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I am Walter Lewin. My lectures will in general, not be a repeat of your book, but it will be complementary to the book. The book will support my lectures, the lectures will support the book. You will not see any tedious derivation in my lectures. for that we have the book. But I will stress the concepts, and I will make you see beyond the equations, beyond the concepts. I will show you whether you like it or not, that physics is beautiful and you may even start to like it. I suggest you do not slip it up, not even in one day. 802 is not easy. We have new concepts every week. And before you know, you maybe too far behind. 我是Walter Lewin,大致上,我上課的方式不是重複課本的內容,是配合著課本,這本書會輔助我的演講,我的演講也會輔助這本書.上課中不會有冗長的推導,因為我們有課本.然而我會強調觀念,使你體會甚於方程式及觀念本身.我將展現,不管你喜歡與否,物理是很美的,甚至你可能開始喜歡它.我建議你不要忽略它,一天也不要.802(這們課)並不簡單,我們每週會有新的觀念.在你知道之前你可能已經落後太多. 電與磁環繞在我們周圍,我們有電燈,電子鐘,我們有麥克風,計算機,電視,錄放影機,收音機及電腦.光本身是一種電磁現象,無線電波也是.因為電的關係,藍天中七彩的彩虹會在那裡,我會在這們課中向大家解釋.汽車飛機火車可以使用是因為電力,馬也需要電力,因為肌肉收縮需要電力.你的神經系統也是由電力所驅動,原子分子以及所有的化學反應都是因為電力而存在.如果沒有電,你將無法「看」,如果沒有電,你的心臟將不會跳動,如果沒有電,你將無法思考,雖然我了解即使有電,你們之中仍有些人在那方面有困難.現代對於原子的概念為:原子核,大小跟原子比起來小很多,原子核中有質子,含正電荷,有中子,不帶電荷.質子的質量大約和中子差不多大約是六點七乘以十的負二十七次方公斤.抱歉,是一點七不是六點七.在這裡正電荷表示核子與中子,然後我們有一團雲狀的電子圍繞著它.如果這個原子是電中性的,則電子的數量和質子的數量一樣.如果你拿去了一個電子,這顆原子就會成為正離子,如果你加入了一個電子,這顆原子就會成為負離子,電子所帶的電量和質子所帶的電量相同,這就是為什麼中性原子的電子及質子的數目相同.電子的質量比質子小一千八百三時倍,因此在大部分的情況下,它的質量是可以被忽略的,原子所有的重量都是在原子核,如果我把六十億顆原子肩並肩排在一起,對吧!我取六十億是相當於全球人口的數目,而你只會得到六十公分的長度.這讓你知道原子有多麼的小.原子核的大小大約是十乘以十的負十二次方公分而原子大約是它的一千倍,電子雲大小大約是十的負八次方.如果你將六十億個它們疊加在一起來你只會得到這麼大.
