Note: See the AC Circuits Answers for the Chapter 21 questions in this set. The questions on displacement current appear with the Magnetism answers.
All answers have been checked against the answer key, and should be presumed to be correct. You should ask for help in the recitations if you are unable to obtain these results.
10. 1.08 cm
16. 4.0 x 1016 m
22. 9.32 x 10-7 W/m2
28. 3.8 x 1026 W
30. 302 pF
2. Sound is not an electromagnetic wave. It is a longitudinal pressure wave in a material or gas. The physics behind the sound wave is in the electrostatic repulsion of molecules as the electrons approach each other, so the wave does have an electromagnetic origin, however. It is not the motion of the free-space fields that is propagating in a sound wave, but the motion of molecules themselves.
3. EM waves can travel through a vacuum, but sound waves cannot.
4. Light and sound both travel as waves, and both are subject to diffraction effects. Sound is a longitudinal wave, while light is transverse. Light can be polarized, but sound cannot.
5. Radio and TV signals travel via waves that are longer than the wavelength of visible light.
6. One wavelength of a 60Hz electromagnetic wave stretches 5000 km. A house a distance of 200 km away is therefore 1/25 of a cycle away, or 14.4 degrees in phase behind the power plant, since there are 360 degrees in a full cycle.
7. For normal speaker placements, the relative wire lengths to the two speakers doesn't matter. If the difference in length between the two speaker wires is L, then there is a time difference of L/c between the two speakers. A difference of 1 m in the length of the speaker wires then leads to a timing difference of 3.3 nanoseconds between the speakers. The highest audible frequencies are about 20 kHz, corresponding to a time difference of 50 microseconds. It would take a difference of 15 km of wire to make a 20 kHz signal out of phase by one cycle. It is possible that the ear is sensitive to phase differences significantly less than the maximum audible frequency. (Bats have been shown to have this capability, and use it to create detailed images of their surroundings via echo-location.) Even so, it would take a great difference in wire length to produce a noticeable phase difference between two speakers.
8. The wavelengths given correspond to:
9. You can skip the questions on AM and FM modulation, since we didn't discuss that.
10. You can skip the questions on AM and FM modulation, since we didn't discuss that.
11. The antennas should be aligned so that the electric field produced by the transmitter aligns with the receiving antenna to produce the greatest emf in the antenna.
12. Low frequency waves are diffracted more around obstacles than high frequency waves. Since AM radio is broadcast at a lower frequency than FM radio, it can be detected more easily around buildings or over hills than FM radio.
13. Cordless telephones first convert the sound to an EM wave in the wire, and then use an antenna to broadcast EM waves between the handset and the telephone. The telephone then sends an EM wave through the wire to the phone company switch (at a much lower carrier frequency than used in the broadcast). Cellular phones use EM waves to send a signal to a nearby antenna.
| Physics 222 | Department of Physics | University of Tennessee |