Radio Radio It involve antennas It involve antennas It apparently - PDF document

Radio 1 Radio 2 Observations about Radio Observations about Radio � It can transmit sound long distances wirelessly It can transmit sound long distances wirelessly Radio Radio � It involve antennas It involve antennas � It apparently involves electricity and magnetism It apparently involves electricity and magnetism � Its reception depends on antenna positioning Its reception depends on antenna positioning � Its reception weakens with distance Its reception weakens with distance � There are two styles of radio: AM and FM There are two styles of radio: AM and FM Radio 3 Radio 4 3 Questions about Radio 3 Questions about Radio Question 1 Question 1 � How can a radio wave exist? How can a radio wave exist? � How can a radio wave exist? How can a radio wave exist? � � How is a radio wave emitted and received? How is a radio wave emitted and received? � What is an electromagnetic wave anyway? What is an electromagnetic wave anyway? � � How can an electric field exist without charge? How can an electric field exist without charge? � How can a radio wave represent sound? How can a radio wave represent sound? � � How can a magnetic field exist without pole? � H How can a magnetic field exist without pole? H n n m m n ti fi ld n ti fi ld i t i t ith ith t p l ? t p l ? Radio 5 Radio 6 Electromagnetic Waves Electromagnetic Waves Electromagnetism (Version 3) Electromagnetism (Version 3) � Radio waves are a class of electromagnetic waves Radio waves are a class of electromagnetic waves � Magnetic fields are produced by Magnetic fields are produced by � Electromagnetic waves Electromagnetic waves � magnetic poles (but free poles don’t seem to exist), magnetic poles (but free poles don’t seem to exist), � moving electric charges, moving electric charges, � are structures in space that consist only of electric are structures in space that consist only of electric fields and magnetic fields. fields and magnetic fields. fields and magnetic fields fields and magnetic fields � nd h n in � and changing electric fields and changing electric fields. nd h n in l l tri fi ld tri fi ld . � They are emitted or received by charge or pole, They are emitted or received by charge or pole, � Electric fields are produced by Electric fields are produced by � but are self but are self- -sustaining while traveling in vacuum sustaining while traveling in vacuum � electric charges, electric charges, � because their electric and magnetic fields endlessly because their electric and magnetic fields endlessly � moving magnetic poles, moving magnetic poles, recreate one another. recreate one another. � and changing magnetic fields. and changing magnetic fields. 1

Radio 7 Radio 8 Structure of a Radio Wave Structure of a Radio Wave Question 2 Question 2 � Electric field is perpendicular to magnetic field Electric field is perpendicular to magnetic field � How is a radio wave emitted and received? How is a radio wave emitted and received? � � Changing electric field Changing electric field creates magnetic field creates magnetic field � Changing magnetic field Ch Ch Changing magnetic field i i i fi ld i fi ld creates electric field creates electric field � Polarization of the wave Polarization of the wave � is associated with the is associated with the wave’s electric field wave’s electric field Radio 9 Radio 10 Launching a Radio Wave Launching a Radio Wave A Tank Circuit A Tank Circuit � Accelerating charge emits electromagnetic waves Accelerating charge emits electromagnetic waves � A tank circuit A tank circuit � � The more charge and the more it accelerates, the The more charge and the more it accelerates, the � is a harmonic oscillator for charge is a harmonic oscillator for charge � stronger the resulting electromagnetic wave stronger the resulting electromagnetic wave � its charge flows rhythmically through the inductor, its charge flows rhythmically through the inductor, from one capacitor plate to the other and back again. from one capacitor plate to the other and back again. from one capacitor plate to the other and back again from one capacitor plate to the other and back again � Radio stations need to emit strong waves R di Radio stations need to emit strong waves R di i i d d i i � Its energy alternates between Its energy alternates between � so they move large amounts of charge so they move large amounts of charge � magnetic field in its inductor magnetic field in its inductor � with the help of resonant devices: tank circuits with the help of resonant devices: tank circuits � electric field in its capacitor electric field in its capacitor Radio 11 Radio 12 Tank Circuit Oscillation Tank Circuit Oscillation Tank Circuits in Radio Tank Circuits in Radio � The frequency of the tank The frequency of the tank � Tanks build up energy at a specific frequency Tanks build up energy at a specific frequency � circuit is determined only by its circuit is determined only by its � Two tanks help radios Two tanks help radios emit emit radio waves radio waves inductance and its capacitance. inductance and its capacitance. � The first tank circuit feeds the antenna The first tank circuit feeds the antenna � It can store a great deal of energy � It can store a great deal of energy It can store a great deal of energy It can store a great deal of energy � The second tank circuit IS the antenna The second tank circuit IS the antenna Th Th d d k i k i i IS h i IS h and have an enormous amount and have an enormous amount � Two similar tanks help radios Two similar tanks help radios detect detect radio waves radio waves of charge moving through it. of charge moving through it. � Tank circuits come in many Tank circuits come in many � shapes, including straight lines! shapes, including straight lines! 2

