Magnets are materials that produce a magnetic field of their own. Extreme examples of magnets are "hard", or "permanent" magnets (like refrigerator magnets), which remember how they have been magnetized, and "soft", or "impermanent" magnets (like the material of the refrigerator door), which lose their memory of previous magnetizations. "Soft" magnets are often used in electromagnets to enhance (often by factors of hundreds or thousands) the magnetic field of a current-carrying wire that has been wrapped around the magnet; when the current increases, so does the field of the "soft" magnet, which is much larger than the field sue to the current. Permanent magnets occur naturally in some rocks, particularly lodestone, but they are now more commonly manufactured.
Normal matter is composed of protons, neutrons, and electrons, and all of these have the fundamental property of quantum mechanical spin. Spin, or intrinsic angular momentum, gives each one of these particles an associated magnetic moment, which produces a magnetic field. Given that each particle has spin and the fact that the average microscopic piece of matter contains huge numbers of these particles, it would be expected that all matter would be magnetic. Even antimatter would have magnetic characteristics. However, everyday experience shows that many materials do not.
Within each atom and molecule, the spin of each of these particles is highly ordered as a result of the Pauli Exclusion Principle. However, there is no long-range ordering of these spins between atoms and molecules. Without long-range ordering, there is no net magnetic field because the magnetic moment of each one of the particles is canceled by the magnetic moment of other particles.
Permanent magnets are special in that long-range order does exist. The greatest amount of order exists within magnetic domains. These domains can be likened to microscopic neighborhoods in which there is a strong reinforcing interaction between particles, and as a result, a great deal of order. The greater the amount of order within and between domains, the greater the resulting field.
Long-range order of the magnetization (and the resulting strong net magnetic field) characterizes a ferromagnetic material.
Electronic generation of magnetism
Electrons play the primary role in generating a magnetic field. Within an atom, electrons can exist either individually or in pairs within any given orbital. When they are paired, the individuals in that pair always have opposite spin?one up, one down. The fact that the spins have opposite orientation means that the two cancel one another. If all electrons are paired, no net magnetic field will be generated.
In some atoms, there are electrons that are unpaired. All magnets have unpaired electrons, but not all atoms with unpaired electrons are ferromagnetic. In order for the material to become ferromagnetic, not only must there be unpaired electrons present, but those unpaired electrons must interact with one another over long ranges such that they are all oriented in the same direction. The specific electron configuration of the atoms (as well as the distance between atoms) is what leads to this long-range ordering. Electrons exist in a lower energy state if they share the same orientation.
An electromagnet, in its simplest form, is a wire that has been coiled into one or more loops. This coil is known as a solenoid. When electric current flows along the coil, a magnetic field is generated around the coil. The orientation of this field can be determined via the right hand rule. The strength of the field is influenced by several factors. The number of loops determines the surface area of interaction, the amount of current determines the amount of activity, and the material in the core determines electrical resistance. The more loops of wire and the greater the current, the stronger the field will be.
If the coil of wire is empty in the center, it will tend to generate a very weak field. Different ferromagnetic or paramagnetic items can be placed in the center of the core with the effect of magnifying the magnetic field, for example an iron nail. In addition, soft iron is commonly used for this purpose. The addition of these types of materials can result in a several hundred- to thousand-fold increase of field strength.
At distances which are large compared to the magnet's dimension, the observed magnetic field obeys an inverse cube law. This means that the field strength is inversely proportional to the third power of the distance from the magnet.
In the case of an electromagnet in contact with a flat metal plate, the force needed to separate the two will be greatest if the two surfaces are machined as flat as possible. The flatter the surfaces, the more points of contact between them, and the smaller the magnetic circuit's reluctance to the magnetic field.
Uses for electromagnets include particle accelerators, electric motors, junkyard cranes, and magnetic resonance imaging machines. Some applications involve configurations more than a simple magnetic dipole; for example, quadrupole magnets are used to focus particle beams.
If enough electric current is passed through the coil of an electromagnet, the attractive magnetic force between adjacent loops of wire can cause the electromagnet to be crushed by its own magnetic field.
Common uses for magnets and electromagnets
* Magnetic recording media: Common VHS tapes contain a reel of magnetic tape. The information that makes up the video and sound is encoded on the magnetic coating on the tape. Common audio cassettes also rely on magnetic tape. Similarly, in computers, floppy disks and hard disks record data on a thin magnetic coating.
* Credit, debit, and ATM cards: All of these cards have a magnetic strip on one of their sides. This strip contains the necessary information to contact an individual's financial institution and connect with their account(s).
* Common televisions and computer monitors: The majority of TVs and computer screens rely in part on an electromagnet to generate an image--see the article on cathode ray tubes for more information. Plasma screens and LCDs rely on different technology entirely.
* Loudspeakers and microphones: Loudspeakers actually rely on a combination of a permanent magnet and an electromagnet. A speaker is fundamentally a device to convert electric energy (the signal) into mechanical energy (the sound). The electromagnet carries the signal, which generates a changing magnetic field that pushes and pulls on the field generated by the permanent magnet. This pushing and pulling moves the cone, which creates sound. Not all speakers rely on this technology, but the vast majority do. Standard microphones are based upon the same concept, but run in reverse. A microphone has a cone or membrane attached to a coil of wire. The coil rests inside a specially shaped magnet. When sound vibrates the membrane, the coil is vibrated as well. As the coil moves through the magnetic field, a voltage is generated in the coil (see Lenz's Law). This voltage in the wire is now an electric signal that is representative of the original sound.
* Electric motors and generators: Some electric motors (much like loudspeakers) rely upon a combination of an electromagnet and a permanent magnet, and much like loudspeakers, they convert electric energy into mechanical energy. A generator is the reverse: it converts mechanical energy into electric energy.
* Transformers: Transformers are devices that transfer electric energy between two windings that are electrically isolated but are linked magnetically.
* Chucks: Chucks are used in the metalworking field to hold objects. If these objects can be held securely with a magnet then a permanent or electromagnetic chuck may be used. Magnets are also used in other types of fastening devices, such as the magnetic base, the magnetic clamp and the refrigerator magnet.
* Magic: Naturally magnetic Lodestones as well as iron magnets are used in conjunction with fine iron grains (called "magnetic sand") in the practice of the African-American folk magic known as hoodoo. The stones are symbolically linked to people's names and ritually sprinkled with magnetic sand to reveal the magnetic field. One stone may be utilized to bring desired things to a person; a pair of stones may be manipulated to bring two people closer together in love.
* Art: 1 mm or thicker vinyl magnet sheets may attached to paintings, photographs, and other ornamental articles, allowing them to be stuck to refrigerators and other metal surfaces.
* Science Projects : Many topic questions are often based on magnets. For example; how is the strength of a magnet affected by glass, plastic, and cardboard?
This is an extract from Wikipedia, the Free Encyclopedia
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