment of cancer X-rays Less penetrating than y-rays can penetrate through sheets of metal and paper Affect photographic plates Produce fluorescence in ZnS 10 11 m to 10-8 m wavelength Used for studying crystals Used to distinguish real diamonds and artificial ones UV rays Can pass through quartz, rock salt Affect photographic plates Produce fluorescence in ZnS 10-8 m to 10-7 m wavelength Used in burglar alarms Used in study of molecular structure
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    Microwaves Obey laws of reflection and refraction They heat an object on which they are incident 10-4 m to 10-2 m wavelength Used in microwave ovens Used for transmission of TV signals Used sometimes in radar technology Radiowaves Obey laws of reflection and refraction They do not heat an object on which they are incident instead they diffract from them. 10-4 m to 105 m wavelength Radio broadcasting Used for wireless communication Used in cellular transmission and reception
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    Layers of propagation Troposphere Stratosphere Mesosphere Ionosphere (thermosphere) exosphere
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    Stratosphere ozono layer Troposphere Thermosphere 90 km 80 km 70 km Mesosphere 60 km 40 tcm 20 km IO km km —
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    Atmosphere Exosphere—The last layer of the atmosphere, Difficult to tell where it stops and space begins, Very few atoms in this layer of the atmosphere The air is really thin that high up- The temperature changes with the act' . It,- If the sun is active, temperatures in the thermosphere can get up to 1,500+C or higher! Mesosphere—The drops when you go higher, like it does in the troposphere. Coldest part of the atmosphere tratosp 10 km to 50 km above Earth's surface, this layer contains ozone that absorbs much of the Sun's ultraviolet radiation. roposp nere—-Contains most clouds and weather. , Most of the troposphere's heat is from Earth, Temperature cools about 6.5 degrees Celsius per kilometer of altitude.
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    Types of propagation of electromagnetic waves Ground wave propagation : The radio waves from the transmitting antenna travel along the surface to reach the receiving antenna Surface wave propagation Cannot be transmitted over large distances 2 MHz Used for local broadcasting
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    Space wave propagation : The radio waves from the transmitting antenna reach the receiving antenna directly or after reflection from the ground after reflection from troposphere Tropospheric waves 100-200 MHz Space wave propagation is best option for TV signals
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    Sky wave propagation : When the radio waves from transmitting antenna reach the receiving antenna directly or after reflection in the ionosphere. 3 MHz-30 MHz Can be transmitted over large distances
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    Modes Of propagation of EM waves Ionosphe transmitter Earth
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    Geostationary satellites Geostationa Orbit 22300 miles 36,000 kilometers

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