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Magnetism

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  • Similar to electric charges: negative and positive - you can never have a monopole magnet (with just one pole)
  • If you bring two like-poles of magnets together, you can feel the force opposing them. Similarly, you feel (and see) the attraction between two opposite poles.
  • Gadolinium is ferromagnetic at temperatures below 20 C (68F) - Must be frozen to be magnetic
  • Flexible magnets are manufactured by mixing Ferrite or Neodymium magnet powders and synthetic or natural rubber binders. Flexible is manufactured by rolling (alendaring)or extrusion methods. Versatility, low cost, and ease of use are among the reasons to choose ferrite based flexible magnets for your application. This magnet material is usually manufactured in strip or sheet form and it is used in micro-motors, gaskets, novelties, signs, and displays. Ferrite flexible magnet material is very low energy and it does not usually replace fully dense magnet materials. Ceramic magnet material (Ferrite) is Strontium Ferrite. This material is one of the most cost effective magnetic materials manufactured in industry. The low cost is due to the cheap, abundant, and non-strategic raw materials used in manufacturing this alloy. The permanent magnets made with this material lend themselves to large production runs. This magnet material has a fair to good resistance to corrosion and it can operate in moderate heat. The majority of the world¡¯s Ceramic magnetic material comes from China because of the alloy¡¯s commodity nature and the high tooling costs found in the west. Ceramic magnets have a low Energy Product and they are usually used in an assembly containing mild steel. Alnico magnet alloy is largely comprised of Aluminum, Nickel, Cobalt and Iron. Alnico is a moderately expensive magnet material because of the Cobalt and Nickel content. This alloy has very good corrosion resistance and a high maximum operating temperature. Some grades of this alloy can operate upwards of 550•C. Magnets made with this alloy are available in a variety of grades and dimensions and they are usually cast and finish ground to size. Alnico magnet material is a mature technology and it has a relatively low Energy Product (BHmax) . This material is now mainly used in military, aerospace, older proprietary designs and in applications where the magnet will be exposed to elevated temperatures Rare Earth Magnets are called such because Neodymium and Samarium (of which they are made are found in the rare earth elements on the periodic table. Both Neodymium and Samarium Cobalt alloys are powdered metals which are compacted in the presence of a magnetic field and are then sintered. Neodymium , or Neo, is made up of Neodymium Iron and Boron and is moderate in price. With poor corrosion resistance this alloy is usually plated or coated (Examples: Nickel Plated, Epoxy Coated, Parylene Coated). Neodymium is offered in a range of operating temperatures depending on your application (80•C to 200•C). Premium Neodymium Alloys capable of operating above 120•C can become quite expensive. This permanent magnet material has many intellectual property rights associated with it and there are a limited number of licensed manufacturers in the world. Many infringing manufacturers from the Pacific-rim dump sub par material into the Western markets. This magnet material is extremely powerful and it has allowed for the miniaturization of many products from HDD (Hard Disc Drives) and motors to novelties and audio devices. Neodymium permanent magnets usually offer the best value when comparing price and performance. Samarium Cobalt (SmCo) Magnets are alloys of the Lanthanide group of elements. SmCo magnets are available in a number of different grades that span a wide range of properties and application requirements. Rare Earth magnets are the most advanced commercialized permanent magnet materials today. SmCo magnets are brittle and machining operations should be performed prior to magnetization, using diamond tools. SmCo magnets are anisotropic, and can only be magnetized in the orientation direction.
  • Fe(II)Fe(III) 2 O 4
  • A cow magnet is a preventive veterinary medical device for cattle . Traditionally, cow magnets were strong alnico magnets about 1cm by 8cm (0.4 by 3.1 inches) in the shape of a smoothed rod, but today they are more commonly several ring-shaped ferrite magnets attached to a stainless-steel or plastic core, in the same shape as the single-piece original. A rancher or dairy farmer feeds a magnet to each calf at branding time; the magnet settles in the rumen or reticulum and remains there for the life of the animal. When the cow grazes, it often consumes and swallows what is called tramp iron : baling and barbed wire , staples , nails , and other metallic objects. These objects are indigestible and would lodge in the reticulum and cause inflammation resulting in lower milk production (for dairy cattle ) or lower weight gain (for feeder stock ). This condition is called hardware disease . The cow magnet attracts such objects and prevents them from becoming lodged in the animal's tissue. While the resultant mass of iron remains in the cow's rumen as a pseudobezoar (an intentionally introduced bezoar ), it does not cause the severe problems of hardware disease. Cow magnets cannot be passed through a cow's 4th bonivial meta-colon.
  • The term was first used by (then) Cmdr Charles F. Goodeve , RCNVR , during World War II while trying to counter the German magnetic mines that were playing havoc with the British fleet. The mines detected the increase in magnetic field when the steel in a ship concentrated the Earth's magnetic field over it. Admiralty scientists, including Goodeve, developed a number of systems to induce a small "N-pole up" field into the ship to offset this effect, meaning that the net field was the same as background. Since the Germans used the Gauss as the unit of the strength of the magnetic field in their mines' triggers (this was not yet a standard measure), Goodeve referred to the various processes to counter the mines as degaussing. The term became a common word. Until recently, the most common use of degaussing was in CRT -based TV sets and computer monitors . For example, many monitors use a metal plate near the front of the tube to focus the electron beams from the back. This plate, the shadow mask , can pick up strong external fields and from that point produce discoloration on the display.

