磁
维基百科，自由的百科全书
磁 是物質間相互吸引或排斥的一種物理現象。常見的磁性物質是鐵和磁石，以及某些鋼鐵類。雖然一般物質並不具有磁性，然而在磁場中的物質仍會受到輕微磁力的作用，甚至於必須用特殊儀器才能測得。
磁力是由於電荷運動所產生的基本力。馬克士威爾方程組描述了這種力的來源及其行為定律（另參考Biot-Savart Law）。當電荷或帶電物體在運動狀態下將產生磁。例如「電磁感應」是電子在電路中移動時具有的現象；而「永久磁鐵」是電子中的次原子固定的自旋運動所造成的現象。
目录 |
[编辑] 磁偶极
一般來說，磁場被視為具有雙極性（或稱磁偶極），也就是具有南極和北極。這種稱呼是來自於古代使用的羅盤，而羅盤是由盤上的磁針與地球磁場的相互吸引才能正確指出南北方向的。
磁場具有勢能，而能量愈低者在物理系統中愈穩定。亦即當磁偶極置放於磁場中時，其極性將會與磁場的極性相對，使得其所受的「淨磁力」與「存儲能」能夠降到最低。舉例來說，兩個棒狀的磁鐵通常會順著彼此的南北極排列，而使得淨磁場降低為零，並且將會有一股斥力阻止其重新轉向。使其重新轉向所需的能量則儲存在其磁場中，所需要的力則是個別磁鐵的兩倍。
[编辑] 磁单极
Contrary to normal experience, theoretical physics predicts the existence of Magnetic monopoles. 保罗·狄拉克 observed in 1931 that, because electrical theory and magnetic theory show a certain symmetry, just as 量子理论 predicts that individual positive or negative electric charges can be observed without the opposing charge, isolated South or North magnetic poles should be observable. In practice, however, although charged particles like protons and electrons can be easily isolated as individual electrical charges, magnetic south and north poles do not appear in isolation. Using quantum theory Dirac showed that if magnetic monopoles exist, then one could explain why the observed 基本粒子 carry charges that are integral multiples of the charge of the electron. 夸克 carry fractional electric charge, but they do not appear as free particles.
In modern elementary particle theory, the "quantization" of charge is realised in a spontaneous breakdown of a non-abelian gauge symmetry. It should be noted that the monopoles predicted in certain Grand unified theories are different from the one originally thought of by Dirac. These monopoles, unlike that of elementary particles are solitons, namely localised energy packets. These monopoles, if at all they exist, contradict cosmological observations. A solution to this monopole problem in Cosmology gave rise to the currently interesting idea of inflation.
[编辑] 原子磁偶极子
The physical cause of the magnetism of objects, distinct from electrical currents, is the atomic magnetic dipole. Magnetic dipoles, or magnetic moments, result on the atomic scale due to the two kinds of movement of electrons. The first is the orbital motion of the electron around the nucleus; this can be considered as a current loop, resulting in a magnetic moment along the axis of the nucleus. The second, much stronger, source of electronic magnetic moment is due to a quantum mechanical property called spin (although current quantum mechanical theory states that electrons do not actually physically spin, or orbit the nucleus for that matter).
The overall magnetic moment of the atom is the net sum of all of the magnetic moments of the individual electrons. Because of the tendency of magnetic dipoles to oppose each other to reduce the net energy, in an atom the opposing magnetic moments of some pairs of electrons will cancel each other, both in orbital motion and in spin magnetic moments. Thus, in the case of an atom with a completely filled electron shell or subshell, the magnetic moments normally completely cancel each other out and only atoms with partially filled electron shells will have a magnetic moment, whose strength depends on the number of unpaired electrons.
The differences in configuration of the electrons in various elements thus determine the nature and magnitude of the atomic magnetic moments, which in turn determine the differing magnetic properties of various materials. Several forms of magnetic behavior have been observed in different materials, including:
- Diamagnetism
- Paramagnetism
- Molecular magnet
- Ferromagnetism
- Antiferromagnetism
- Ferrimagnetism
- Metamagnetism
- Spin glass
- Superparamagnetism
Highly magnetic stars called magnetars are also believed to exist.
