1. E&M Demo and More Chris Tully Princeton NJAAPT, March 16, 2013 1

2. The Electrophorus A device that uses induction to produce a substantial charge. Insulating handle Metal disk Hard rubber 1. The hard rubber pallet is charged (-) by rubbing with fur. Positive charge moves down and negative moves up. 2. The electrophorus is moved to contact the metal case. Negative charge, repelled by the negative charge on the rubber, tries to get as far away as possible. 3. Once again isolated from the case, the electrophorus carries a substantial net (+) charge. Metal plate 2

3. The Electroscope Demo: Charge flows onto the conductor and causes the foil to deflect. A simple device for measuring charge. 3

4. Charging an electroscope by induction. 1. Bring charge close, without touching. 2. Momentarily ground the electroscope, thus allowing some of the positive charge to escape. 3. Move the charge away, leaving the electroscope charged. 4

5. Wimshurst machine 5

6. Rainforest + + + + + + + + + + + + - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Negative Charge Positive Charge + + + 6

7. Ohm’s Law 7

8. Charging a Capacitor 8 I V I Sunlight Current Source (For example, solar panel) Voltage Source Q3. Starting from the same initial charging current and ending at the same maximum voltage, which will charge up a capacitor faster? a) b) c) Tie – no difference in charging rate

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10. Storing energy in capacitors Capacitors can store energy The stored energy can be quickly released with spectacular effect. 10

11. 11 Field of Permanent Dipole Magnet There are no “magnetic charges” analogous to electric charges (monopoles). Fields are due to currents and do not terminate on “magnetic charges” Field lines are continuous inside objects. North pole: where field comes out South pole: where the field goes in

12. Magnetic Levitation 12

13. The “Ring Toss” 13

14. Force on a current ring B increasing I clockwise from above Force up B Bx I F B B induced 14

15. Alternating current with Inductor 15

16. Alternating current with capacitor 16

17. Transporting and Rotating B field flux I 1 (t) I 2 (t) I 3 (t) 17

18. 3-Phase Power 18

19. 3-Phase Power 19 47Ω connected from V1 to ground 33Ω to ground 3.3KΩ to ground 1µF connected to V1 100mH connected to V2.15µF connected to V2

20. Eddy Currents Lenz’s Law holds for “eddy currents”. Here, part of the current loop is outside the B-field and has no force on it, and part is inside the B-field with a force. The net result is a force that tries to oppose the change in flux, acting opposite the velocity. No force here (B=0) 20

21. Tesla Coil 21

22. EM waves from accelerating charge on antenna B 22

23. Anatomy of a EM Wave 23 Note that the velocity is in the direction of ExB:

24. The dipole antenna oscillator and receiver The 200 MHz oscillator (at rear) generates EM waves with the dipole antenna. The polarization (E-field) is horizontal. The dipole receiver (in front) detects the waves. The bulb lights up when the signal is strong. Expected polarization and intensity pattern are observed. 24

25. Polarization The orientation of the E field could be vertical, horizontal, “circular,” or random. 25

26. Polarization Radio waves will be polarized along the direction of the radiating element. The receiving antenna should have the same polarization to work efficiently. 26

27. Standing Waves Just as one can have standing waves on a string, one can have standing electromagnetic waves. 27

28. Standing Waves- Photo 28

29. Heat Dissipation of Light Bulbs Heat capacity of water: –Takes 4.2 Joules to raise 1gram=1cm 3 of water by 1 degree C –1 Watt = 1 Joule/sec 29

30. 30 The Universe was not always as cold and dark as it is today – there are a host of landmark measurements that track the history of the universe None of these measurements, however, reach back as far in time as ~1 second after the Big Bang –At ~1 second the hot, expanding universe is believed to have become transparent to neutrinos –In the present universe, relic neutrinos are predicted to be at a temperature of 1.9K (1.7x10 -4 eV) and to have an average number density of ~56/cm 3 of electron neutrinos EVERYWHERE IN SPACE! Dicke, Peebles, Roll, Wilkinson (1965) 1 sec Looking Back in Time

31. 31 Princeton Tritium Observatory for Light, Early-Universe, Massive-Neutrino Yield