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Integrated Coordinated Science for the 21st Century

Active Physics
+ Chapter 5
Long-Distance Communication
Answers
Physics To Go
  1. In the radio, radio waves and chemical energy from the battery became an electrical signal to the earphones. The LED changed this electrical signal to a changing light signal. The solar cell converted the changing light signal back to a changing electrical signal. Then the amplifier/speaker changed this electrical signal into sound.
  2. a) The telegraph can transmit only one letter at a time. The transmission system in Activity 6 can transmit not only the words but also the tone, the speed, the volume, the pauses—everything that makes up verbal expression. The telegraph is better for transmitting a “yes” or “no” answer, because that could simply be a dot or a dash.
    b) The system of Activity 6 is better for transmitting a speech. (See the answer to Part (a) above.)
    c) The telegraph would be better, since the system of Activity 6 is limited to transmitting only a few millimeters.
  3. a) It transmits music over only a tiny distance. Also, the sound quality after transmission is not as good as the original program.
    b) The system could be expanded by adding identical LEDs to increase the strength of the light signal. A lens system could make a beam of light, and a second lens system could focus the beam on the solar cell (the LED and the solar cell would be at the focus of these lenses). A more sensitive light detector than the solar cell would improve the transmission.
  4. a) If the room light is constant, it probably has little effect. But if it changes substantially at a high frequency, like the output of fluorescent lights, that could add noise and reduce the quality of the transmission.
    b) Turn off the room lights. Use an infrared LED and an infrared detector (like a remote control).
  5. a) The solar cell is designed to make electricity from sunlight, so it is designed for bright light.
    b) The light from the LED is dim compared to sunlight.
    c) No. Ideally the LED would be matched to a more sensitive detector [see answer to Part (b)].
  6. If you could find a way to change the brightness of the laser beam to match the music, a laser could certainly be included. It would vastly increase the distance of your transmissions