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10 Most Recent Questions and Answers (out of 37)

1457. What is the function of a magnet in an audio speaker? — EB
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An audio speaker generates sound by moving a surface back and forth through the air. Each time the surface moves toward you, it compresses the air in front of it and each time the surface moves away from you, it rarefies that air. By doing this repetitively, the speaker forms patterns of compressions and rarefactions in the air that propagate forward as sound.

The magnet is part of the system that makes the surface move. Attached to the surface itself is a cylindrical coil of wire and this coil fits into a cylindrical channel cut into the speaker's permanent magnet. That magnet is carefully designed so that its magnetic field lines radiate outward from the inside of the channel to the outside of the channel and thus pass through the cylindrical coil the way bicycle spokes pass through the rim of the wheel.

When an electric current is present in the wire, the moving electric charges circulate around this cylinder and cut across the magnetic field lines. But whenever a charge moves across a magnetic field line, it experiences a force known as the Lorenz force. In this case, the charges are pushed either into or out of the channel slot, depending on which way they are circulating around the coil. The charges drag the coil and surface with them, so that as current flows back and forth through the coil, the coil and surface pop in and out of the magnet channel. This motion produces sound.


1450. If I wanted to magnetize a screwdriver, what would be the best way of doing this? I know it can be done by rubbing magnets across the screwdriver's tip, but I would like to know a way of doing it with a piece of coiled wire and a battery. I have heard that this can be done with a car battery. — MS, West Virginia
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Iron and most steels are intrinsically magnetic. By that, I mean that they contain intensely magnetic microscopic domains that are randomly oriented in the unmagnetized metal but that can be aligned by exposure to an external magnetic field. In pure iron, this alignment vanishes quickly after the external field is removed, but in the medium carbon steel of a typical screwdriver, the alignment persists days, weeks, years, or even centuries after the external field is gone.

To magnetize a screwdriver permanently, you should expose it briefly to a very strong magnetic field. Touching the screwdriver's tip to one pole of a strong magnet will cause some permanent magnetization. Rubbing or tapping the screwdriver also helps to free up its domains so that they can align with this external field. But the better approach is to put the screwdriver in a coil of wire that carries a very large DC electric current.

The current only needs to flow for a fraction of a second—just long enough for the domains to align. A car battery is a possibility, but it has safety problems: it can deliver an incredible current (400 amperes or more) for a long time (minutes) and can overheat or even explode your coil of wire. Moreover, it may leak hydrogen gas, which can be ignited by the sparks that will inevitably occur while you are magnetizing your screwdriver.

A safer choice for the current source is a charged electrolytic capacitor—a device that stores large quantities of separated electric charge. A charged capacitor can deliver an even larger current than a battery can, but only for a fraction of a second—only until the capacitor's store of separated charge is exhausted. Looking at one of my hobbyist electronics catalogs, Marlin P. Jones, 800-652-6733, I'd pick a filter capacitor with a capacity of 10,000 microfarads and a maximum voltage of 35 volts (Item 12104-CR, cost: $1.50). Charging this device with three little 9V batteries clipped together in a series (27 volts overall) will leave it with about 0.25 coulombs of separated charge and just over 3.5 joules (3.5 watt-seconds or 3.5 newton-meters) of energy.

Make sure that you get the polarity right—electrolytic filter capacitors store separated electric charge nicely but you have to put the positive charges and negative charges on the proper sides. [To be safe, work with rubber gloves and, as a general rule, never touch anything electrical with more than one hand at a time. Remember that a shock across your heart is much more dangerous than a shock across you hand. And while 27 volts is not a lot and is unlikely to give you a shock under any reasonable circumstances, I can't accept responsibility for any injuries. If you're not willing to accept responsibility yourself, don't try any of this.]

If you wrap about 100 turns of reasonably thick insulated wire (at least 18 gauge, but 12 gauge solid-copper home wiring would be better) around the screwdriver and then connect one end of the coil to the positively charged side of the capacitor and the other end of the coil to the negatively charged side, you'll get a small spark (wear gloves and safety glasses) and a huge current will flow through the coil. The screwdriver should become magnetized. If the magnetization isn't enough, repeat the charging-discharging procedure a couple of times, always with the same connections so that the magnetization is in the same direction.


1406. Do VCR's work on the same principle as audio tape players? If so, how does a VCR generate a signal while it's on pause?
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Yes, VCR's work on the same principle as an audio tape player: as a magnetized tape moves past the playback head, that tape's changing magnetic field produces a fluctuating electric field. This electric field pushes current back and forth through a coil of wire and this current is used to generate audio signals (in a tape player) or video and audio signals (in a VCR).

