Radio 1 Radio Radio 2 Observations about Radio
Radio 1 Radio
Radio 2 Observations about Radio Transmit sound long distances without wires n Involve antennas n Seem to involve electricity and magnetism n Reception depends on antenna positioning n Reception weakens with distance n Two styles of radio: AM and FM n
Radio 3 3 Questions about Radio How can a radio wave exist? n How is a radio wave emitted and received? n How can a radio wave represent sound? n
Radio 4 Question 1 n How can a radio wave exist? What is an electromagnetic wave anyway? n How can an electric field exist without charge? n How can a magnetic field exist without pole? n
Radio 5 Electromagnetic Waves n Radio waves are a type of electromagnetic wave n n a structure in space consisting only of an electric field and a magnetic field Electromagnetic waves are emitted or received by charge or pole n but are self-sustaining while traveling in vacuum. n Their electric and magnetic fields create one another n
Radio 6 Electromagnetism (Version 3) n Magnetic fields are produced by magnetic poles (but free poles don’t seem to exist), n moving electric charges, n and changing electric fields. n n Electric fields are produced by electric charges, n moving magnetic poles, n and changing magnetic fields. n
Radio 7 Structure of a Radio Wave Electric field is perpendicular to magnetic field n Changing electric field creates magnetic field n Changing magnetic field creates electric field n Polarization of the wave is associated with the wave’s electric field n
Radio 8 Question 2 n How is a radio wave emitted and received?
Radio 9 Launching a Radio Wave Accelerating charge emits electromagnetic waves n The more charge and the harder it accelerates, the stronger the resulting electromagnetic wave n Radio stations need to emit strong waves n so they move large amounts of charge n with the help of resonant devices: tank circuits n
Radio 10 A Tank Circuit is a harmonic oscillator for charge and current n Charge flows regularly back and forth from one capacitor plate to the other, through the inductor. n Its total energy shifts back and forth between its inductor and its capacitor. n
Radio 11 Tank Circuit Oscillation The frequency of the tank circuit is determined only by its inductance and its capacitance. n It can store a great deal of energy and have an enormous amount of charge moving through it. n Tank circuits come in many shapes, including straight lines! n
Radio 12 Tank Circuits in Radio Tanks build up energy at a specific frequency n Two tanks help radios emit radio waves n The first tank circuit feeds the antenna n The second tank circuit is the antenna n n Two similar tanks help radios detect radio waves
Radio 13 Emitting Radio Waves (Part 1) A transmitter uses a tank circuit to “slosh” charge up and down its antenna, which acts as a second tank. n A receiver uses a tank circuit to detect charge “sloshing” on its tank-circuit antenna. n Transmitter antenna charge affects receiver antenna charge n Antenna orientations matter! n
Radio 14 Emitting Radio Waves (Part 2) n Accelerating charge emits radio waves Charge produces electric field n Current produces magnetic field n Changing current produces changing magnetic field, produces changing electric field, prod… n A radio wave consists only of an electric and magnetic field n A radio wave travels through empty space at the speed of light n
Radio 15 Question 3 n How can a radio wave represent sound?
Radio 16 AM Modulation Information can be encoded as a fluctuating amplitude of the radio wave n The air pressure variations that are sound cause changes in the amount of charge moving on the antenna n
Radio 17 FM Modulation Information can be encoded as a fluctuating frequency of the radio wave n The air pressure variations that are sound cause slight shifts in the frequency of charge motion on the antenna n
Radio 18 Summary about Radio waves are emitted by charge accelerating on a transmitting antenna n Radio waves are detected when they cause charge to accelerate on a receiving antenna n Radio waves consist only of self-sustaining electric and magnetic fields n Radio waves can represent sound information as variations in amplitude or frequency n
- Slides: 18