Electricity and MagnetismWord下载.docx

1、Electricity and magnetism is all around us.We have electric lights, Electric clocks.We have microphones, calculators, televisions, VCRs, radio, computers.Light itself is an electromagnetic phenomenon as radio waves are.The colors of the rainbow in the blue sky are there because of electricity.And I

2、will teach you about that in this course.Cars, planes, trains can only run because of electricity.Horses need electricity because muscle contractions（肌肉收缩） require electricity.Your nerve system is driven by electricity.Atoms, molecule, all chemical reactions exist because of electricity.You could no

3、t see without electricity.Your heart would not beat without electricity.And you could not even think without electricity, though I realize that even with electricity some of you may have a problem with that.尽管我认为即使不缺少电，某些同学也成问题The modern picture of an atom is a nucleus, which is very small compared

4、to the size of the atom.The nucleus has protons, which are positively charged and it has neutrons, which have no charge.The mass of the proton is approximately the same as the mass of the neutron.Its about six point seven times ten to the minus twenty-seventh kilograms.One point seven.The positive c

5、harges here with the nucleons（核子）, with the neutrons, and then we have electrons in a cloud around it.And if the atom is neutral, the number of electrons and the number of protons is the same.If you take one electron off you get a positive ion.If you add an electron then you get a negative ion.The c

6、harge of the electron is the same as the charge of the proton.Thats why the number is the same for neutral atoms.The mass of the electron is about eighteen hundred thirty times smaller than the mass of the proton.s therefore negligibly small in most cases.All the mass of an atom is in the nucleus.If

7、 I take six billion atoms lined up touching other, I take six billion because thats about about the number of people on earth.Then you would only have a length of sixty centimeters.Gives you an idea of how small the atoms are.The nucleus has a size of about ten to the minus twelve centimeters.And th

8、e atom itself is about ten thousand times larger.The cloud of electrons, which is about ten to the minus eight centimeters.And if you line six billion of those up you only get this much.Already in six hundred BC, it was known that if you rub amber that it can attract pieces of dry leaves.And the Gre

9、ek word for amber is electron.So thats where electricity got its name from.In the sev- sixteenth century there were more substances known to do this.For instance glass and sulfur.And it was also known and written that when people were bored at parties that the women would rub their amber jewelry and

10、 would touch frogs, which then would start jumping of desperation, which people considered to be fun, not understanding what actually was happening to the amber nor what was happening to the frogs.In the eighteenth century it was discovered that there are two types of electricity.One if you rub glas

11、s and another if you rub rubber or amber for that matter.Lets call one A and the other B.It was known that A repels A and B repels B but A attracts B.And it was Benjamin Franklin without any knowledge of electrons and protons who introduced the idea that all substances are penetrated with what he ca

12、lled electric fluid, electric fire.And he stated if you get too much of the fire then youre positively charged and if you have a deficiency（缺点，缺乏）of that fire, then youre negatively charged.He introduced the sign convention and he decided that if you rub glass that that is an excess of fire and he c

13、alled that therefore positive.You will see later in this course why this choice he had fifty percent chance is extremely unfortunate but we have to live with it.So if you take this fluid according to Benjamin Franklin and bring it from one substance to the other, then the one that gets an excess bec

14、omes positively charged but automatically as a consequence of that the one from which you take the fluid becomes negatively charged.And so thats the whole idea behind the conservation of charge.You cannot create charge.If you create plus then you automatically create minus.Plus and plus repel each o

15、ther.Minus and minus repel each other.And plus and minus attract.And Benjamin Franklin who did experiments also noticed that the more fire you have the stronger the forces.The closer these objects are to each other, the stronger the forces.And there are some substances that he noticed, which conduct

16、 this fluid, which conduct this fire, and they are called conductors.If I have a glass rod as I have here and I rub it then it gets this positive charge that we just discussed.So here is this rod and I rub it with some silk and it will get positively charged.What happens now to an object that I brin

17、g close to this rod and I will start off with从（用）开始, end up with结束；以而结束；以告终 taking a conductor.And the reason why I choose a conductor is that conductors have a small fraction of their electrons, which are not bound to atoms but which can freely move around in the conductor.s characteristic for a co

18、nductor, for metals.s not the case with nonconductors.There the all electrons are fixed to individual atoms.So here we have a certain fraction of electrons that can wander around.Whats going to happen that electrons want to be attracted by these positive charges.Plus and minus attract each other.And

19、 so some of these electrons, which can freely move will move in this direction and so the plus stay behind.This process we call induction.You get sort of a polarization.You get a charge division.s a very small effect, perhaps only one in ten to the thirteen electrons that was originally here will en

20、d up here but thats all it takes.So we get a polarization and we get a little bit more negative charge on the right side than we have on the left side.And so whats going to happen is, since the attraction between these two will be stronger than the repelling force between these two because the dista

21、nce is smaller and Franklin had already noticed the shorter the distance the stronger the force.What will happen is that if this object is free to move it will move towards this rod.And this is the first thing that I would like you to see.I have here a conductor that is a balloon, helium-filled ball

22、oon.And I will rub this rod with silk.And as I approach that balloon you will see that the balloon comes to the rod.I will then try to rub with that rod several times on that balloon.It will take a while perhaps because the rod itself is a very good nonconductor.s not so easy to get charge exchange

23、between the two.But if I do it long enough I can certainly make that balloon positive.Then theyre both positive.And then they will repel each other.But first, the induction part, whereby you will see the balloon come to the glass rod.These experiments work best when it is dry, in the winter.They don

24、t work so well when it is humid so its a good time to teach eight o two in the winter.OK there we go this should be positively charged now.And the balloon wants to come to the glass.You see that?Very clearly.Come on baby.OK.So now I will try to get this balloon charged a little so there is a change

25、of electrons that go from the balloon to the glass.And the glass doesnt, its not a conductor itself so it is not always so easy to get charge exchanges.OK lets see whether I have succeeded now in making the balloon positively charged as well as the glass rod.If thats the case then the balloon is not

26、 going to like me.The balloon will now be repelled.And you see that very clearly.To show you now that there are indeed two different kinds of electricity.If I now rub with cat fur by tradition we do that with cat fur, I dont know why by tradition we use silk for the glass.So if we do this with cat f

27、ur now then this becomes negatively charged.Remember there were two types of electricity.And since that balloon is positively charged now the balloon will come to me.And there it is.Now it comes to me.So youve seen for the first time now clearly that there are two different kinds of electricity.The

28、positive charge is chosen by Franklin on the glass rod and the negative charge on the rubber.So now you may think that if I approach a nonconducting balloon with a glass rod and I have a nonconducting balloon here you may think now that this balloon will not come to the glass rod because there are n

29、o free electrons.So these electrons cannot freely move and so you dont get this polarization.You dont get this induction.But that is not the case.And this is actually quite subtle.You have to look now at the atomic scale.If I take an atom like you have here.You have positive charge and you have the electrons here in a cloud around the positive nucleus.If I bring a glass rod positively charged nearby, then these electrons which are stuck to the atoms, they cannot freely move like in cond