Understanding Radios and Electromagnetic Waves: The Role of Tank Circuits and Antennas, Study notes of Classical Physics

Download Understanding Radios and Electromagnetic Waves: The Role of Tank Circuits and Antennas and more Study notes Classical Physics in PDF only on Docsity! 13.1 Radios and Electromagnetic waves Transmitter building Transmission line Antenna Transmission tower Guy wire Collapsible antenna = Frequency selector Volume controls Frequency indicator Anchors a i New ideas for today: •Electrical resonators (tank circuits) •Electromagnetic waves (light) •How FM and AM radio works Tank circuits are used to turn small amounts of sloshing charges into large amounts of sloshing charges! Tank circuits are all around you: • Cell phone • Any watch with a battery / electronic clock • Computers… (b) Current Current Inductor Capacitor i Magnetic field Inductor Capacitor Inductor Capacitor Magnetic field Current (b) Current 8 5 8 g g é Inductor Inductor Capacitor Bla) .arellmel lh Magnetic field Magnetic field Capacitor Current Inductor Inductor Current Magnetic field Inductor Magnetic field Capacitor Just two plates of metal •Stores charges •Stores energy – it takes work to separate charges that attract! This is just like a pendulum! Potential energy Kinetic energy Potential energy Ever wonder how a traffic light “knows” that a car is waiting? That loop of wire is part of a tank circuit…and your car changes its inductance! Alice and Bob each use a hand- cranked generator to charge a capacitor. Who has done more work? + + + - - - + + ++++++ - - - - - - - - (A) (B) Clicker question Antennas that are far apart… …are connected by electromagnetic radio waves! •Accelerating charges make electromagnetic waves •Electromagnetic waves cause charges to accelerate •Electromagnetic waves and light are the same thing! •They move at the speed of light: 186,282 miles/sec •They have two “polarizations” •Are not like sound waves: do not require a “medium” Electromagnetic waves Heinrich Hertz: discovered radio waves (1885) and showed that they are light 1856-1943 Nikola Tesla Invented AC power, fluorescent lights…and the Tesla coil Wanted to transmit AC power using radio waves Tesla Coil _ ri nw — Steve Ward's and Jeff Lay Sled Twin Solid State MusicalJ eglalGoils wid Playing MarioBros , * Lightning on the Yawn 2007 aud Baraboo Wi . Ln Physics 140: How Things Work EM waves with AM frequencies bounce off of the ionosphere and can travel really far (1000s of miles) at night •37-190 miles above the earth •Very electrically charged! •FM frequency waves do not reflect from the ionosphere AM = Amplitude modulation FM = Frequency modulation Frequency modulation: information encoded in slight changes in the frequency Amplitude modulation: information is encoded in changes in the size of the electrical signal demodulator carrier signal sound