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第十一章稳恒电流

VIEWS: 19 PAGES: 28

									Chapter 11 Magnetic Field
      in the Medium

      (Magnetic Property of Matter)
§ 11-1 Classifications of magnetic media
       Magnetic permeability


§ 11-2 Molecular theory of paramagnetism &
       Diamagnetism

§ 11-3 Ampere’s Law in the magnetic matter
       and magnetic intensity H

§ 11-4 Ferromagnetism
§ 11-1 Classifications of magnetic media
            Magnetic permeability
1. Magnetization of Media
 When a magnetic medium is brought into a
 magnetic field,
  (1)The matter will be magnetized;
 (2) The magnetic field will also be influenced
     and changed:
              
       B  B  B      additional field due to the
                        magnetization of matter
      External field
2.Classification of Magnetic Media
                                     
 (1)Paramagnetic substance( 顺 磁 质 ): B  has
                 
 same direction of B , --- B  B .
                                 
 (2) Diamagnetic substance(抗磁质): B  is
             
 opposite to B0, --- B  B0 .

                                           
  (3)Ferromagnetic substance(铁磁质):
                                          B has
  the same direction of B0 and B  B .
3. Permeability(磁导率)
        B ---relative permeability of
   r 
        B   the material.

           for vacuum
     
             for paramagnet ic subs tan ce
 r 
           for diamagnetic subs tan ce
      
             for ferromagne tic subs tan ce

    r 0   ---Permeability
 § 11-2 Molecular Theory of Paramagnetism
                 & Diamagnetism
1. The basic theory:



  Molecule               atom


                                Charged
                                and spin

              electron
   There are the elementary current loops
 resulting from electron spinning ( 自 旋 )
 about their axes and revolving (轨道旋转)in
 orbits around nuclei, --- Molecular Current.


                     Paramagnetic substance
             pm                     
                                   pm  
Molecular magnetic dipole
                     Diamagnetic substance
                                    
                                    pm  0
                                  
2. Paramagnetic substance:        pm  0
 Mg, Al, W, Pt, O2 ,etc.
                                           
                                            B0
                             pm
                        
                        B0

                       In the external M-field,
                                   
Randomly due to
thermal motion,
                        pm turns to B0
 no magnetism           B>B0

  Surface magnetized
  current:
                           
 3. diamagnetic substance: pm  0
The electrons in the atom is in precession(进动)around
                 
the direction of B because of the Lorentz force .
               0



                                       axis
    B0             B0



                        
                       B
         B
     §11-3    Ampere’s Law in a Medium
1. The extension of two laws:
   Like the conduction current,molecular
current will produce the magnetic field. For the
magnetic medium, the Gauss’s magnetic law
             
         
          S
            B  dS  0 does not change;

however, the Ampere’s law
    
  B  d    0  I should be changed
                   into another form.
   
 B  dl   0 (  I c   I m )
l


    molecular (or polarizing ) current

                 unknown

     B
2. Ampere’s Law in a Medium
 Example: the long solenoid


                        B0   0 nI

                                js

         r

                              B   0 j s
   B   r B0  nI
From
  B  B0  B   0 nI   0 j s  nI
we have
              0
       js         nI  (  r  1 )nI
              0
  Depend on r and conduction current I.
Taking the loop as                    r
shown in Fig., apply
Ampere’s law ,
    
   B  dl   0 (  I  I m )
                               I    I m
Using
                0
         js         nI  (  r  1 )nI
                0

       I m    (  r   ) I    I     I
We have
    
  B  dl    I               r P
        
        B 
        dl   I
 Introduce a new quantity
           
         B                        (auxiliary
   H        ---magnetic intensity parameter)
          
we have                 conduction currents
                                     
     H  dl   I ---Ampere’s law for H
Example: A specimen of ferromagnetism with 
is formed in a toroidal ring, A wire carrying I is
uniformly close packed over the ring. The
number of winds per unit length is n. find B=?
                d<<r
                             
        r            As  H  dl   I
                          l
             d
                        2 rH  NI
                     NI
                  H      nI
                     r

       B  H   nI    Here: n  N
                                 r
           §11-4       Ferromagnetic
1. Properties of Ferromagnetic material
                           
 (1) r >>1,   B  B  B  B
    B      
                               ferromagnetism
                    B  0 H
                               paramagnetism
                              diamagnetism
     O
                   H
         B-H curve
                           
(2) r is not a constant  B is not a linear
                  
    function of H , it is depends on the
    magnetizing process.
                 
             B  H
(3) hysteresis (磁滞现象)
   ----B’s changing behinds the exterior field H);

(4) The temperature effect of ferromagnetism:
    Curie temperature Tc
    T < Tc , ferromagnetism
     T > Tc , paramagnetism
2. Hysteresis Loop 磁滞回线
(1) Initial magnetization curve
                                     Measure B
Magnetizing
current
                                    BG
       I
                              secondary coil
        Primary coil

                       Ferromagnetic substance

  H  nI
   B
               B~H


   O                  H

Initial magnetization curve
(2)Hysteresis Loop
    The process of magnetization;

B
          B~H
                                    H   H


O             H            磁滞回线

  
 B depending on not only H but on the
 magnetic history of sample ;
 The B-H curve for decreasing H does not
  coincide with that for increasing H, which is
  called hysteresis(滞后);

               剩磁
                    Br


                    Hc            H
         矫顽力

                         磁滞回线

RemanenceBr(剩磁)and Coercive force (矫顽
力)H c
3. Hard & soft M-materials

    B             B                B



          H               H               H



 a)soft       b)hard          c)矩磁铁氧体材料


 The different material can be used for
 different purpose.
              H
Soft material: c is small, be suitable for
              alternating M-field.
 Hard material: c is large, be suitable for
               H
 permanent magnet.
 矩磁铁氧体材料:反向磁场一超过矫顽力
                   磁化方向立即反转—只有两种磁
                   化状态,对应电流的开与关两种
                   状态,适用于做计算机的存储器
                   元件的环形磁芯。
4. Magnetic Domains 磁畴
 The magnetism of ferromagnetic materials
 result from spinning of the electrons.
The interactions between the atoms in the
ferromagnetic materials are intensive. 量子理论
证明,在这种作用下,铁磁质的内部形成了一些
自发磁化的小区域-磁畴:
4. Magnetic Domains 磁畴
 The magnetism of ferromagnetic materials
 result from spinning of the electrons.

 The electronic spinning magnetic moments of
 the adjacent atoms align spontaneously to
 form many of small magnetization volume –
 M-domain.
                                    

                                     
 (1) H=0:M-domains are                 
   randomly oriented.
--no magnetism in
  macroscopic size.                    
(2) when the external M-field H
  is supplied,
  the domains that are
  oriented favorably with H
  increase in size – the walls of
  domains are moving.
 the orientation of the domains turn to the
  direction of H.
  (3) when the H is intensive, all dimains turn to
   the direction of H.




(4) When the H is removed, the domains disrupt
  again. But they can not restore their initial
  states because of internal strain.
                            --Hysteresis

								
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