[09:18~12:35] This is the first thing I would like you to see. I have here a conductor as a balloon and I will rub this rod with silk. And as I approach to the 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 non-conductor, it's not so easy to get charge exchange between the two. But if I do it long enough, I can certainly make that balloon positive. Then, they're both ( both 在此指rod 跟balloon 都被charge成帶正極的物體) positive and then they will repelling each other. First, the induction part, here by you will see the balloon comes to the glass rod. This experiment works best when it is dry in the winter. They don’t work well when it is humid, so it is a good time to teach 802 in winter. OK, there we go! These should be positive charged now and the balloon will come to the rod. See that, very clearly!! (Come on, baby!! ^^ Students laughing!! ) OK! So now I will try to get this balloon charge a little, so it doesn’t change over with the electron that will go from the balloon to the glass, and the glass doesn’t, isn’t a conductor itself and it is not always easy to get charge exchanges. OK, let's see whether I had succeeded now in making the balloon positively charged as well as the glass rod. If that’s the case, then the balloon is not going to like me, and the balloon now will be repelled. And you see that very clearly. To show you now there are indeed two different kinds of electricity, if I now rub with cat fur by tradition, we do that with cat fur, I don’t know why, by tradition we used the silk for the glass. So if we do this with cat fur now, then this becomes negatively charged then we are going to have two types of electricity. And since that balloon is positively charged, now the balloon will come to me. There it is!! So you have seen the first time now clearly, that there are two different kinds of electricity. The positive charge chosen by Franklin on the glass rod, and the negative charge on the rubber. So now, you may think, that if I approached a non-conducting balloon with a glass rod, and I have a non-conducting balloon here. You may think now the balloon will not come to the glass rod because there are no free electrons. So, these electrons cannot freely move, and so you don’t get this polarization, you don’t get this induction. That is not the case and this is actually quite subtle. [12:36~14:36] You have to look now at the atom’s scale. If I take an atom that you have here, you have positive charge and you have the electron is here in a cloud around the positive nucleus. If I bring a glass rod positively charge near by, then these electrons which are stuck to the atoms, they cannot freely moved like in conductors, however will spend a little bit more time on the side where the glass rod is, because they feel attracted by the glass rod , whereas the nuclei, if anything, want to go away from the glass rod. So what you are going to see is that in a way if I started off with the spherical atom, let's suppose this is a spherical atom or a spherical molecule. Then what will happen is that you get sort of a shape like this, and the electrons spend a little bit more time here than they spend here. And that means I had actually polarized that atom. If the electrons spend more time on this side of the atom than on this side, I have also created phenomenon of the induction and I therefore expect that this side becomes more negative than that side. I can show you that in a nice way with a transparency. Whereby, I have plus and minus signs, and I have equal numbers of plus and minus signs. So they represent neutral atoms. There you see. (Oh~ Boy! It's a little dirty but maybe I can clean it a little! OK! ^^ So, Here we go! ) [14:37~17:50] So, notice that there are equal amount of "+"s and "-"s. So think of plus and minus as one neutral atom. There is the representation. Now I am holding a glass rod on this side, which is positively charged. So each atom, the electron wants to go a little bit to this side, so the nucleus stays behind. And if each atom does that, this is what is going to happen. And now notice what you end up with, in the middle of the substance, plus and minus cancel each other out again. But on the right side, you have created a negatively charged layer, and on the left side, you have created a positively charged layer. So in a way you have again induction. So even in the no-conducting object, this side will go to negative, this side will go to positive, and therefore, if I approach a non-conducting balloon with a glass rod, I will also see the balloon come to me. So I can easily show you that it doesn’t make any difference whether I choose glass or I choose rubber. I can do it with both. Non-conducting balloon always have a potential problem. The potential problem is that, they can be charged by themselves just like the metal balloon can be charged by themselves. However, if I touched the metal balloon, then any charged is there will immediately flow me to the earth, we will understand that later, because this is a conductor. Remember, the electric fluid, is conducted by a metal, but not by a non-conductor. So, this is more difficult, even if I kiss it and touch it, it is not clear that I can take all the charge off. In fact, by doing that, I may make it worse. Let's hope it is not charged too much, and let's approach this glass rod and see whether I can convince you that indeed it (the balloon) is coming to the rod. Not because of the free electrons, but because of that process. (Oh~ Boy~~ ho!! ^^ ) And it should also do the same with rubber. (I hope… if it were negatively charged, it will go away!!) Oh! It does go away, so it is negatively charged. Did