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

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

9. Tank circuit Electric energy Magnetic energy Tank circuit

10. This is just like a pendulum! Potential energy Kinetic energy Potential 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. Alice and Bob each use a hand- cranked generator to charge a capacitor. Who has done more work? + + + - - - + + ++++++ - - - - - - - - (A) (B) Clicker question

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… WILL FM: 90.9 MHz WILL AM: 580 kHz cellphone: 900 MHz 90.9 million oscillations per second 580 thousand oscillations per second 900 million oscillations per second!!!

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

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. 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” Electromagnetic waves Polarization

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

20. 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 Clicker question

21. 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 Faraday cage

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

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

24. demodulator carrier signal sound

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