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**Coursera 课程：电磁学 - 磁学与物质** 本课程深入探讨了磁学领域，重点关注磁性材料及其行为。 **课程主要内容包括：** * **磁学基础：** * 电流环作为磁偶极子及其磁偶极矩。 * 旋转电子的磁偶极矩。 * 磁偶极子（条形磁铁）轴线及其垂直轴线上的磁场强度。 * 均匀磁场中磁偶极子（条形磁铁）的受力、力矩及势能。 * 条形磁铁的等效螺线管模型。 * 磁场线。 * **地磁场：** * 地磁场的性质类似于地球中心的磁偶极子。 * 地磁场的三要素：水平分量、磁倾角和磁偏角。 * **磁性材料：** * 磁化强度（M）：材料的单位体积偶极矩。 * 磁场强度（H）与磁感应强度（B）的关系：B = µ₀ (H + M)。 * 线性材料的磁化：M = χ H，以及 B = µ H。 * 磁化率（χ）、相对磁导率（µr）和磁导率（µ）之间的关系：µ = µ₀ µr，µr = 1 + χ。 * 磁性材料的三大分类：抗磁性（Diamagnetic）、顺磁性（Paramagnetic）和铁磁性（Ferromagnetic）。 * 抗磁性材料：χ 负且小。 * 顺磁性材料：χ 正且小。 * 铁磁性材料：χ 大，B-H 关系非线性，具有磁滞现象。 * **永磁体：** * 在室温下能长期保持铁磁特性的物质称为永磁体。 **课程总结关键要点：** 1. 磁学历史悠久，基本规律（同极相斥、异极相吸、磁极不可分离）自古已知。 2. 磁偶极子在均匀磁场中的受力、力矩和势能。 3. 地磁场可近似为地球中心的磁偶极子。 4. 地磁场的三要素用于描述地表磁场。 5. 材料的磁化强度（M）和磁场（B）的关系，以及磁化率（χ）和磁导率（µ）的概念。 6. 磁性材料分为抗磁性、顺磁性和铁磁性，铁磁性材料具有磁滞特性。 7. 永磁体是能长期保持铁磁特性的物质。
Magnetism and MatterCurrent loop as a magnetic dipole and its magnetic dipole momentMagnetic dipole moment of a revolving electronMagnetic field intensity due to a magnetic dipole (bar magnet) along its axis and perpendicular to its axisTorque on a magnetic dipole (bar magnet) in a uniform magnetic field −Bar magnet as an equivalent solenoidMagnetic field linesEarth's magnetic fieldMagnetic elementsPara-, dia- and ferro - magnetic substances, with examplesElectromagnets and factors affecting their strengthsPermanent magnetsSUMMARY1. The science of magnetism is old. It has been known since ancient times that magnetic materials tend to point in the north-south direction; like magnetic poles repel and unlike ones attract; and cutting a bar magnet in two leads to two smaller magnets. Magnetic poles cannot be isolated. 2. When a bar magnet of dipole moment m is placed in a uniform magnetic field B, (a) the force on it is zero, (b) the torque on it is m × B, (c) its potential energy is -m.B, where we choose the zero of energy at the orientation when m is perpendicular.3. The earth's magnetic field resembles that of a (hypothetical) magnetic dipole located at the centre of the earth. The pole near the geographic north pole of the earth is called the north magnetic pole. Similarly, the pole near the geographic south pole is called the south magnetic pole. This dipole is aligned making a small angle with the rotation axis of the earth.4. Three quantities are needed to specify the magnetic field of the earth on its surface - the horizontal component, the magnetic declination, and the magnetic dip. These are known as the elements of the earth's magnetic field.5. The magnetisation M of the material is its dipole moment per unit volume. The magnetic field B in the material is, B = µ0 (H + M).6. For a linear material M = χ H. So that B = µ H and χ is called the magnetic susceptibility of the material. The three quantities, χ, the relative magnetic permeability µr , and the magnetic permeability µ are related as follows: µ = µ0 µr µr = 1+ χ 7. Magnetic materials are broadly classified as: diamagnetic, paramagnetic, and ferromagnetic. For diamagnetic materials χ is negative and small and for paramagnetic materials it is positive and small. Ferromagnetic materials have large χ and are characterised by non-linear relation between B and H. They show the property of hysteresis. 9. Substances, which at room temperature, retain their ferromagnetic property for a long period of time are called permanent magnets.