you see that? By touching it, I actually probably charged it and there is not much I can do about it! Very difficult to get charge off. I already have the suspicion when I approach it with the glass, it was too eager to come to the glass. Still negatively charged!! That is the way goes! It is not because of the demonstration fail but is because of the balloon is negatively charged and doesn’t want to give it up because it (this balloon) is a non-conductor. [17:51~21:12] Friction can cause electric charge. And that is exactly what happened when I touch this balloon and try to discharge it, through friction I may have charged it. If I take this party balloon that all of you may have seen and you just rub them on your shirt or on your trousers, they stick to my hands. They have charge on them, whether it is positive or negative I don’t know. I don’t even remember, it is not important. And so when I bring them to my hands, my hand is not a good conductor, but you get an induction. This phenomena that we just discussed then so the two attract each other. The positive and the negative side attract each other. You can stick them on the ceiling, you can stick them on the board, you can decorate your room that way. Very pretty, isn’t it? All that you can do now because of 802. Now this heavy balloon maybe a little bit more difficult, also I am wearing cotton, if you wear nylon or polyester it's much better, it's much easier to get … oh~ that’s good. That’s a nice one! I think we need a blue one! ^^ Here we go! So you see, friction, causes electricity; that is, of course, by the silk when we rub the glass and the catfur we rub the rubber, then we create charge on one. Of course, if you made the glass positively charged, your silk, will be automatically negatively charged. When you comb your hair, you may have noticed with dry weather that you can hear some cracking noise, cracking noise means sparks. You will learn all about sparks in this course though not today. But you can hear it if you are very quiet. As you do that, you charge the comb. I can hear the cracking. Interesting! So the comb is now charged, probably so am I, and there it comes. See! Not as good as the glass, but same idea. If you take your shirt off, and you make it dark in your dormitory, and you stand in front of the mirror, an amazing experience (all student laughing … ^^ ), I will be happy to do it for you because that I told you I already wear the cotton and it doesn’t work with cotton so well. You really have to do with the nylon shirt. And when you take your nylon shirt off, not only do you hear the cracking, but you actually see the glow with the teeny tiny sparks. You actually like a light ball. This experiment that you cannot miss, and I will suggest you try that this weekend. Do it with a friend, that is even more fun! [21:13~25:45] We all perhaps remember when you just walk around, do your normal things during the day, the rugs in rooms and you want to leave the room when you touch the door knob and you get a shock. The spark that flies over. It’s electricity. Even when you touch a person, you sometimes feel this shock. When you cook and you take Saran wrap (塑料薄膜) off these rolls, the damn stuff just doesn't want to come off because as you roll if off, there is a friction and it gets charged, and it often gets crumbled up, and it is very bad, very difficult to handle it. You've all experienced that! Also, cellophane around boxes with chocolate. The same thing happens, as you take it off, you charged it, whether you like it or not! I now want to do an experiment and I need a volunteer. I need a student who actually is wearing preferred not all cotton, but I think Simon you have a beautiful wonderful nylon parker. So if you are willing to sacrificed a little bit for the sake of the science and come over here and sit down here. Just relax, make sure your feet off to the ground, ok! So, what I am going to do now, Simon I am going to beat you with rag fur. (student laughing!) And as I beat you with the rag fur, you will get charged! And, since I don’t want you to be the only person who suffers on this experiment, I will also stand on an isolated stool, so if you become for instance positive charged, I don’t know whether it is positive or negative, I will get the other amount of charged. So we shared in the charge. And as I beat you, you will charged more and more, and I will charged more and more, and then we will have to convince the class that we are both charged. And we will do that in the way that will be hopefully rather convincing. I …. Ohm…. Let me just start beating you a little bit (student laughing ^^ ) to make you feeling at home. We know each other, right ? OK! Now of course as I mention to you this experiment works well when it is dry, so if you are too wet won’t worked, but lets see if you sweat too much that it doesn’t work too well. So, are you ready? (student laughing loudly!! ) I have heard in my hand an nylon flash tone. And we don’t know yet whether the voltage is because we will learn that in this course. To get a good flash out you need about a few thousand volts. So we will see when we make it dark shortly, and I will hold the flash light, the flash I did on the one hand, and nylon discharge to, and Simon will touch it on the other side. If we succeed it, then you may see some light. So, Simon look at me first, don’t touch it yet because we are going to make all the way dark. You know where it is? It’s there. Ok, make it dark, Marcos. Touch it! Touch it! WOW~ (Student exclaim!) Good! Try it again, touch it again!! OK, thank you! Can we have some light? (clapping!) Thank you very much!