Radio 13 Radio 14 Emitting Radio Waves Emitting Radio Waves (Part 1) Emitting Radio Waves Emitting Radio Waves (Part 2) (Part 1) (Part 2) � A transmitter uses a tank circuit to “slosh” A transmitter uses a tank circuit to “slosh” � Accelerating charge emits radio waves Accelerating charge emits radio waves charge up and down its antenna, charge up and down its antenna, � Stationary charge produces an electric field Stationary charge produces an electric field which acts as a second tank. which acts as a second tank. � Steady current produces a magnetic field Steady current produces a magnetic field � A receiver uses a tank circuit to A receiver uses a tank circuit to � Changing current produces a changing magnetic Changing current produces a changing magnetic g g g g p p g g g g g g detect charge “sloshing” on its detect charge “sloshing” on its field, produces a changing electric field, prod… field, produces a changing electric field, prod… tank- tank -circuit antenna. circuit antenna. � A radio wave consists only of an electric and A radio wave consists only of an electric and magnetic field magnetic field � Transmitter antenna charge Transmitter antenna charge affects receiver antenna charge affects receiver antenna charge � A radio wave travels through empty space at the A radio wave travels through empty space at the speed of light speed of light � Antenna orientations matter! Antenna orientations matter! Radio 15 Radio 16 Introductory Question Introductory Question (revisited) Question 3 Question 3 (revisited) If you took an electrically charged ball and If you took an electrically charged ball and � How can a radio wave represent sound? How can a radio wave represent sound? � shook it up and down rapidly, charges in a shook it up and down rapidly, charges in a nearby metal object would move in response. nearby metal object would move in response. How far away could that metal object be and How far away could that metal object be and still respond? still respond? 1 meter 1 meter A. A. 1 kilometer 1 kilometer B. B. The other side of the universe The other side of the universe C. C. Radio 17 Radio 18 AM Modulation AM Modulation FM Modulation FM Modulation � Information can be encoded as a fluctuating Information can be encoded as a fluctuating � Information can be encoded as a fluctuating Information can be encoded as a fluctuating � amplitude of the radio wave amplitude of the radio wave frequency of the radio wave frequency of the radio wave � The air pressure variations The air pressure variations � The air pressure variations The air pressure variations � that are so nd ca se changes that are so nd ca se changes that are sound cause changes that are sound cause changes that are sound cause slight that are sound cause slight that are so nd ca se slight that are so nd ca se slight in the amount of charge in the amount of charge shifts in the frequency of shifts in the frequency of moving on the antenna and moving on the antenna and charge motion on the antenna charge motion on the antenna thus the intensity of the wave thus the intensity of the wave and the frequency of the wave and the frequency of the wave � The receiver detects these The receiver detects these � The receiver detects these The receiver detects these � changes in radio wave intensity. changes in radio wave intensity. changes in radio wave frequency. changes in radio wave frequency. 3