. How does heat affect magnetism? — MC, Capitol Heights, MD
The magnetism we associate with a permanent magnet or with steel's response to that permanent magnet involves the careful ordering of tiny magnetic electrons within the materials. Just as heat tends to destroy all forms of order in a newspaper when you put it in the fire, so heat tends to destroy the magnetic order in a permanent magnet or in steel when you bake them. Many permanent magnets lose their magnetism when heated to oven temperatures and even steel becomes non-magnetic when heated red-hot.
. Can an object conduct electricity but be nonmagnetic? Are these independent properties? You said during lecture that copper is nonmagnetic but doesn't it conduct electricity?
Electric conductivity and magnetism are pretty much independent properties. There are good conductors that are magnetic (iron) and good conductors that are nonmagnetic (copper). There are also insulators that are magnetic (iron oxide) and insulators that are nonmagnetic (glass).
. How does a magnetic tape record the difference in timbre or sound quality of the sounds? How does it represent a piano versus an electric guitar? Also, how does more than one tone get recorded (e.g., an entire band or symphony)?
Even a single instrument playing a single note produces a complicated sound. The air pressure fluctuations produced by the instrument aren't as simple and smooth as you might think. While the instrument may produce mostly the fundamental tone—the main pitch associated with the note being played—it also produces other tones that are usually integer multiples of the fundamental tone. These higher pitched "harmonics" contribute to the sound we hear and allow us to determine what instrument is playing that sound. We also hear the temporal shape of the sound—the sound envelope. A piano produces a sound that starts loud and gradually becomes softer while a violin produces a sound that starts soft and gradually becomes louder. An electric guitar offers its player even more control over the pitch and sound envelope. The tape recorder detects the pressure fluctuations associated with all these tones and volume changes and records them all as the magnetization of the tape's surface. When many instruments are playing at once, the pressure fluctuations are even more complicated and they add together to create a complicated pressure pattern at the microphone. Nonetheless, the recorder simply detects the air pressure changes at the microphone and records them on the tape, and that's all it needs to do to keep an accurate record of the sound. When the magnetization of the tape is used to reproduce sound, you again hear all the instruments playing.
. How long will a magnetic tape stay magnetized? Won't it lose its magnetization very fast, like we saw with the iron nails?
At room temperature, a magnetic tape will remain magnetized for years and years. It is made of much harder magnetic materials than the nails are made of and it is much harder to demagnetize than the nails. In effect, it is covered with tiny permanent magnets and you have seen permanent magnets that remain magnetic for decades or centuries.
. When you talk about the magnetic tape and recording, is it the pressure or frequency that is being recorded? Are pressure and frequency interrelated?
Sound consists of pressure fluctuations. The stronger those pressure fluctuations, the louder the sound. The rapidity with which the air goes between a pressure increase and a pressure decreases determines the frequency of the sound and the pitch that we hear. So the extent of the pressure fluctuations, their amplitude, determines the sound volume while the number of pressure fluctuations each second, their frequency, determines the sound pitch. The tape recorder detects both and records both. The louder the sound, the deeper the recorder magnetizes the tape. The higher the frequency of the sound, the more often the tape recorder reverses the magnetization of the tape's surface.
. When making an electromagnet, why does a hard core stay permanently magnetized while a soft core does not? — CD, Houston, TX
Iron and steel are intrinsically magnetic materials, meaning that at the atomic scale they exhibit magnetic order and have magnetic poles present. Most materials, including copper and aluminum, have no such magnetic order—they are nonmagnetic all the way to the atomic scale. But while it is composed of magnetic atoms, a large piece of iron or steel normally doesn't appear magnetic. That's because a large piece of iron or steel contains many tiny magnetic domains. Although each of these magnetic domains is highly magnetic, with a north pole at one end and a south pole at the other end, the metal appears nonmagnetic at first because these domains point equally in all directions and their magnetizations cancel one another. Before the magnetic character of a piece of iron or steel will become visible, something must align its magnetic domains.
In an electromagnet, an iron or steel core is surrounded by a coil of wire. When you run current through that coil of wire, the magnetic field of the current causes the core's magnetic domains to change sizes—the domains that are aligned with the field grow at the expense of the domains misaligned with the field and the whole piece of iron or steel becomes highly magnetic. When you stop current from flowing through the coil of wire, the domains may return to their original sizes and shapes and the iron or steel may become nonmagnetic again.
The abilities for magnetic domains to change sizes depends on the chemical and physical properties of the metal, particularly its crystalline structure. In some magnetic materials, the domains change size extremely easily. These materials are considered to be "soft"—they magnetize easily in the presence of a magnetic field and demagnetize easily when that field is removed. Most electromagnets are made from such soft magnetic materials because it takes only a small current in a wire coil to magnetize the electromagnet's soft core and that core quickly becomes nonmagnetic when you stop the current from flowing.
But in other magnetic materials, the domains don't change size easily. These materials are considered to be "hard"—they are both difficult to magnetize and difficult to demagnetize. You must put lots of current through the coil of wire around a hard magnetic material in order to magnetize that material. But once you turn off the current, the material will retain its magnetization and it will be a permanent magnet.
. How did wire recorders work? — MW, San Diego, CA
The original recording scheme invented by Poulson used a wire as the recording medium, rather than a tape. It recorded audio information as the magnetization of a steel wire in much the same way that a modern tape recorder records audio information as the magnetization of iron particles on the surface of a plastic tape. Both devices record the air pressure changes associated with sound as magnetization changes in a magnetizable surface—the higher the air pressure, the deeper the magnetization in a particular direction; the lower the air pressure, the deeper the magnetization in the opposite direction.
. What kind of tape recorders are the best: cassette recorders or the ones with bigger spools? — HB, Stde, Sweden
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.
. How are magnets made and what are they made of? — S, San Francisco, CA
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.
. How does a video recorder work? — SH, Sault Ste. Marie, Ontario
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.