Magnetism Magnetism Presentation Transcript

  • Magnetism
  • Magnets
    • A magnet is any material that produces a magnetic force (push or pull) on a magnetic material (such as iron)
      • closely related to electricity, but different
        • Where there is electricity, there's a magnetic field
        • Where there's magnetic field, electricity can be created
  • Magnetic Poles
    • Every magnet has two poles
      • North & South
        • same poles repel
        • opposites attract
  • Magnetic Fields
    • Lines show areas of force exerted in a region around a magnet
      • closer lines show greater force
      • force is strongest near the magnet
    • Force originates from N pole and moves towards S pole
  • Magnet Interactions
    • Like poles repel
      • North repels North
      • South repels South
    • Opposite poles attract
  • Magnetic Domains
    • The spin of electrons create tiny magnetic regions called domains
      • In some atoms, these domains cancel out
      • In magnets, domains are all lined up in same direction
    • Whenever all electrons spin in the same direction, a magnetic field is produced
    • Domains stay aligned within the magnet
      • dipoles present in each new piece
    What would happen to the poles if we cut this magnet in half?
  • Magnetic Materials
    • Not all metals are magnetic
      • 3 Are naturally magnetic ( ferromagnetic )
        • Iron
        • Nickel
        • Cobalt
    • Aluminum is NOT magnetic
    • Some Lanthanides form strong magnets
      • Rare Earth magnets
      • Neodymium & Samarium
      • Gadolinium is magnetic only at low temperatures
        • Must be cooled to be magnetic
        • Used in MRIs (Magnetic Resonance Imaging)
  • Types of Magnets Alnico Magnets Flexible Magnets Ceramic Magnets Neodymium Rare Earth Magnets: (very strong) Samarium Cobalt (SmCo)
  • Magnetite
    • Lodestone is magnetized magnetite
      • means "leading stone"
      • used in early compasses
      • possibly made by lightning
    • Naturally magnetic igneous rock
      • found on black sand beaches and deserts
      • Largest US deposit in NY (Adirondacks)
  • Cow Magnets
    • Fed to young cows to prevent "hardware disease"
    Alnico Cow Magnets
  • Iron In Our Food
    • Iron-fortified
      • Cereal
      • Rice
      • Baby food
    • Can extract from food with a strong magnet
      • Neodymium
  • Permanent Magnets
    • Field can be lost or removed
      • Heating to the Curie Point
      • Sending an Electric Current through
        • Degaussing
        • Magnetic storage media erased
      • Banging It
        • dislodges aligned domains
    • Electron spins (domains) are aligned
    • Retains its own magnetic field
        • Examples: Refrigerator Magnets, Bar Magnets, Lodestone, Horseshoe Magnets, Hematite, etc…
    622 Iron Oxide (Fe 2 O 3 ) 358 Nickel (Ni) 1130 Cobalt (Co) 770 Iron (Fe) Curie temp °C Substance
  • Temporary Magnets
    • Domains temporarily line up
    • Will keep magnetic field only until tampered with
      • Examples:
        • Paperclips, scissors, staples, thumb tacks, pins, screwdrivers, refrigerator door, car door locks, etc…
        • Anything that is magnetic, but will not keep its field
  • Making a Temporary Magnet
    • Magnetic Induction
      • domains are aligned by touching or bringing it near a magnet
      • only in some materials
    • Strength of magnet determines:
      • strength of induced field
      • how long induced magnetism lasts
    • Object loses its magnetism once magnet is removed
      • Temporary magnets can also be made using electric currents