浙江大学大学物理甲下 chapter 35

1、Chapter 35 Magnetic Properties of Materials1Chapter 35 Magnetic Properties of MaterialsChapter 35 Magnetic Properties of Materials235-1 The Magnetic Dipole 1. DefinitionAi loop theofmoment dipole magnetic Thedirection and magnitude of 30303202322202 22 )( )(2zBiAzzRiRzzRiRBMagnetic dipole produces m

2、agnetic field.Chapter 35 Magnetic Properties of Materials3 The effect of magnetic field on the magnetic dipole momentB)(EpmaxstableunstableChapter 35 Magnetic Properties of Materials4Interacting energy of magnetic dipole in magnetic field If U=0, when = /2, then . BU2/dsindsinddiBAWiBAW)(EpU)cos(cos

3、 )d(cos 2iBAiABcos)()( 2BUUcos)(BUWhen = 0, U is minimum.Chapter 35 Magnetic Properties of Materials5 2. The field of a dipoleA bar magnet can be considered to be a magnetic dipole.Chapter 35 Magnetic Properties of Materials635-2 The Force on a Dipole in a Nonuniform fieldzBBzzUFBUzzzzzdd)(dddd12122

4、1Permanent dipoles are attracted to the source of the field.Permanent magnetic dipole momentsChapter 35 Magnetic Properties of Materials7 In summary, in a nonuniform magnetic field, permanent dipoles are rotated into alignment with the field and attached to the source of the field, but induced dipol

5、es are repelled from the source of the field.Induced magnetic dipole momentsInduced dipoles are repelled from the source of the field.Chapter 35 Magnetic Properties of Materials835-3 Atomic and Nuclear Magnetism 1. Orbital magnetic dipole momentAtomic dipole tiny current looprereeefTei22)2(vvlmel2B

6、is Bohr magneton lmemmerrmll22 ,vvBlnhnmehnl22 ,2 if222vverrreiAChapter 35 Magnetic Properties of Materials9 2. Intrinsic magnetic moment (spin moment)Smes2 The total magnetic moment of an atom is obtained from the vector sum of the orbital and spin magnetic moments of all its electrons.Chapter 35 M

7、agnetic Properties of Materials10 3. Nuclear magnetism The nucleus has a magnetic moment with two parts: orbital magnetic moment of protons and intrinsic magnetic moment of protons and neutrons. 4. Nuclear magnetic resonanceBUsSmes2Nobel prize in physics 1952:Bloch & Purcell for their developmen

8、t of new methods for nuclear magnetic precision methods and discoveries of connection therewith.Chapter 35 Magnetic Properties of Materials11NMRIChapter 35 Magnetic Properties of Materials12Paul C.Lauterbur USPeter Mansfield UK The Nobel prize in physiology or medicine for their discoveries concerni

9、ng magnetic resonance imaging. (2003)Chapter 35 Magnetic Properties of Materials1335-4 Magnetization(a) In an unmagnetized sample:(b) In an magnetized sample:Define the magnetization M:VVMnThere are three field: applied magnetic field B0, magnetized field BM, net magnetic field B.MBBB0Chapter 35 Mag

10、netic Properties of Materials14a long solenoid of circular cross section with a magnetic materialMBBMBM000 0BBm)1(0EEe)(0m00) 1(material. theofty permeabili is BMmmChapter 35 Magnetic Properties of Materials15Magnetization of materials surface current Chapter 35 Magnetic Properties of Materials16nIB

11、nlIlBsB00000 dMMnInIBIInlBlsB000 )(dMmmmnInInInIBB00000 ,Magnetic Material and Amperes LawAmperes law for a solenoid:magnetic material is inserted,IM is surface current. Chapter 35 Magnetic Properties of Materials17Amperes law should be amended as: d0IsBm) 1(mMII d0IsBm ) , , d(00HHBBHIsHmmChapter 3

12、5 Magnetic Properties of Materials1835-5 Magnetic Materials 1. Paramagnetism The atoms of this material have permanent magnetic dipole moments. B0 and 0M are parallel, B B0.Curies law: when B0/T is smallTBCM0Adiabatic demagnetization to decrease the temperature of system.Chapter 35 Magnetic Properti

13、es of Materials19 2. Diamagnetism Such materials might be those having atomic magnetic dipole moments of zero. B0 and 0M are antiparallel, B 1, B B0.m is not a constant.Chapter 35 Magnetic Properties of Materials21magnetic domains B=0 B B B BChapter 35 Magnetic Properties of Materials22Hysteresis an

14、d magnetic domainsPoint c: remains magnetized even when the applied field B0 is zero.Point d: a saturated magnetization in the opposite direction.Point e: retains a permanent magnetization.Point b: a saturated magnetization in the original direction.Ferromagnetism can “remember” how it became magnet

15、ized.BChapter 35 Magnetic Properties of Materials23fPoint f: Hf Coercive forcePoint c, e: Bc Remanent magnetizationsoft magnetic material BChapter 35 Magnetic Properties of Materials24hard magnetic material moment magnetic material Chapter 35 Magnetic Properties of Materials25Chapter 35 Magnetic Pro

16、perties of Materials26Quick quiz What happens when a paramagnetic substance is placed in an external magnetic field?(1). The atomic dipole moments weaken slightly, but tend to line up with the external field.(2). The atomic dipole moments strengthen slightly, and tend to line up with the external fi

17、eld.(3). The atomic dipole moments weaken slightly, and tend to line up against the external field.(4). The atomic dipole moments strengthen slightly, but tend to line up against the external field.Chapter 35 Magnetic Properties of Materials27 Quick quiz Three curves a, b, and c show the magnetizing process of paramagnetic, diamagnetic and ferromagnetic substance respectively. The dashed line denoted relationship B-H in vacuum. Which curve is represent diamagnetic substance?(1). a(2). b(3). cChapter 35 Magnetic Propertie