[25:46~29:47] Equal charges repel each other! I showed that demonstration with the balloons. Here we have an instrument which called Van da Graaff. It is named after Prof. Van da Graaff who invented it, who was an MIT professor. And this instrument which I will not discuss in any detail though, but you will understand it latter on this course. I will tell you all about it later. This thing of this instrument has a super amber rod, and yet we don’t know what the voltage is, I mentioned already 20,000 volts between Simon and me. In this instrument you have to think in terms of several hundred thousand volts. So this instrument is not without danger, but of course that makes it more exciting to work with it! So, it is a super amber rod, and what I will do first now is to put some confetti on top and then we turn on the Van da Graaff, the confetti may at first go to the charge though, and it already on the top of it, and when it picks some of the charge, it will then spread out because it will repel each other. So lets go get some light there. We will make it better to see. Let me put some of this (confetti) on top. It is just some regular confetti, pieces of paper. All right, now all of I have to remember is how to start the Van da Graaff. (Machine running.) Most of the actions have already occurred. I will put a little bit more on! If he sparks, then don’t worry yet. (students laughing) Let's put some more on! Woops! Nothing left for the second class. Make it perhaps a little bit darker. Ahhh~ that’s too dark! Ok, try it once more, give it a zap. So look at the confetti on top, and I think it's quite convincing. Some of the confetti will stay there. Well, that's for reason that is not a good conductor, so it gets it first sucked in and if it doesn’t get charge of the Van de Graaff then it will not spread out. All right! So now let's try for the first time a little bit more quantitative. <!--[if !supportLineBreakNewLine]--> <!--[endif]-->
<!--[if !supportLineBreakNewLine]--> <!--[endif]--> [29:48~32:06] If I take two charges, and we use in general, we use for charge the symbol “q”. So here we have q1, and here we have q2. And let's say they are separated by a distance “r”. The unit vector in the direction from 1 to 2 are called that r-roof-12. The roof stands for unit vector. If these charges are equal, both minus or both plus, then they will repel each other. So, here, there is a force “F”, which I called F1,2, is a force due to number 1, and since action equals minus reaction, force here is F2,1, is equal in magnitude but 180o in opposite direction. Coulomb, France physicist, who did a lot of research on this in 19th, 18th century actually. Coulomb found the following relationship that the force is proportional to the product of the two charges. So, it is q1 times q2, times a constant which nowadays we call Coulomb's Constant K, divided by the distance between theses charges squared; and this is in direction of the unit vector that goes 1 to 2. This is the force on number 2 due to 1. And notice that this equation is sign sensitive, because if q1 and q2 are both negative, the force will be in this direction (from number 1 to 2). If both are positive is also in this direction (from number 1 to 2) as I have it. However, if one is positive and one is negative, you get a minus direction so this force flips over, and that one (left side force F2,1) then also obviously flips over. [32:07~40:02]
In the SI units, in this course, we will use for the unit of charge the coulomb named after this great man. One coulomb charge is a horrendous amount of charge, more than you'll ever see in your lifetime. We normally work with micro-coulombs, sometimes even less than that! The charge of one proton, which is exactly the same as the charge of one electron, is approximately 1.6 times 10 to the minus nineteen coulomb (1.6*10-19). So one coulomb is something like 6 times 10 to the eighteen (6*1018) protons or electrons if the charge is negative. This constant K in SI unit is 9 times 10 to the ninth (K=9*109). And the unit you can find out because you know that this (F) is newtons, this(q1*q2) is coulomb squared, and this(r2) is square meters. So, the unit is Newton-square-meters divided by square-coulomb (Nm2/C2). But that's not so important. No one ever thinks of it that way. For historical