However, there is one big difference between an audio player and a VCR. In an audio player, the tape moves past a stationary playback head. In a VCR, the tape moves past a spinning playback head. When you pause an audio tape player, the tape stops moving and there is no audio signal. But when you pause a VCR, the playback head continues to spin. As the playback head (actually 2 or even 4 heads that trade off from one another) sweeps across a few inches of the tape, it experiences the changing magnetic fields and fluctuating electric fields needed to produce the video and audio signals. That's why you can still see the image from a paused VCR. To prevent the spinning playback heads from wearing away the tape, most VCRs limit the pause time to about 5 minutes.


1381. Before you speak into the tape recorder, is the tape non-magnetic because half of the magnets face one way and half the other way?
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Exactly. When you switch your tape recorder to the record mode, it has a special erase head that becomes active. This erase head deliberately scrambles the magnetic orientations of the tape's magnetic particles. The erase head does this by flipping the magnetizations back and forth very rapidly as the particles pass by the head, so that they are left in unpredictable orientations. There are, however, some inexpensive recorders that use permanent magnets to erase the tapes. This process magnetizes all the magnetic particles in one direction, effectively erasing a tape. Because it leaves the tape highly magnetized, this second technique isn't as good as the first one. It tends to leave some noise on the recorded tape.

1268. How does a cassette tape recorder work? — TW, Ottawa, Ontario
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Like any tape recorder, a cassette recorder uses the magnetization of the tape's surface to represent sound. The tape is actually a thin plastic film that's coated with microscopic cigar-shaped permanent magnets. These particles are aligned with the tape's length and can be magnetized in either of two directions—they can have their north magnetic poles pointing in the direction of tape motion or away from that direction. In a blank tape, the particles are magnetized randomly so that there are as many of them magnetized in one direction as the other. In this balanced arrangement, the tape is effectively non-magnetic. But in a recorded tape, the balance is upset and the tape has patches of strong magnetization. These magnetized patches represent sound.

When you are recording sound on the tape, the microphone measures the air pressure changes associated with the sound and produces a fluctuating electric current that represents those changes. This current is amplified and used to operate an electromagnet in the recording head. The electromagnet magnetizes the tape—it flips the magnetization of some of those tiny magnetic particles so that the tape becomes effectively magnetized in one direction or the other. The larger the pressure change at the microphone, the more current flows through the electromagnet and the deeper the magnetization penetrates into the tape's surface. After recording, the tape is covered with tiny patches of magnetization, of various depths and directions. These magnetized patches retain the sound information indefinitely.

During playback, the tape moves past the playback head. As the magnetic fields from magnetized regions of the tape sweep past the playback head, they cause a fluctuating electric current to flow in that head. The process involved is called electromagnetic induction; a moving or changing magnetic field produces an electric field, which in turn pushes an electric current through a wire. The current from the playback head is amplified and used to operate speakers, which reproduce the original sound.

The rest of the cassette recorder is just transport mechanism—wheels and motors that move the tape smoothly and steadily past the recording or playback heads (which are often the same object). There is also an erase head that demagnetizes the tape prior to recording. It's an electromagnet that flips its magnetic field back and forth very rapidly so that it leaves the tiny magnetic particles that pass near it with randomly oriented magnetizations.


1264. What happens to a permanent magnet's magnetic field if its temperature is lowered? What happens to a magnetic field at absolute zero?
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Thermal energy is actually bad for permanent magnets, reducing or even destroying their magnetizations. That's because thermal energy is related to randomness and permanent magnetization is related to order. Not surprisingly, cooling a permanent magnet improves its ordering and makes its magnetization stronger (or at least less likely to become weaker with time). At absolute zero, a permanent magnet's magnetic field will be in great shape—assuming that the magnet itself doesn't suffer any mechanical damage during the cooling process.

1246. How does a VCR Plus system work? Are codes built in for every possibility of channel and time or does it calculate somehow? I know that if you enter a random number (including single digits) that some program is scheduled. — LK, Huntington, West Virginia
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The VCR Plus codes contain just enough information to tell the VCR what time and day a program starts, what channel that program is on, and how long it will last. What is remarkable about these codes is not that they exist, but that many of them are so short. A long number that contained the complete date, the entire channel number, and the length of the program in minutes would obvious fulfill the requirements, but the actually numbers are never that long. While I don't know the precise encoding scheme, the date is clearly compressed—a daily or weekly program is represented by a very small code—and so is the record time for programs with a common duration. The VCR Plus codes get significantly longer when they must represent one-time only shows and shows with complicated durations. Even then, the date is truncated so that there are no current codes to represent a show five years in the future.

