Radio Radio nearby metal object would move in response. nearby - PDF document

Radio 1 Radio 2 Introductory Question Introductory Question If you took an electrically charged ball and If you took an electrically charged ball and � shook it up and down rapidly, charges in a shook it up and down rapidly, charges in a Radio Radio 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 3 Radio 4 Observations about Radio Observations about Radio 3 Questions about Radio 3 Questions about Radio � It can transmit sound long distances wirelessly It can transmit sound long distances wirelessly � How can a radio wave exist? How can a radio wave exist? � It involve antennas It involve antennas � How is a radio wave emitted and received? How is a radio wave emitted and received? � It apparently involves electricity and magnetism It apparently involves electricity and magnetism � How can a radio wave represent sound? How can a radio wave represent sound? � 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 5 Radio 6 Question 1 Question 1 Electromagnetic Waves Electromagnetic Waves � How can a radio wave exist? How can a radio wave exist? � Radio waves are a class of electromagnetic waves Radio waves are a class of electromagnetic waves � � What is an electromagnetic wave anyway? What is an electromagnetic wave anyway? � Electromagnetic waves Electromagnetic waves � How can an electric field exist without charge? How can an electric field exist without charge? � 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 � H � How can a magnetic field exist without pole? H How can a magnetic field exist without pole? n n m m n ti fi ld n ti fi ld i t i t ith ith t p l ? t p l ? � They are emitted or received by charge or pole, They are emitted or received by charge or pole, � but are self but are self- -sustaining while traveling in vacuum sustaining while traveling in vacuum � because their electric and magnetic fields endlessly because their electric and magnetic fields endlessly recreate one another. recreate one another. 1

Radio 7 Radio 8 Electromagnetism Electromagnetism (Version 3) Structure of a Radio Wave Structure of a Radio Wave (Version 3) � Magnetic fields are produced by Magnetic fields are produced by � Electric field is perpendicular to magnetic field Electric field is perpendicular to magnetic field � magnetic poles (but free poles don’t seem to exist), magnetic poles (but free poles don’t seem to exist), � Changing electric field Changing electric field � moving electric charges, moving electric charges, creates magnetic field creates magnetic field � and changing electric fields � nd h n in and changing electric fields. nd h n in l l tri fi ld tri fi ld . � Changing magnetic field Changing magnetic field Ch Ch i i i fi ld i fi ld � Electric fields are produced by Electric fields are produced by creates electric field creates electric field � electric charges, electric charges, � Polarization of the wave Polarization of the wave � moving magnetic poles, moving magnetic poles, is associated with the is associated with the � and changing magnetic fields. and changing magnetic fields. wave’s electric field wave’s electric field Radio 9 Radio 10 Clicker Questions Clicker Questions Question 2 Question 2 � If you place a positive test charge in a radio If you place a positive test charge in a radio � How is a radio wave emitted and received? How is a radio wave emitted and received? wave while there are no other charges nearby, wave while there are no other charges nearby, will that test charge experience a force? will that test charge experience a force? A. Yes Yes A. B. No No B. Radio 11 Radio 12 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 2

Radio 13 Radio 14 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 Th The second tank circuit IS the antenna 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! Radio 15 Radio 16 Emitting Radio Waves Emitting Radio Waves (Part 1) Emitting Radio Waves (Part 2) Emitting Radio Waves (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 17 Radio 18 Clicker Question Clicker Question Introductory Question (revisited) Introductory Question (revisited) If you took an electrically charged ball and If you took an electrically charged ball and � Does a flashlight emit radio waves? Does a flashlight emit radio waves? � 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. A. No No A. How far away could that metal object be and How far away could that metal object be and still respond? still respond? B. Yes, whenever it is on. Yes, whenever it is on. B. C. Yes, but only as you turn it on or off. Yes, but only as you turn it on or off. C. 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. 3