今日課程內容 CH21 電荷 庫倫定律 CH22 電場 點電荷 電偶極

21.2 Electric Charge:

21.3 Conductors and Insulators Conductors( 導體 ) are materials through which charge can move freely; examples include metals (such as copper in common lamp wire), the human body, and tap water. Nonconductors( 非導體 )—also called insulators( 絕緣體 )—are materials through which charge cannot move freely; examples include rubber, plastic, glass, and chemically pure water. Semiconductors( 半導體 ) are materials that are intermediate between conductors and insulators; examples include silicon and germanium in computer chips. Superconductors( 超導體 ) are materials that are perfect conductors, allowing charge to move without any hindrance. The properties of conductors and insulators are due to the structure 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 a central nucleus. When atoms of a conductor come together to form the solid, some of their outermost (and so most loosely held) electrons become free to wander about within the solid, leaving behind positively charged atoms ( positive ions).We call the mobile electrons conduction electrons. There are few (if any) free electrons in a nonconductor.

Conductor: Charge flows freely Metals Insulator: Almost no charge flows Most other materials Some materials are semiconductors.( 半導體 ) 21-3 Insulators( 絕緣體 ) and Conductors( 導體 ) Some materials are superconductors.

21.3 Conductors and Insulators

21.5 Charge is Quantized( 量子化 ). Since the days of Benjamin Franklin, our understanding of of the nature of electricity has changed from being a type of ‘continuous fluid’ to a collection of smaller charged particles. The total charge was found to always be a multiple of a certain elementary charge, “e”: The value of this elementary charge is one of the fundamental constants of nature, and it is the magnitude of the charge of both the proton and the electron. The value of “e” is: 1 C = ×10 18 e 1 mol e = C

21.5 Charge is Quantized Elementary particles either carry no charge, or carry a single elementary charge. When a physical quantity such as charge can have only discrete values, rather than any value, we say the quantity is quantized. It is possible, For example, to find a particle that has no charge at all, or a charge of +10e, or -6e, but not a particle with a charge of, say, 3.57e.

21.4 Coulomb’s Law Coulomb’s law: The SI unit of charge is the coulomb. The constant The quantity  0 is called the permittivity( 電容率 ) constant

21.4 Coulomb’s Law Current( 電流 ) is the rate dq/dt at which charge moves past a point or through a region in which i is the current (in amperes) and dq (in coulombs) is the amount of charge moving past a point or through a region in time dt (in seconds). Therefore,

21.4 Coulomb’s Law If there are n charged particles, they interact independently in pairs, and the force on any one of them, say particle 1, is given by the vector sum in which, F 1,4 is the force acting on particle 1 due to the presence of particle 4, etc. As with gravitational force law, the shell theorem has analogs in electrostatics:

Example, The net force due to two other particles:

Example, The net force due to two other particles, cont.:

Example, Equilibrium of two forces:

Example, Charge Sharing:

Example, Charge Sharing, cont.:

Example, Mutual Electric Repulsion in a Nucleus:

21.6 Charge is Conserved If one rubs a glass rod with silk, a positive charge appears on the 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 charged bodies and for atoms, nuclei, and elementary particles. Example 1: Radioactive decay of nuclei, in which a nucleus transforms into (becomes) a different type of nucleus. A uranium-238 nucleus ( 238 U) transforms into a thorium- 234 nucleus ( 234 Th) by emitting an alpha particle. An alpha particle has the same makeup as a helium-4 nucleus, it has the symbol 4 He. Here the net charge is 238. Example 2: An electron e (charge -e) and its antiparticle, the positron e (charge +e), undergo an annihilation process, transforming into two gamma rays (high-energy light):. Here the net charge is zero. Example 3: A gamma ray transforms into an electron and a positron. Here the net charge is again zero.

Chapter 22 Electric Fields

22.2 The Electric Field: The Electric Field is a vector field. The SI unit for the electric field is the newton per coulomb (N/C).

22.2 The Electric Field:

22.3 Electric Field Lines( 電場線 ): At any point, the direction of a straight field line or the direction of the tangent to a curved field line gives the direction of at that point. The field lines are drawn so that the number of lines per unit area, measured in a plane that is perpendicular to the lines, is proportional to the magnitude of E. Thus, E is large where field lines are close together and small where they are far apart.

22.3 Electric Field Lines:

22.4 The Electric Field due to a Point: To find the electric field due to a point charge q (or charged particle) at any point a distance r from the point charge, we put a positive test charge q 0 at that point. The direction of E is directly away from the point charge if q is positive, and directly toward the point charge if q is negative. The electric field vector is: The net, or resultant, electric field due to more than one point charge can be found by the superposition principle. The net electric field at the position of the test charge is

Example, The net electric field due to three charges:

22.5 The Electric Field due to an Electric Dipole( 電偶極 ):

22.5 The Electric Field due to an Electric Dipole: From symmetry, the electric field E at point P—and also the fields E + and E - due to the separate charges that make up the dipole—must lie along the dipole axis, which we have taken to be a z axis. From the superposition principle for electric fields, the magnitude E of the electric field at P is The product qd, which involves the two intrinsic properties q and d of the dipole, is the magnitude p of a vector quantity known as the electric dipole moment of the dipole.

Example, Electric Dipole and Atmospheric Sprites: We can model the electric field due to the charges in the clouds and the ground by assuming a vertical electric dipole that has charge -q at cloud height h and charge +q at below-ground depth h (Fig. 22-9c). If q =200 C and h =6.0 km, what is the magnitude of the dipole’s electric field at altitude z 1 =30 km somewhat above the clouds and altitude z 2 =60 km somewhat above the stratosphere? Sprites (Fig. 22-9a) are huge flashes that occur far above a large thunderstorm. They are still not well understood but are believed to be produced when especially powerful lightning occurs between the ground and storm clouds, particularly when the lightning transfers a huge amount of negative charge - q from the ground to the base of the clouds (Fig. 22-9b).

22.6 The Electric Field due to a Continuous Charge( 連續電荷分佈 ): When we deal with continuous charge distributions, it is most convenient to express the charge on an object as a charge density rather than as a total charge. For a line of charge, for example, we would report the linear charge density (or charge per unit length), whose SI unit is the coulomb per meter. Table 22-2 shows the other charge densities we shall be using.

22.6 The Electric Field due to a Line Charge:

Example, Electric Field of a Charged Circular Rod

22.6 The Electric Field due to a Charged Disk:

22.8: A Point Charge in an Electric Field When a charged particle, of charge q, is in an electric field, E, set up by other stationary or slowly moving charges, an electrostatic force, F, acts on the charged particle as given by the above equation.

22.8: A Point Charge in an Electric Field: Measuring the Elementary ChargeInk-Jet Printing

Example, Motion of a Charged Particle in an Electric Field

22.9: A Dipole in an Electric Field

22.9: A Dipole in an Electric Field: Potential Energy

Example, Torque, Energy of an Electric Dipole in an Electric Field

習題 Ch21: 35 Ch22: 9, 14, 19, 24, 30, 37, 50, 54, 59