浙江大学大学物理甲下 chapter 29

1、Chapter 29 The Electrical Properties of Materials129-1 Types of Materials1. Conductors Conductors (including most metals) are materials in which electric charges readily flow. In conductors, each atom gives up one or more of its valence electrons to the entire material, which are free to move when a

2、n external electric field is applied to the material. gas of electrons22. Insulators In a insulator, the electrons are bound rather tightly to the atoms and are not free to move under the applied electric fields. An insulator can carry any distribution of electric charges on its surface or in its in

3、terior. The electric field in the interior of an insulator can have nonzero value. An insulator can be regarded as a collection of molecules that are not easily ionized. 33. Semiconductors In a semiconductor, perhaps one atom in 1010 to 1012 might contribute an electron to the flow of electricity (e

4、lectric current) in the material. Understanding the behaviors of semiconductors requires the knowledge of Quantum Mechanics. Semiconductors have played an important role in modern electronic technology. Commonly used semiconductors include silicon, germanium and lots of compounds. 44. Superconductor

5、s Under certain conditions, often involving cooling to very low temperatures, electric charge can flow through some material with no resistance at all . The materials under these conditions are called superconductors. Understanding superconductivity does also need Quantum Mechanics. 529-2 A Conducto

6、r in an Electric Field ( Static Conditions ) The content in this section is only a review. Suppose we place a rectangular slab of a conductor in a uniform electric field E0, 6 1.The electric field is zero at any point inside a conductor in electrostatic equilibrium. The conductor becomes an equipote

7、ntial, and its surfaces become equipotential surfaces. 2.When a conductor is in electrostatic equilibrium, only points on its surface can have net charge. 3.The electric field at the surface of the conductor is perpendicular to the surface. 4.The excess charge on a conductor in electrostatic equilib

8、rium is more concentrated at regions of greater curvature. The properties of conductor in electrostatic equilibrium7In general829-3 A Conductor in an Electric Field: ( Dynamic Conditions )1. Electric Current In dynamic situation, there is no accumulation of electrons on the conductor boundaries. By

9、some mechanisms, the induced electrons are removed from one boundary of the conductor, through an external path, to another boundary:9 Two new concepts: Electric circuit: the continuous loop of flowing electrons. Electric current: the flow of electrons. The direction of the current i is opposite to

10、the motion of the electrons.10 The electric current i is defined as the net charge that flows through the cross-section of a conductor per unit time interval: i is a scalar. The SI unit of current is ampere (A):1 ampere = 1 coulomb/secondIf the current is constant, we have: 11+- The EMF of a source

11、is defined as the work on per unit positive charge moved from negative terminal to positive inside the battery by the source. Joule/Coulomb = Volt2. Electromotive Force (EMF)F is nonelectric force.123. Current Density Current density j is a vector quantity related to electric current i, whose direct

12、ion is defined to be the direction of the flow of the positive charge, and whose magnitude is defined as current per unit area: electronsAjMore accurately, i is the flux of j.13 In dynamic case, there is a nonzero electric field E inside the conductor. It provides an acceleration a to individual ele

13、ctrons:3. Drift speed of moving electrons The net motion of electrons is a drift in the direction opposite to the field. 14 On the average, electrons can be described as moving with a constant drift velocity vd in a direction opposite to the field: 15 Consider the motion of electrons in a portion of

14、 the conductor of length L. If the number density of electrons is n, then:16 The direction of the current density is opposite to the drift of the electrons. Therefore, Typically, in conductor, vd10-5 m/s, in semiconductor, vd 103 m/s. Question: If the electrons drift at such a low speed, why do elec

15、trical effects seem to occur immediately after the switch is thrown?1729-4 Ohmic Materials In the external path of a circuit, the direction of current density j is the same as that of the electric field E. So, it is reasonable that j should be proportional to E:electrical conductivityRemark:The larg

16、er the value of is, the better the conductor is for electric current.2. The SI unit for conductivity is the siemens per meter (S/m), where, 1 siemens = 1 ampere/volt = (1 ohm )-1 183. The inverse of the conductivity is called resistivity:SI unit: ohmmeterOhmic materials The resistivity (or conductiv

17、ity) of an ohmic material is independent of the magnitude and direction of the applied electric field: For ohmic materials, the following equation is called ohms law:19Resistance The Ohms law has an alternative expression. Consider a homogenous, isotropic conductor of length L and uniform cross-sect

18、ional area A, to which a field with a potential difference V is applied: Here, 20Ohms law is recast as: where the quantity R is called the resistance of the conductor: For ohmic materials, the resistance is independent of the magnitude and the sign of the applied potential difference. DVi21 The foll

19、owing plot illustrates the difference between the ohmic materials and non-ohmic materials: 2229-5 Ohms Law: A Microscopic View Ohms law is not a fundamental law of electromagnetism. It can be interpreted by the principle of Mechanics, Thermodynamics and Coulombs law. 1. Illustration of electrons dri

20、ft232. The drift speed vd The electron acceleration acquired in the applied electric field is, During the time-interval between two one-after-another collisions, the electrons speed changes from zero into: By Maxwell velocity distribution, T: temperature of the conductor: mean free path of electrons

21、24 On an average sense, the electrons drift velocity is defined as above vf: 3. The Conductivity By substituting the above expression of the drift velocity into the microscopic expression of current density,25Therefore, Because we see:The lower the temperature, the less the resistivity. This is in a

22、greement with experiments qualitatively.26 The electrical properties of an insulator may depend on the sum of the electric dipole moments of the molecules: 1 Polar insulators: Insulators in which the molecules have permanent dipole moments are called polar. 29-6 An Insulator in an Electric Field 27E

23、0E0-qqOBefore the external electric field is applied, After the external electric field is applied, The effect of aligning of the molecule dipole moments is called polarization. 28 2 Ferroelectric insulators: In ferroelectric insulators, the alignment of the dipoles remains even when we remove the a

24、pplied field. 3 Nonpolar insulators: In nonpolar insulators, there is no nonzero dipole moments of a molecule before the external electric field is applied, 29 So that the polarization happens: After the external electric field is applied,Before the external electric field is applied, 30 If a slab of insulating material is placed into an electric field, the dipole moments of the molecules will be aligned along the field: 31