[编辑] 磁鐵的種類
[编辑] 電磁鐵
電磁鐵是種被普遍應用的磁鐵，因為我們可以容易地將它的磁性啟動或是消除；例如：大型起重機利用電磁鐵將廢棄車輛抬起。
當一條導線內部通有電流時，會在周圍的空間中，根據"右手定則"建立磁場。若將右手掌張開當做模型，拇指所指的是導線上電流的流動方向——由正極流往負極("慣用的電流流向"，與實際的電子流動方向相反)。再將拇指以外的四指握成拳頭狀，他們代表著包覆在導線外的磁場方向：由手指根部順著手指，指往手指尖端的方向。 我們也可以用另一種幾何圖形來理解：在一個環形或是螺旋線圈狀的導線裡，電流將沿著導線以畫圓圈的方式通過，所以，按照先前所說的"右手定則"，當電流通過一個圓圈中的每一小塊圓弧時，位於圓心的磁場方向都會指向同一方；現在我們假設電流是沿著一個鐘面逆時針流動，有一個人面對鐘面，站在正前方，則這一圈電流在圓心所產生的磁場方向是"由圓心垂直指向這個人"。這種環形電流會產生磁偶極，其強度將視圓環上的電流量，或是螺旋線圈的圈數乘上電流量的值而定。 In the case of such a loop, if the fingers of the right hand are directed in the direction of conventional current flow (i.e. positive to negative, the opposite direction to the actual flow of electrons), the thumb will point in the direction corresponding to the North pole of the dipole.
[编辑] 永磁体
[编辑] Magnetic metallic elements
Due to their unpaired electron spins, some metals are magnetic when found in their natural states, as ores. These include iron ore (magnetite or lodestone), cobalt, and nickel. Such naturally occurring magnets were used in the first experiments with magnetism. Technology has expanded the availability of magnetic materials to include various manmade products, all based, however, on naturally magnetic elements.
[编辑] Composites
[编辑] Ceramic or ferrite
Ceramic, or ferrite, magnets are made of a sintered composite of powdered iron oxide and barium/strontium carbonate ceramic. Due to the low cost of the materials and manufacturing methods, inexpensive magnets (or nonmagnetized ferromagnetic cores, for use in electronic components such as radio antennas, for example) of various shapes can be easily mass produced. The resulting magnets are noncorroding, but brittle and must be treated like other ceramics.
[编辑] 铝镍钴(AlNiCo)
铝镍钴(AlNiCo)永磁材料采用将铝、镍、钴以及少量其他增强磁性能的成分做成合金，再通过铸造或者烧结处理成形。烧结处理为永磁体提供了良好的机械特性，而铸造的方法则可以提供更高的磁场特性，并且方便设计较为特殊的形状。铝镍钴永磁体具有不错的抗腐蚀性能。虽然还无法和和金属相比，但较铁氧体永磁材料而言其具有更适于加工的物理特性。
[编辑] Injection molded
Injection molded magnets are a composite of various types of resin and magnetic powders, allowing parts of complex shapes to be manufactured by injection molding. The physical and magnetic properties of the product depend on the raw materials, but are generally lower in magnetic strength and resemble plastics in their physical properties.
[编辑] Flexible
Flexible magnets are similar to injection molded magnets, using a flexible resin or binder such as vinyl, and produced in flat strips or sheets. These magnets are lower in magnetic strength but can be very flexible, depending on the binder used.
[编辑] Rare earth magnets
Rare earth elements have an f electron shell, filled with 14 electrons. The spin of these electrons can be aligned, resulting in very strong magnetic fields, therefore these elements are used in compact high strength magnets where their higher price is not a factor.
[编辑] 钐钴(Samarium cobalt)
钐钴(SmCo)永磁材料的抗氧化性较高，磁场强度和温度稳定性比铝镍钴(AlNiCo)和陶瓷材料高。烧结型钐钴极易碎裂，如果在暴露温度变化剧烈的情况下可能破裂。由于钐元素在地壳中含量稀少，造成此种材料价格昂贵。该永磁体抗腐蚀能力较强，不需要进行表面处理。一般用于环境比较苛刻或高端产品中。
[编辑] 钕铁硼(Neodymium iron boron)
钕铁硼(NdFeB)永磁材料具有很高的磁场强度，但是其抗氧化性和温度稳定性与钐钴(SmCo)相比都较差。 由于原料和专利许可的原因该永磁材料的价格也较为昂贵。 使用粉末冶金方式制造成形所需要的较高成本限制了它的广泛应用。 为了避免腐蚀和高温的损害，使用时需要在该永磁材料表面做保护处理,例如用金、镍、锌、锡进行电镀，以及表面喷涂环氧树脂等。
[编辑] Single molecule magnets (SMMs)
It has recently been discovered that certain molecules containing paramagnetic metal ions are capable of storing a magnetic moment at very low temperatures. These are very different to conventional magnets that store information at a "domain" level and theoretically could provide a far denser storage medium than conventional magnets. Very briefly, the attributes for an SMM are:
- a large ground state spin value (S), which is provided by ferromagnetic or ferrimagnetic coupling between the paramagnetic metal centres.
- a negative value for the anisotropy of the zero field splitting (D)
- It is also required for the ground state to be relatively well separated from its excited states.
Most SMM's contain manganese, but can also be found with vanadium, iron, nickel and cobalt clusters.
[编辑] SI magnetism units
Template:SI magnetism units
[编辑] Other magnetism units
- gauss
- oersted
- maxwell
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