reasons, which may at time be a pain in the neck for you, we write for K is 1 divided by 4 pi epsilon-zero. There is nothing magic about that! It’s just a historical reason. So 1 divide 4 epsilon-zero is 9 times 10 to the ninth. (1/(4 πε 0) = 9*109). That’s all that matters. This epsilon-zero has a name, it's called “permittivity of free space” (ε0 = 8.854*10-12 (Farad/meter)). But you can forget about that. It is not important the name. Notice that there is a clear parallel with gravity. Newton's law of gravity that the force which in that case is always attracting, gravity never repels, is the product of two masses, and then you have here gravitational constant and again you have the distance square. So there is an enormous parallel between the two. It is a great beauty that electricity acts in the way is very parallel to the way that gravity works. If I added a third charge, for instance here q3, and if now I want to know what the forces are on q2, then I use the superposition principle which we used many times in 801 class. And we say ok the net force on number-2 (q2), is the force due to number-1 (q1), plus the force from number-3 (q3). If number-3, if this is positive and this is positive, and this is negative, then this force will be in this direction F3,2, and then the net force on number-2 will be the vectorial sum of these two (指來自q1和q3的力). Is it obvious that the superposition principle works? Not at all! It’s not at all obvious. Do we believe in it? Yes, we do! Why do we believe it? Because it's consistent with all the experiments that we have done. But the superposition principle, which is very powerful, is really not a matter of course. But it works. We can always use it and we will! If you compare 801 with 802, thereby comparing electricity with gravity, you will see that electric forces are way more powerful than gravitational forces. And the way I can best show you that is by taking two protons, which are distances d apart. Here is a proton, and here is a proton, and they are separate by a distance d. They repel each other, and the force by which they repel each other, is of course extremely easy to calculate. We know Coulomb’s Law, that law is called after Coulomb. And so the force, the electric force with which repel each other, this is now just the magnitude of the force, is the charge the proton, which is 1.6 times 10 to the negative nineteen (1.6*10-19) but I have to square that! I have to multiply by Coulomb’s Constant which is 9 times 10 to the ninth (9*109), and I divide it by d square (d2). That’s the electric force. If I want to know the gravitational force, which is the force with which they attract each other. These (electric force) are repelling force. I just want the magnitude here, then I have to take the mass of the proton, which is 1.7 times 10 to the minus twenty-seven (1.7*10-27), and I have to square that. Remember m1 times m2 times the gravitational constant, the gravitational constant in SI unit, is 6.7 times 10 to the minus 11 (6.7*10-11), and I divide that by d2. If now I compare the electric force with the gravitational force, so I divide it one by the other. Notice that d cancels. They both have d2 downstairs. So you will easily be able to show that this ratio is roughly 10 to the 36th (1036). So the electric force is thirty-six orders of magnitude more potent than the gravitational attraction. This teaches you some respect perhaps for 802. If these were the only forces that acted on the protons, and you bring them in the nucleus, which has the size of the 10 to the minus 12(10-12) centimeters. Then the acceleration that the proton will experience is the electric force divided by the mass of the proton. F equals m*a! Basics of 801. And if you take this electric force, when you make d 10 to the minus 12 (10-12) centimeters, which is 10 to the minus fourteen(10-14) meters, and you calculate this ratio and you will find that it is twenty-six order of magnitude higher than the gravitational acceleration on earth. Twenty-six order of magnitude higher. So you wonder what the hell holds the nucleus together if there is a such tremendous force on these protons. Well, what is holding them together are the nuclear forces which we do not fully understand, but thank goodness the nuclear forces are not part of 802. So I will leave that alone for now.
