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《物理双语教学课件》Chapter14ElectricCharge电荷Chapter 14 Electric ChargeWe now begin a study of the branch of physics concerned with electric and magnetic phenomena. The laws of electricity and magnetism play central roles in operation of many devices such as radios, televisions, electric motors, computers, and high-energy accelerators. More fundamentally, we now know that the inter-atomic and inter-molecular forces that are responsible for the formation of solids and liquids are electric in origins. Furthermore, such forces as the pushes and pulls between objects and the elastic force in a spring arise from electric forces at the atomic level.The early Greek philosophers knew that if you rubbed a piece of amber, it would attract bits of straw. The Greeks also recorded the observation that some natura lly occurring “stone” known today as the mineral magnetite, would attract iron. From these modest origins, the sciences of electricity and magnetism developed separately for centuries-until 1820.The new science of electromagnetism,the combination of electrical and magnetic phenomena, was developed further by workers in many countries. One of the best was Michael Faraday, a truly gifted experimenter with a talent for physical intuition and visualization. In the mid-19th century, James Clerk Maxwellput Fa raday’s ideals into mathematical form, introduced many new ideas of his own, and put electromagnetism on a sound theoretical basis.14.1 Electric Charge1.The definition of electric charge: If you walk across a carpet in dry weather, you can produce a spark by bring your finger close to a metal doorknob. On a grander scale, lighting is familiar to everyone. Each of those phenomena represents a tiny glimpse of the vast amount of electric charge that is stored in the familiar objects that surround us. Electric charge is an intrinsic characteristic of the fundamental particles making up those objects.2.The classification of charge(1)The vast amount of charge in an everyday object is usually hidden because the object contains equal amounts of two kinds of charge: positive charge and negative charge. With such equality-or balance-of charge, the object is said to be electrically neutral; that is, it contains no net charge to interact with other objects.(2)If the two types of charge are not in balance, then there isa net charge that can interact with other objects. And webecome aware of the existence of the net charge. We say that an object is charged to indicate that it has a charge imbalance, or net charge. The imbalance is always very small compared to the total amount of positive charge and negative charge contained in the object.3.To see the demonstration ofthe figure, we come to theconclusion that charges withthe same electrical sign repeleach other, and charges with opposite signs attract each other.4.The SI unit of charge is derived from the SI unit of electric current, the ampere (A). The SI unit of charge is the coulomb (C).14.2 Conductors and Insulators1.In some materials, such as metals, tap water, and the humanbody, some of the negative charge can move rather freely. We call such materials conductors. In other materials, such as glass, chemically pure water, and plastic, none of the charge can move freely. We call these materials nonconductors or insulators.2.The properties of conductors and insulators are due to thestructure and electrical nature of atoms. Atoms consist of positively charged protons, negatively charged electrons, and electrically neutral neutrons. The protons and neutrons are packed tightly together in the central nucleus.3.The charge of a single electron and that of a single protonhave the same magnitude but are opposite in sign. Hence an electrically neutral atom contains equal numbers of electrons and protons. Electrons are held near the nucleus because they have the electrical sign opposite that of the protons in the nucleus and thus are attracted to the nucleus.4.When atoms of a conductor likecopper come together to form the solid,some of their outmost electrons do notremain attracted to the individualatoms but become free to wanderabout within the solid, leaving behind positively charged atoms. We call the mobile electrons conduction electrons. The experiment of the figure demonstrates the mobility of charge in a conductor.5.Semi-conductor, such as silicon and germanium, are materialsthat are intermediate between conductors and insulators. The microelectronic revolution that has changed our lives in so many ways is due to devices constructed of semi-conducting materials.6. Finally, there are superconductors , so called because they present no resistance to the movement of electric charge through them . When charge moves through a material, we say that an electric current exists in the materials. Ordinary materials, even good conductors, tend to resist the flow of charge through them, In a superconductor, however, the resistance is not just small; it is precisely zero .14.3 Coulomb ’s law1. Let two charged particles (also called point charges ) have charge magnitudes q 1 and q 2 and be separated by a distance r.(1) The electrostatic force of attraction or repulsion between them has the magnitude )'(412210221slaw Coulomb r q q r q q k F πε==,in which k is a constant with the value 229/1099.8C m N ??, called the electrostatic constant , and the quantity0ε, called the permittivity constant , with the value 2212/1085.8m N C ??-.(2) Each particle exerts a force of this magnitude on the other particle; the two forces form an action-reaction pair . If the particles repel each other, the forces on each particle point away from the other particle as in figures (a) and (b). If theparticles attract each other, the forceon each particle points toward theother particle as in figure (c).2.Coulomb’s law has the same form as that of Newton’sgravitational law.(1)Both describe inverse square laws that involve a propertyof the interacting particles-the mass in one case and the charge in the other.(2)The laws differ in that gravitational forces are alwaysattractive but electrostatic forces may be either attractive or repulsive, depending on the sign of the two charges. This difference arises from the fact that, although there is only one kind of mass, there are two kinds of charge.3.Coulomb’s law has survived every experimental test; noexceptions to it have ever been found. It holds even within the atom, correctly describing the force between the positively charged nucleus and each of the negatively charged electrons, even though classical Newtonian mechanics fails in that realm and is replaced there by quantum physics. This simple law also correctly accounts for the forces that bind atoms and molecules together to form solids and liquids.4. Still another parallel between the gravitational force and the electrostatic force is the both obey the principle of superposition . If we have n charged particles, they interact independently in pairs, and the force on any one of them, let us say particle 1, is given by the vector sum n F F F F F 11413121 ++++=, in which, for example, 14F is theforce acting on particle 1 owing to the presence of particle 4. An identical formula holds for the gravitational force.5. The two shell theorems that we found so useful in our study of gravitation have analogs in electrostatics: (1) A shell of uniform charge attracts or repels a charged particle that is out side the shell as if all the shell ’s charge were concentrated at its center. (2) A shell of uniform charge exerts no electrostatic force on a charged particle that is located inside the shell .6. Spherical conductors : If excess charge is placed on a spherical shell that is made of conducting material , the excess charge spreads uniformly over the external surface . This arrangement maximizes the distances between all pairs of the excess charge. According to the first shell theorem, the shell then will attract or repel an external charge as if the excess charge on the shell were concentrated at its center.14.4 Charge is quantized and conserved1.The experiment shows that electric charge is made up ofmultiples of a certain elementary charge. That is, any positive or negative charge q that can be detected can be written as = =nq, in which e, the elementary charge, has ±ne±,1±,3,2the value C19. The elementary charge is one of the .1-6010important constants of nature. The electron and proton both have a charge of magnitude e.2.When a physical quantity such as charge can have onlydiscrete values rather than any value, we say that the quantity is quantized. Energy, angular momentum are quantized; Charge adds one more important physical quantity to the list.3.If you rub a glass rod with silk, a positive charge appears onthe rod. Measurement shows that a negative charge of equal magnitude appears on the silk. This suggests that rubbing does not create charge but only transfers it from one body to another, upsetting the electrical neutrality of each body during the process. This hypothesis of conservation of charge has stood up under close examination, both for large-scale chargedbodies and for atoms, nuclei, and elementary particles. No exceptions have ever been found. Thus we add electric charge to our list of quantities-including energy and both linear and angular momentum-that obey a conservationlaw. We have a lot of examples: Radioactive decay of nuclei, annihilation process of electron and positron, and the pair production, the converse of annihilation.。
