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Magnet

2007 Schools Wikipedia Selection. Related subjects: Electricity and
Electronics

   Iron filings in a magnetic field generated by a bar magnet
   Enlarge
   Iron filings in a magnetic field generated by a bar magnet

   A magnet is an object that has a magnetic field. It can be in the form
   of a permanent magnet or an electromagnet. Permanent magnets do not
   rely upon outside influences to generate their field. They occur
   naturally in some rocks, but can also be manufactured. Electromagnets
   rely upon electric current to generate a magnetic field - when the
   current increases, so does the field.

   Magnets are attracted to, or repelled by, other materials. A material
   that is strongly attracted to a magnet is said to have a high
   permeability. Examples of materials with very high permeability include
   iron and steel. Liquid oxygen is an example of something with a low
   permeability, and it is only weakly attracted to a magnetic field.
   Water has such a low permeability that it is actually slightly repelled
   by magnetic fields. Everything has a measurable permeability: people,
   gases, and even the vacuum of outer space.

   The SI unit of magnetic field strength is the tesla, and the SI unit of
   total magnetic flux is the weber. 1 weber = 1 tesla flowing through 1
   square meter, and is a very large amount of magnetic flux.

Physical origin of magnetism

Permanent magnets

   Normal pieces of matter are composed of particles such as protons,
   neutrons, and electrons; and all of these have the fundamental property
   of quantum mechanical spin. Spin gives each one of these particles an
   associated magnetic field. Because of this, 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 this is not the case.

   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 ordering does exist.
   The highest degree of ordering 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 degree of ordering within and
   between domains, the greater the resulting field will be.

   Long-range ordering (and the resulting strong net magnetic field) is
   one of the hallmarks of 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.

Electromagnets

   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 centre, it will tend to generate a
   very weak field. Different ferromagnetic or paramagnetic items can be
   placed in the centre 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.

   Electromagnets find uses in many places, ranging from particle
   accelerators, to electric motors, to junkyard cranes, to magnetic
   resonance imaging machines. There are also specialized applications
   that involve more than a simple magnetic dipole, such as the quadrupole
   magnets used to focus particle beams.

   If enough electric current is passed through the coil of an
   electromagnet, the magnetic force between neighboring loops of wire can
   cause the electromagnet to be crushed by its own magnetic field.

Characteristics of magnets

Permanent magnets and dipoles

   All magnets have at least two poles: that is, all magnets have at least
   one north pole and at least one south pole. The poles are not a pair of
   things on or inside the magnet. They are a concept used to discuss and
   describe magnets. In the image at the top of this page, the poles look
   like specific locations, because the highest surface intensity of the
   field occurs at the poles, but this does not mean that they are
   specific locations.

   To understand the concept of pole, it can be imagined that a row of
   people who are all facing the same direction and standing in line.
   While there is a "face" end of the line and a "back" end of the line,
   there is no one place where all of the faces are and all of the backs
   are. The person at the front of the face end has a back; and the person
   at the back end has a face. If the line is divided into two shorter
   lines, each one of the shorter lines still has a face end and a back
   end. Even if the line is pulled completely apart so that there are just
   individuals standing around, each one of the individuals still has a
   face and a back. This can continue without end.

   The same holds true with magnets. There is not one place where all of
   the north or south poles are. If a magnet is divided in two, two
   magnets will result and both magnets will have a north and a south
   pole. Those smaller magnets can then be divided, and all of the
   resulting pieces will have both a north and south pole. In most
   instances, if the material continues to be broken into smaller and
   smaller pieces there will be a point where the pieces are too small to
   retain a net magnetic field. They won't become individual north or
   south poles though; instead, they will just lose the ability to
   maintain a net field. Some materials, however, can be divided down to
   the molecular level and still maintain a net field with both a north
   and a south pole. There are theories involving the possibility of north
   and south magnetic monopoles, but no magnetic monopole has ever been
   found.

North-south pole designation and the Earth's magnetic field

   A standard naming system for the poles of magnets is important.
   Historically, the terms north and south reflect awareness of the
   relationship between magnets and the earth's magnetic field. A freely
   suspended magnet will eventually orient itself north-to-south, because
   of its attraction to the north and south magnetic poles of the earth.
   The end of a magnet that points toward the Earth's geographic North
   Pole is labeled as the north pole of the magnet; correspondingly, the
   end that points south is the south pole of the magnet.

   The Earth's current geographic north is thus actually its magnetic
   south. Confounding the situation further, magnetised rocks on the ocean
   floor show that the Earth's magnetic field has reversed itself in the
   past, so this system of naming is likely to be backward at some time in
   the future.

   Fortunately, by using an electromagnet and the right hand rule, the
   orientation of the field of a magnet can be defined without reference
   to the Earth's geomagnetic field.

   To avoid the confusion between geographic and magnetic north and south
   poles, the terms positive and negative are sometimes used for the poles
   of a magnet. The positive pole is that which seeks geographical north.

