February 27, 1995
One Minute Papers - Questions and Answers
What is the difference between an electric and a magnetic field?
An electric field exerts forces on electric charges while a magnetic field exerts forces on magnetic poles. If you place a positive electric charge in an upward-pointing electric field, that electric charge will accelerate upward (in the direction of the electric field). But if you place a stationary north magnetic pole (if you could find one) in that same electric field, nothing will happen. An electric field exerts no force on a stationary magnetic pole. On the other hand, if you place a north magnetic pole in an upward-pointing magnetic field, that pole will accelerate upward (in the direction of the magnetic field). But if you place a stationary positive electric charge in that same magnetic field, nothing will happen. So electric fields act on stationary electric charges and magnetic fields act on stationary magnetic poles.
How good are store bought antennas and if they are better than factory issue, which ones are most advantageous?
Ultimately the only things that matter about an antenna are (1) how much charge it moves in response to the correct radio transmission and (2) how little charge it moves in response to the wrong radio transmissions. Most store bought antennas probably just boost the amount of moving charge by attaching an amplifier to an otherwise undistinguished antenna. While that trick will increase the amount of charge moving in response to the correct transmission, it will also increase the amount moving due to undesired transmissions. Almost everything electrical transmits radio waves and these may well interfere with your reception. For example, your neighbor's lawn mower may send out radio waves and introduce noise into your music. Just amplifying the antenna signal does nothing to eliminate that problem. Your best bet is to find a directional antenna; an antenna that responds most strongly to radio waves coming from a particular direction. TV antennas are typically directional, with many separate antenna elements. Satellite dishes are highly directional.
How is the charge moving in the waves related to what is actually played on the radio?
First, there isn't any charge moving in the waves themselves. The waves contain only electric and magnetic fields. These fields will push on any electric charges or magnetic poles they encounter, but they are not themselves electrically charges or magnetically poled. The amount of fields in a radio used for audio transmission depend on the station's transmitting power and on the encoding format for the music. In AM (Amplitude Modulation) encoding, the music is encoded as the strength of the radio waves. Each time the radio wave's strength goes up and down once, the speaker cone in your receiver goes forward and backward once. In FM (Frequency Modulation) encoding, the radio wave's strength remains steady but its precise frequency changes slightly. Each time the radio wave's frequency goes up and down once, the speaker cone in your receiver goes forward and backward once.
If electric and magnetic field are forever recreating one another - in radio waves - how do you change the sounds they produce?
Within each portion of the wave, the local electric and magnetic fields endlessly recreate one another. But this portion of the wave heads outward from the transmitting antenna at the speed of light and is soon far away from the earth. As the transmitter changes the amount of charge on the antenna or its frequency of motion up and down, it creates new portions of the wave that may differ from the portions sent out a minute ago, a second ago, or even a few millionths of a second ago. Thus the transmitter's changes very quickly pass outward to all of the receivers nearby. The farther you are from the transmitter, the longer it takes for the various patterns in the wave to reach you and your receiver. All of the music transmitted by radio stations in the 50's is still traveling outward because the patterns emitted back then continue to travel. They are now 40 or 50 light years away from the earth and are so widely dispersed across space that it would take a phenomenally sensitive receiver to detect them. But they are out there nonetheless. Many of the searches for extraterrestrial intelligence have focused on trying to detect this sort of radio transmission across the depths of space. If other peoples have invented radio, they are quite likely to have chosen AM or FM modulation as their encoding schemes, too.