[40:03~47:48] So, what’s hold our world together? Well, on the nuclear scale, 10-12 centimeters, very important are the nuclear forces. On the atomic scale, up to thousands of kilometers is really electric forces that hold our world together. But on a much larger scale, planets, stars, and galaxies, it is the gravity that holds our world together. And now you may say ah that is very inconsistent with what you just told us because didn’t you just tell us the d cancels, if you compare the gravity with the electricity. Yes! However, most objects are neutral or very close to neutral. And so if you take the earth, it is very unlikely even that the earth as a whole will have a charge of more than 10 coulomb. That probably is already an exaggeration. So if I take the earth and I take the moon, and I put on both a charge of 10 coulomb. Here is the earth and here is the moon, and I could say just arbitrary 10 coulomb here, and let us put on here either –10 coulomb so they will attract each other. But given the distance so much nothing, the force is negligibly small. But, of course, the force of the gravity, which is proportional to their masses wins. And in this particular case, you take the earth and the moon, the gravitational force wins over the electric force by 25 orders of magnitude. So, even though our immediate surroundings, are dominated by electric forces, including your own body for that matter, the behavior of the universe on the large scaleis dictated by gravity. We will use various instruments to measure charge in a quantitative way. And one of these instruments that you will see, we will use it often the lectures that are to come, is called the electroscope. It is a very simple instrument. In general, it is just a conducting rod. It could be aluminum, metal. And at the end, are two pieces of tinsels, two pieces of aluminum foil, and often there is a nice knob here, and if I touch this with a charge object, then because this can conduct electricity, this can conduct the fire, as found by Benjamin Franklin. If I touch this object which is positively charged, then this object will become positively charged. If I charged it with negatively charge, it will become negatively charged. And you see now here these two very light pieces of aluminum foil will repel each other. And so you will see that this shows a certain angle, and the more charges there is and the larger that angle. So it gives a way doing some quantitative measurement. There are other electrical scopes which are not too different, just one central rod and there would have only one leave hanging there. That’s why you charge that one of then this leave will go up and if you charge more, it will go up even further. I don’t have electroscope now here, but what I want you to see that if I charge myself up, and I holding my hand this Christmas tree tinsels that in the way if I get enough charge only. And these tinsels will spread out. So the idea that immediately follows from the fact that you get certain amount of charge, whether it is negative charged form me or whether I am positively charged that didn’t make any difference. These tinsels will spread out. Of course, the best way I can do that is by charge myself with the Van de Graaff. And as I said earlier, the experiments of this nature are not entirely without risks. And so there is always the possibility that I don’t survive this demonstration. But don’t worry, in that case there will be someone else who will lecture 802 except he is not likely to show this demonstration again. So you might want to take a close look because this maybe the only time you will ever see it. So, I will give you some nice light on the Van de Graaff, and it's always a scary moment for me. Slip nice about the Van de Graaff, Now am I going to turn it on or Marco you have the courage to turn it on? You will turn it on? OK, hold it, Marco. This is too close for comforts. Are you ready? Are you nervous? (Dr. look at the student and said: ) See you! So look at the tinsels and try not to look at me please. Go ahead! I am now a living electroscope. If the weather is cooperating today, and if I have a long hair, you will even see my hair will start to act like an electroscope. We can try that, too. Or we can show it. (laughter and applause) Is it working? (yes! Student response) Ok, well this weekend make sure you take this nylon shirt off in front of the mirror and enjoy the experiment at home. Don’t try this ever. See you Friday. (Applause)
Comments:From glin - 11/16/05 9:13 PM Thanks!!! Please email me your information: name, brief bio for our volunteer database. gracelin_tx at msn dot com
From welkin25 - 11/16/05 4:24 PM Done.
From glin - 11/15/05 10:55 PM welkin25, why don't you finish proofreading the entire video and put your name on the table on the top of the page? This way, we can say this video is done - transcribed & proofread!!! Thanks.
From welkin25 - 11/15/05 9:32 PM 添加/修改了Robert Wu所說的紅字部份﹐但是沒有全篇聽完。
Last Modified 12/28/05 9:35 AM |