Common uses for magnets and electromagnets

   Magnets have many uses in toys. M-tic uses magnetic rods connected to
   metal spheres for construction
   Enlarge
   Magnets have many uses in toys. M-tic uses magnetic rods connected to
   metal spheres for construction
     * 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.

   Magnetic hand separator for heavy minerals
   Enlarge
   Magnetic hand separator for heavy minerals
     * 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: 30 millimetre 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.

Magnetization of materials

   Ferromagnetic materials can be magnetized in the following ways:
     * Placing the item in an external magnetic field will result in the
       item retaining some of the magnetism on removal. Vibration has been
       shown to increase the effect. Ferrous materials aligned with the
       earth's magnetic field and which are subject to vibration (e.g.
       frame of a conveyor) have been shown to acquire significant
       residual magnetism.
     * Placing the item in a solenoid with a direct current passing
       through it.
     * Stroking - An existing magnet is moved from one end of the item to
       the other repeatedly in the same direction.
     * Placing a steel bar in a magnetic field, then heating it to a high
       temperature and then finally hammering it as it cools. This can be
       done by laying the magnet in a North-South direction in the Earth's
       magnetic field. In this case, the magnet is not very strong but the
       effect is permanent.

Demagnetizing materials

   Permanent magnets can be demagnetized in the following ways:
     * Heating a magnet past its Curie point will destroy the long range
       ordering.
     * Contact through stroking one magnet with another in random fashion
       will demagnetize the magnet being stroked, in some cases; some
       materials have a very high coercive field and cannot be
       demagnetized with other permanent magnets.
     * Hammering or jarring will destroy the long range ordering within
       the magnet.
     * A magnet being placed in a solenoid which has an alternating
       current being passed through it will have its long range ordering
       disrupted, in much the same way that direct current can cause
       ordering.

   In an electromagnet which uses a soft iron core, ceasing the flow of
   current will eliminate the magnetic field. However, a slight field may
   remain in the core material as a result of hysteresis.

Types of permanent magnets

   A stack of ferrite magnets
   Enlarge
   A stack of ferrite magnets
     * Rare Earth types:
          + Neodymium magnets, some of the most powerful permanent magnets
          + Samarium-cobalt magnets
     * Other types:
          + Ceramic magnets
          + Plastic magnets
          + Alnico magnets

Magnetic forces

   Magnetized items interact with other items in very specific ways.

Magnets and ferromagnetic materials

   If a magnet is brought close enough to a ferromagnetic material (that
   is not magnetized itself), the magnet will strongly attract the
   ferromagnetic material regardless of orientation. Both the north and
   south pole of the magnet will attract the other item with equal
   strength.

Magnets and diamagnetic materials

   By definition, diamagnetic materials weakly repel a magnetic field.
   This occurs regardless of the north/south orientation of the field.

Magnets and paramagnetic materials

   By definition, paramagnetic materials are weakly attracted to a
   magnetic field. This occurs regardless of the north/south orientation
   of the field.

Calculating the magnetic force

   Calculating the attractive or repulsive force between two magnets is,
   in the general case, an extremely complex operation, as it depends on
   the shape, magnetization, orientation and separation of the magnets.

Force between two monopoles

   The force between two magnetic monopoles is as follows:

          F={{\mu m_1m_2}\over{4\pi r^2}}

   where

          F is force (SI unit: newton)
          m is pole strength (SI unit: ampere-meter)
          μ is the permeability of the intervening medium (SI unit: tesla
          meter per ampere or henry per meter)
          r is the separation (SI unit: meter).

   Since magnetic monopoles are only a theoretical construction, this
   equation does not describe a physically realisable arrangement. It is
   stated here because it is the simplest possible calculation of magnetic
   forces. In reality, one of the more complex formulae given below will
   be more useful.

Force between two very close attracting surfaces

          F=\frac{AB^2}{2\mu_0}

   where

          A is the area of each surface, in m^2
          B is the flux density between them, in teslas
          μ[0] is the permeability of space, which equals 4π x 10^-7
          tesla∙meter/ampere

Force between two bar magnets

   The force between two identical cylindrical bar magnets placed
   end-to-end is given by:

          F=\left[\frac {B_0^2 A^2 \left( L^2+R^2 \right)}
          {\pi\mu_0L^2}\right] \left[{\frac 1 {x^2}} + {\frac 1
          {(x+2L)^2}} - {\frac 2 {(x+L)^2}} \right]

   where

          B[0] is the flux density at each pole, in T,
          A is the area of each pole, in m^2,
          L is the length of each magnet, in m,
          R is the radius of each magnet, in m, and
          x is the separation between the two magnets, in m

   Retrieved from " http://en.wikipedia.org/wiki/Magnet"
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   with only minor checks and changes (see www.wikipedia.org for details
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