1110. How does a video recorder work? — SH, Sault Ste. Marie, Ontario
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A video recorder is much like a normal tape recorder, except that it records far more information each second. When you play an audiotape in a normal tape recorder, small magnetized regions of tape move past a playback head. This playback head consists of an iron ring with a narrow gap in it and there is a coil of wire wrapped around the ring. As the magnetized regions of the tape pass near the ring's gap, they magnetize the ring. The ring's magnetization changes as the tape moves and these changing magnetizations cause currents to flow in the coil of wire. These currents are amplified and used to reproduce sound. When you record the tape, the recorder sends currents through the wire coil, magnetizing the iron ring and causing it to magnetize the region of tape that's near the gap in the ring.

In a video recorder, the tape moves too slowly to produce the millions of the magnetization changes needed each second to represent a video signal. So instead of moving the tape past the playback head, the video recorder moves the playback head past the tape. As the tape travels slowly through the recorder, the playback head spins past it on a smooth cylindrical support. The tape is wrapped part way around this support and two or more playback heads take turns detecting the patches of magnetization on the tape's surface. The tape is tilted slightly with respect to the spinning heads so that the heads sweep both along the tape and across its width. That way, the entire surface of the tape is used to record the immense amount of information needed to reproduce images on a television screen. During recording, currents are sent through the heads so that they magnetize the tape rather than reading its magnetization.


1109. How are magnets made and what are they made of? — S, San Francisco, CA
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The strongest modern magnets are made by assembling lots of tiny magnetic particles into a solid object. These magnetic particles are "intrinsically" magnetic, meaning that the atoms from which the particles are formed retain their magnetism in coming together as a solid. Electrons are naturally magnetic and most atoms exhibit the magnetism of their electrons. But as these atoms come together to form a solid, most of them lose their magnetism. For example, copper, aluminum, gold, and silver are all nonmagnetic solids built from magnetic atoms. There are only a few materials that don't lose their atomic magnetism and might be suitable for making permanent magnets. However, most of these magnetic materials only exhibit their magnetism when exposed to other magnets—when they're alone, their magnetism is mostly hidden. For example, iron and steel are magnetic materials but they only appear strongly magnetic when you bring a permanent magnet near them.

To make a strong permanent magnet, you must find a material that is both intrinsically magnetic and that is able to stay magnetic when it's by itself. Materials that hide their magnetism when alone do this by allowing their magnetic structure to break up into tiny pieces that all point in different directions. Each of these tiny magnetic pieces is called a magnetic domain, and iron and steel are normally composed of many magnetic domains. A good permanent magnet material is one that is intrinsically magnetic and that resists the formation of randomly oriented magnetic domains. A very effective way to make such permanent magnet materials is to assemble lots of tiny magnetic particles. Each of these particles is shaped in a way that makes one of its ends a north pole and its other end a south pole, and that makes it extremely hard for these two poles to exchange places. The particles are then aligned with one another and bonded together to form a permanent magnet. To make sure that the particles all have their north poles at one end and their south poles at the other end, the finished magnet is exposed to an extremely strong magnetic field—one so strong that it flips any misaligned magnetic particles into alignment with the others. After being magnetized in this manner, the permanent magnet is very hard to demagnetize, which is just what you want in a permanent magnet.

The most common magnet materials are Ferrite and Alnico. Ferrite magnets are made from a mixture of iron oxide and barium, strontium, or lead oxide. Alnico magnets are made from aluminum, nickel, iron, and cobalt, and consist of tiny particles of an iron-nickel-aluminum alloy inside an iron-cobalt alloy. But the strongest modern magnets are made from an iron-neodymium-boron alloy. The latter magnets are very resistant to demagnetization and the forces they exert on one another are amazingly strong.


1076. What kind of tape recorders are the best: cassette recorders or the ones with bigger spools? — HB, Stde, Sweden
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The audio quality of analog tape recording improves as the tape moves faster past the recording and playback heads. That's because the faster tape motion spreads out the magnetized regions of tape over greater distances on the tape's surface. A cassette tape moves so slowly that oppositely magnetized regions are often bunched tightly together and they demagnetize one another. This demagnetization produces high-pitched noise in the recording. In contrast, a reel-to-reel tape that moves rapidly past the heads has magnetized regions that are widely spaced on the tape's surface and that are much less susceptible to demagnetization and noise.

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