第一节有趣的静电现象课件.ppt

8.1 Electrical Chargesa,In winter,when we take off our pullover,it is easy to see sparks and hear cracks.b,When you touch somebody,it may even give you a shock in winter.c,After a long time,you will find that there is a lot of dust on your TV screen.How to explain above phenomena?repelrepelattract1.Two kinds of chargesThe polythene（聚乙烯 塑料）is negatively charged by rubbing it with a dry,woolen cloth.The perspex（有机玻璃）is positively charged by rubbing it with a dry,woolen cloth.2 Charge interactions+attractrepelrepelattractF FUnlike charges attractF FF FLike charges repelLike charges repel;unlike charges attract.3.Where charges come fromElectrons are orbiting the nucleus and much lighter than protons and neutrons.+Neutral/unchargedatoms have equal numbers of electrons and protonsNormally the net(overall)charge on a material is zeroElectrons are often removed from and added to an atom by rubbingimbalance of chargePositively charged+Negatively charged+Rubbing materials together does not make electric charge.It just separates charges that are already there.When two materials are rubbed together,one material ends up with more electrons than normal,so it has net negative charge.The other lost electrons,it is left with a net positive charge.Polythene rod(-)cloth(+)Polythene cloth Perspexelectron affinity（吸引力）When polythene is rubbed with a cloth,polythene pulls electrons from clothA plastic strip was rubbed with cotton.The correct explanation for why the plastic strip becomes positively charged is that.A the plastic strip acquired extra protons from the cotton.B the plastic strip acquired extra protons during the charging process.C protons were created as the result of the charging process.D the plastic strip lost electrons to the cotton during the charging process.Saran Wrap has a larger electron affinity than Nylon.If Nylon is rubbed against Saran Wrap,which would end up with the excess negative charge?_ Explain.The material with the greater affinity for electrons can take electrons away from the other material.Saran wrap takes electrons from nylon and acquires the negative charge.In turn,the nylon loses electrons and becomes charged positively.Saran wrap 4.Conductors and insulatorsIn some materials,part of their electrons are loosely held and can move freely through the material.Such materials are called conductors of electricity.conductors human bodyGood poormetal water(silver earth copper aluminium)carbon Plastics glass e.g:rubber PVC dry airPolythene Perspex In some materials,electrons are tightly held to atoms and are not free to move-although they can be transferred by rubbing.They are called insulators.When they are charged by rubbing,the charges will only appear on the rubbed portion and cannot escape easily.insulatorsSemi-conductors:these are in-between materials.They are poor conductors when cold,but much better conductors when warm.e.g silicon硅,germanium锗