Month: December 1969

How does the distance between the transmitting antenna and the receiving antenna…

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How does the distance between the transmitting antenna and the receiving antenna affect the amount of current flowing between the two systems?

Actually, there is no current flowing between the two systems. Current flowing up and down the transmitting antenna causes current to flow up and down the receiving antenna, but there is no direct connection between the two and they do not share any current. That explains how an isolated radio can still receive music. But the amount of current flowing in the receiving antenna does depend on its distance from the transmitting antenna. When the two are very close, the charge in the receiving antenna responds directly to the charge moving on the transmitting antenna. As they move apart, this direct response quickly dwindles to virtually nothing. In its place, a new effect appears. The transmitting antenna creates radio waves that exist apart from the accelerating charges that created them. The strength of the radio wave diminishes in power roughly as the square of the distance from the transmitting antenna. The electric and magnetic fields diminish in power roughly in proportion to this distance. The current flowing in the receiving antenna also falls roughly in proportion to this distance.

How can an antenna be short and still work as well as a long one?

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How can an antenna be short and still work as well as a long one?

The length of an antenna is very important. If the antenna is too short, the charges will reach its end too soon and the charge will not flow very smoothly back and forth in it. If the antenna is too long, the charges will not reach its end before it is time for them to reverse directions and some of the antenna will not be used (it will actually cause more trouble than help). Thus there is an ideal length for the antenna and this length depends on the frequency of the radio wave it is trying to create. But it is also possible to shorten an antenna by delaying the flow of charge to its ends. Adding a coil to the antenna (an inductor) will slow the flow of current through the antenna and make a short antenna behave like a longer antenna. Most portable AM radios use a coiled antenna that behaves as though it were much longer than its physical length. FM radios work best with antennas that are about 1 meter long.

Is it possible to have memory in a computer monitor?

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Is it possible to have memory in a computer monitor?

Yes. In fact, many modern monitors do have memory in them. However, this memory isn’t used for the same information that’s handled in the computer itself. Instead, the monitor’s memory is used to control the monitor’s behavior. Many sophisticated monitors are equipped with digital controllers that are almost full-fledged computers themselves. These controllers can adjust the size and position of the screen image and the manner in which that image is built. This work by the controller allows the monitor to respond properly when the computer changes the screen resolution or the refresh rate (the frequency with which the image you see is rebuilt). The controller requires memory to operate and it also needs to store data that it can expect to recover next time you turn the monitor on. On a sophisticated monitor, you adjust the image size by pushing buttons under the screen and the monitor uses special memory to record your button presses. When the monitor is turned on, it recalls its record of your adjustments and uses them to return the image size to what it was last time the monitor was on.

How do notebook computer monitors work?

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How do notebook computer monitors work?

These displays use liquid crystals, liquids that contain long chain or disk-shaped molecules. These molecules can be aligned by external electric fields or by their own interactions with one another to form very orderly arrays; hence the name “liquid crystals”. The extent to which these molecules are oriented determines their optical properties. A notebook computer uses electric fields to orient or disorient the liquid crystals and control their optical properties. With some help from other optical devices, the notebook computer can make these liquid crystals block or unblock light to appear dark or light. Adding color filters allows them to produce colored images on their screens.

Is the transistor in an audio amplifier used even during silent moments?

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Is the transistor in an audio amplifier used even during silent moments?

During the silent passages of the music, the amplifier does not vary the amount of current passing through the speaker so that the speaker doesn’t move and doesn’t produce sound. To conserve energy and to avoid heating up the speaker, a good amplifier doesn’t send any current through the speaker during a quiet passage. Whether or not the amplifier actually consumes power during the quiet passage depends on the exact design of the amplifier. Some stereo experts claim that they can hear the differences between amplifiers the do or do not consume power with their output transistors during the quiet times and claim that the power wasting amplifiers sound better.

How does the coil in a microphone turn sound into electric current?

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How does the coil in a microphone turn sound into electric current?

The coil in a microphone is attached to a movable surface that is pushed back and forth by the sound. Near the coil is a magnet so that, as the coil moves, the magnet induces electrical currents in it. Whenever a magnet moves past a coil of wire or a coil of wire moves past a magnet, a current is induced in that coil of wire.

Does an (audio) amplifier benefit from using matched pairs of power transistors?

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Does an (audio) amplifier benefit from using matched pairs of power transistors?

A decade or two ago, it was important to match the power transistors used to control currents leaving an audio amplifier. If the transistor that controlled current flowing one direction through the speaker was significantly different from the transistor that controlled current flowing in the opposite direction, then the sound reproduction would be poor. That’s because the current flows would be asymmetric and asymmetric currents lead to distorted sounds from the speaker. The most common measure of this sort of error is called “total harmonic distortion,” an indication of how much power the amplifier puts into unwanted high frequency currents. Without carefully matched power transistors, an amplifier might put several percent of its power into these harmonic frequencies.

However, modern audio amplifiers generally use feedback techniques to correct for their own internal imperfections. They can compensate so well for mismatches in their components that total harmonic distortion has virtually disappeared from amplifiers. Amplifiers are still rated according to total harmonic distortion, but now it is rarely more than a few thousandths of a percent and depends more on the feedback techniques used than on the perfection of the power switching components. In short, the power transistors in modern amplifiers don’t have to be matched well any more.

When magnetic tape is put on top of another piece of magnetic tape, the tape on …

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When magnetic tape is put on top of another piece of magnetic tape, the tape on the bottom is demagnetized — its memory is erased. How are we then able to rewind and forward tape, scrolling the tape together, on a cassette tape with out damaging the magnetism?

The process of winding tape up on reels does damage its magnetism slightly. The adjacent layers of tape do interact with one another and they do cause the sound on one layer to appear on the adjacent layers. Fortunately, the effect is very subtle and takes a long time to appear so that the tape must remain tightly wound up for ages before you can hear the damage. Tapes don’t age perfectly anyway because thermal energy slowly erases the magnetization, particularly in a hot environment.

Magnets can be demagnetized by heat. Is that true for permanent magnets or mater…

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Magnets can be demagnetized by heat. Is that true for permanent magnets or materials that have been magnetized?

It’s true for both because permanent magnets are just a special material that has been magnetized. In fact, permanent magnets are often demagnetized more easily than other simpler materials. Anything that spoils the internal order of a material (heat or vibration) can demagnetize it.