13.1 Radios and Electromagnetic waves

1. 13.1 Radios and Electromagnetic waves

2. New ideas for today: • Electrical resonators (tank circuits) • Electromagnetic waves (light) • How FM and AM radio works

3. What do all radios have in common?

4. Antennas! • Charges move in the transmitting antenna • Those charges exert forces on charges in receiving antenna • Charges in the receiving antenna move

5. 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…

6. Tank circuit

7. Capacitor and oil Capacitor Capacitors Just two plates of metal Capacitor and generator • Stores charges • Stores energy – it takes work to separate charges that attract!

8. Inductors Inductor Just a loop of wire Moving charges (current) store energy in magnetic field

9. Magnetic energy Electric energy Tank circuit Electric energy Magnetic energy Tank circuit

10. This is just like a pendulum! Potential energy Potential energy Kinetic energy

11. 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!

12. Clicker question Alice and Bob each use a hand-cranked generator to charge a capacitor. Who has done more work? + + ++++++ + + + - - - - - - - - - - - (A) (B)

13. “Capacitive touch switch” Switch measures how long it takes to charge a capacitor What’s the capacitor?

14. Just like pendulums, tank circuits have a frequency • We call that the “resonant frequency” • Driving current through the tank circuit at the resonant frequency results in big changes in voltage across the capacitor • This is the trick to radios! Antennas are tuned to listen to specific frequencies… 90.9 million oscillations per second WILL FM: 90.9 MHz WILL AM: 580 kHz cellphone: 900 MHz 580 thousand oscillations per second 900 million oscillations per second!!!

15. Tuning forks Antennas that are far apart… Radio broadcast …are connected by electromagnetic radio waves! • Accelerating charges make electromagnetic waves • Electromagnetic waves cause charges to accelerate

16. Electromagnetism II Magnetic fields created by Fundamental particles(dipoles) ―electrons,protons, neutrons… Moving electric charges (current) Electric fields created by Charges Changing magnetic fields (induction)

17. Electromagnetism II Magnetic fields created by Fundamental particles(dipoles) ―electrons,protons, neutrons… Moving electric charges (current) Changing electric fields Electric fields created by Charges Changing magnetic fields (induction)

18. Polarization Electromagnetic waves • 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”

19. Hertz radio waves Heinrich Hertz: discovered radio waves (1885) and showed that they are light Tesla coils Tesla coil and radio

20. Clicker question Alice wants to send a message to Bob. Which method will get her message to Bob faster? A) Using a laser beam B) Using a walkie talkie (radio) C) Using a cell phone D) All of the above are the same

21. Faraday cage 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

22. Information is transmitted by modulation electronic sound Microphone receives sound wave and converts to oscillating electrical current (20 Hz – 20 kHz) modulator An electrical “carrier” signal is produced (90.9 MHz for WILL FM) indepedently AM FM

23. AM = Amplitude modulation FM = Frequency modulation Amplitude modulation: information is encoded in changes in the size of the electrical signal Frequency modulation: information encoded in slight changes in the frequency

24. carrier demodulator signal sound

25. For next class: Read Section 13.2 See you next class!