22西尔斯物理学1.ppt

The principle of superposition of electric fields 3. The electric field made up of many point charges the total force 库仑的介绍参阅：/200406/ca435124.htm 点电荷系的场强 电场强度的叠加原理 * 22 Electric charge and electric field A fundamental branch of physics is known as electromagnetism, the study of electric and magnetic interactions. Q1 Q2 + + Q1 Q2 – – Like charge (two positive charges or two negative charges ) repel; Unlike charge (a positive charge and a negative charge) attract. 22-2 Electric Charge Q1 Q2 + – 22-3 Electric Charge and the structure of matter The structure of atoms can be described in terms of three particles: the negatively charged electron, the positively charged proton, and the uncharged neutron. The protons and neutrons in an atom make up a small, very dense core called the nucleus. 1. Principle of consevation of charge Charge is not created or destroyed, only transferred. The algebraic sum of all the electric charges in any Closed system is constant. 2. The magnitude of charge of the electron or proton is a natural unit of charge. We say that charge is quantized. 22-4 Conductors, Insulators, and induced charges Conductor transfers charge on contact Insulator does not transfer charge on contact Semiconductor might transfer charge on contact Induced Charge Induction: The process that gives another body a charge of opposite sign without losing any of its own charge. The excess charges from induction are called inducted charge. 练习1：研究处于范德格拉夫（Van de Graaff）起电机与人之间的带电小球的运动情况，以获得对电场分布的大致感受。观看视频，解释原因。 Polarization Polarization is similar to induction, except it doesn’t create induction charge, instead, it shifts the charge within the molecules of a neutral insulator. It creates the electric force between charged object and uncharged object. (C.A.Coulomb 1736 ?1806） French physical scientist 22-5 Coulomb’s Law Coulomb’s law Coulomb’s law states that the force between tw