1. Chapter 11 Magnetic Field in the Medium (Magnetic Property of Matter)

2. § 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

3. When a magnetic medium is brought into a magnetic field, § 11-1 Classifications of magnetic media Magnetic permeability (1)The matter will be magnetized; (2) The magnetic field will also be influenced and changed: External field additional field due to the magnetization of matter 1. Magnetization of Media

4. 2.Classification of Magnetic Media (1)Paramagnetic substance( 顺磁质 ): has same direction of, ---. (2) Diamagnetic substance( 抗磁质）： is opposite to, ---. (3)Ferromagnetic substance( 铁磁质 ): has the same direction of and.

5. 3. Permeability （磁导率） ---relative permeability of the material. ---Permeability

6. § 11-2 Molecular Theory of Paramagnetism & Diamagnetism 1. The basic theory: atom Molecule electron Charged and spin

7. Diamagnetic substance Paramagnetic substance 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. Molecular magnetic dipole

8. 2. Paramagnetic substance: Mg, Al, W, Pt, O 2,etc. Randomly due to thermal motion ，  no magnetism Surface magnetized current: In the external M-field, turns to  B>B 0

9. 3. diamagnetic substance: The electrons in the atom is in precession( 进动） around the direction of because of the Lorentz force. axis

10. §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 does not change; however, the Ampere’s law should be changed into another form.

11. BB molecular (or polarizing ) current unknown

12. 2. Ampere’s Law in a Medium Example: the long solenoid rrrr

13. From we have Depend on  r and conduction current I.

14. Taking the loop as shown in Fig., apply Ampere’s law, Using rrrr

15. We have Introduce a new quantity ---magnetic intensity rrrr P we have ---Ampere’s law for conduction currents (auxiliary parameter)

16. 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=? Here: As r d d<<r

17. §11-4 Ferromagnetic 1. Properties of Ferromagnetic material (1)  r >>1, B-H curve H B O    ferromagnetism  paramagnetism diamagnetism  ferromagnetism  paramagnetism diamagnetism

18. (2)  r is not a constant  is not a linear function of, it is depends on the magnetizing process. (3) hysteresis ( 磁滞现象 ) ----B’s changing behinds the exterior field H); (4) The temperature effect of ferromagnetism: Curie temperature T c T < T c, ferromagnetism T > T c, paramagnetism

19. 2. Hysteresis Loop 磁滞回线 (1) Initial magnetization curve Ferromagnetic substance Primary coil secondary coil BG Magnetizing current Determine H Measure B

20. H B O B～HB～H Initial magnetization curve

21. (2)Hysteresis Loop The process of magnetization; depending on not only H but on the magnetic history of sample ; 磁滞回线 H H B O B～HB～H

22. The B-H curve for decreasing H does not coincide with that for increasing H, which is called hysteresis( 滞后）； Remanence ( 剩磁） and Coercive force ( 矫顽 力） 磁滞回线 H 剩磁 矫顽力

23. ３. Hard & soft M-materials a ） softb ） hard c ）矩磁铁氧体材料 H B H B H B The different material can be used for different purpose.

24. 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。 Soft material ： is small, be suitable for alternating M-field. Hard material ： is large, be suitable for permanent magnet.

25. 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：

26. 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.   

27. (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 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.

28. (3) when the H is intensive, all dimains 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