Turn off all electronic devices

Transcription

1 Radio 1 Radio 2 Observations about Radio Radio It can transmit sound long distances wirelessly It involve antennas It apparently involves electricity and magnetism Its reception depends on antenna positioning Its reception weakens with distance There are two styles of radio: AM and FM Turn off all electronic devices Radio 3 3 Questions about Radio Radio 4 Question 1 1. How can a radio wave exist? 2. How is a radio wave emitted and received? 3. How can a radio wave represent sound? Q: How can a radio wave exist? A: Electric and magnetic fields create one another. Radio waves are electromagnetic waves: structures made only of electric and magnetic fields they are emitted or received by charge or pole they are self-sustaining sustaining while traveling, even in vacuum their electric and magnetic fields recreate one another Radio 5 Electromagnetism (Version 3) Radio 6 Structure of a Radio Wave Magnetic fields are produced by magnetic poles and subatomic particles, moving electric charges, and changing electric fields. Electric fields are produced by electric charges and subatomic particles, moving magnetic poles, and changing magnetic fields. Electric field is perpendicular to magnetic field Changing electric field creates magnetic field Changing magnetic field creates electric field Polarization of the wave is associated with the wave s electric field 1

2 Radio 7 Question 2 Radio 8 A Tank Circuit Q: How is a radio wave emitted and received? A: Accelerating charge electromagnetic wave Accelerating charge causes electromagnetic wave It makes an electric field that changes with time It makes a magnetic field that changes with time and the two fields can form an electromagnetic wave Electromagnetic wave causes accelerating charge Its electric field pushes on the charge For bigger wave, slosh charge in a tank Tank circuit is a harmonic oscillator It consists of a capacitor and an inductor Charge cycles through the circuit Tank s energy alternates between magnetic field in its inductor electric field in its capacitor Tank circuit can accumulate energy Frequency set by capacitor & inductor Radio 9 An Antenna is a Tank Circuit An antenna is a straightened tank circuit! Antenna s frequency is set by its length Resonant when it is ½ radio wavelength long A conducting surface can act as half the antenna Above a conducting surface, antenna is resonant when it is ¼ wavelength long Radio 10 Emitting and Receiving Waves A transmitter uses a tank circuit to slosh charge up and down its antenna, which acts as a second tank. A receiver uses a tank circuit to detect charge sloshing on its tank-circuit antenna. Transmitter antenna charge affects receiver antenna charge Antenna orientations matter! Radio 11 Question 3 Radio 12 AM Modulation Q: How can a radio wave represent sound? A: Vary the wave to send sound information. AM or Amplitude modulation Fluctuating amplitude conveys sound information FM or Frequency modulation Fluctuating frequency conveys sound information Information can be encoded as a fluctuating amplitude of the radio wave The air pressure variations that are sound cause changes in the amount of charge moving on the antenna and thus the intensity of the wave The receiver detects these changes in radio wave intensity. 2

3 Radio 13 FM Modulation Radio 14 Summary about Radio Information can be encoded as a fluctuating frequency of the radio wave The air pressure variations that are sound cause slight shifts in the frequency of charge motion on the antenna and the frequency of the wave The receiver detects these changes in radio wave frequency. Accelerating charges cause electromagnetic waves Electromagnetic waves cause accelerating charges Those waves are only electric and magnetic fields Accelerating charge on a transmitting antenna produces a radio wave that causes charge to accelerate on a receiving antenna Radio waves can represent sound information Radio 15 Radio 16 Observations About Microwaves Microwave ovens cook food from inside out They often cook foods unevenly They don t defrost foods well You shouldn t put metal inside them?! Do they make food radioactive or toxic? Turn off all electronic devices Radio 17 4 Questions about 1. Why do microwaves cook food? 2. How does metal respond to microwaves? 3. Why do microwave ovens tend to cook unevenly 4. How does the oven create its microwaves? Radio 18 Question 1 Q: Why do microwaves cook food? A: Water in the food responds to their electric fields. Microwaves are a class of electromagnetic waves Long-wavelength EM waves: Radio & Microwave Medium-wavelength: IR, Visible, UV light Short-wavelength: X-rays & Gamma-rays Microwaves have rapidly fluctuating electric fields. 3

4 Radio 19 Water Molecules Radio 20 Microwave Heating Water molecules are unusually polar An electric field tends to orient water molecules les A fluctuating electric field causes water molecules to fluctuate in orientation Microwaves have alternating electric fields Water molecules orient back and forth Liquid water heats due to molecular friction Ice doesn t heat due to orientational stiffness Steam doesn t heat due to lack of friction Food s liquid water content heats the food Radio 21 Question 2 Radio 22 Introductory Question (revisited) Q: How does metal respond to microwaves? A: Currents flow back and forth in the metal. Non-conductors polarize in the microwaves Conductors carry currents in the microwaves Good, thick conductors reflect microwaves Poor or thin conductors experience resistive heating Sharp conductors initiate discharges in the air If you put a CD in a microwave oven, it will A. do nothing. B. burn up the microwave oven. C. burn up the CD. Radio 23 Question 3 Radio 24 Question 4 Q: Why do microwave ovens tend to cook unevenly? A: Interference produces nonuniform electric fields. Interference is when the fields add or cancel Adding fields are constructive interference Canceling fields are destructive interference Reflections lead to interference in a microwave Most ovens stir the waves or move the food Q: How does the oven create its microwaves? A: A magnetron tube radiates microwaves. Magnetrons are vacuum tubes invented in WWII Electrons travel through empty space obtaining power from a strong electric field bent by a strong magnetic field and the Lorentz force delivering power to an electromagnetic field 4

5 Radio 25 Generating Microwaves Magnetron tube has tank circuits in it Streams of electrons amplify tank oscillations A loop of wire extracts energy from the tanks A short ¼-wave antenna emits the microwaves Radio 26 Summary about They cook food because of its water content Polar water molecules heat in microwave fields Thin or sharp metals overheat or spark The microwaves are produced by a magnetrons 5