1 00:00:00 --> 00:00:00,5 2 00:00:00,5 --> 00:00:04,441 I'm Walter Lewin. My lectures will in general not 3 00:00:04,441 --> 00:00:09,122 be a repeat of your book but they will be complementary to 4 00:00:09,122 --> 00:00:11,832 the book. The book will support my 5 00:00:11,832 --> 00:00:14,871 lectures. My lectures will support the 6 00:00:14,871 --> 00:00:17,581 book. You will not see any tedious 7 00:00:17,581 --> 00:00:21,934 derivations in my lectures. For that we have the book. 8 00:00:21,934 --> 00:00:26,861 But I will stress the concepts and I will make you see beyond 9 00:00:26,861 --> 00:00:29,653 the equations, beyond the concepts. 10 00:00:29,653 --> 00:00:33,924 I will show you whether you like it 11 00:00:33,924 --> 00:00:36,515 or not that physics is beautiful. 12 00:00:36,515 --> 00:00:39,187 And you may even start to like it. 13 00:00:39,187 --> 00:00:42,912 I suggest you do not slip up, not even one day, 14 00:00:42,912 --> 00:00:47,446 eight oh two is not easy. We have new concepts every week 15 00:00:47,446 --> 00:00:51,09 and before you know you may be too far behind. 16 00:00:51,09 --> 00:00:54,491 Electricity and magnetism is all around us. 17 00:00:54,491 --> 00:00:57,73 We have electric lights. Electric clocks. 18 00:00:57,73 --> 00:01:00,321 We have microphones, calculators, 19 00:01:00,321 --> 00:01:02,264 televisions, VCRs, radio, 20 00:01:02,264 --> 00:01:05,099 computers. Light 21 00:01:05,099 --> 00:01:09,795 itself is an electromagnetic phenomenon as radio waves are. 22 00:01:09,795 --> 00:01:14,491 The colors of the rainbow in the blue sky are there because 23 00:01:14,491 --> 00:01:18,216 of electricity. And I will teach you about that 24 00:01:18,216 --> 00:01:20,483 in this course. Cars, planes, 25 00:01:20,483 --> 00:01:23,884 trains can only run because of electricity. 26 00:01:23,884 --> 00:01:28,58 Horses need electricity because muscle contractions require 27 00:01:28,58 --> 00:01:31,981 electricity. Your nerve system is driven by 28 00:01:31,981 --> 00:01:35,544 electricity. Atoms, molecule, 29 00:01:35,544 --> 00:01:40,199 all chemical reactions exist because of electricity. 30 00:01:40,199 --> 00:01:43,576 You could not see without electricity. 31 00:01:43,576 --> 00:01:47,683 Your heart would not beat without electricity. 32 00:01:47,683 --> 00:01:52,065 And you could not even think without electricity, 33 00:01:52,065 --> 00:01:57,815 though I realize that even with electricity some of you may have 34 00:01:57,815 --> 00:02:02,288 a problem with that. The modern picture of an atom 35 00:02:02,288 --> 00:02:07,673 is a nucleus which is very small compared to the size of the 36 00:02:07,673 --> 00:02:13,029 atom. The nucleus has protons which 37 00:02:13,029 --> 00:02:19,725 are positively charged and it has neutrons which have no 38 00:02:19,725 --> 00:02:23,62 charge. The mass of the proton is 39 00:02:23,62 --> 00:02:29,586 approximately the same as the mass of the neutron. 40 00:02:29,586 --> 00:02:35,429 It's about six point seven times ten to the minus 41 00:02:35,429 --> 00:02:41,759 twenty-seventh kilograms. One point seven. 42 00:02:41,759 --> 00:02:45,447 The positive charges here with the nucleons, 43 00:02:45,447 --> 00:02:49,648 with the neutrons, and then we have electrons in a 44 00:02:49,648 --> 00:02:53,592 cloud around it. And if the atom is neutral the 45 00:02:53,592 --> 00:02:58,48 number of electrons and the number of protons is the same. 46 00:02:58,48 --> 00:03:02,853 If you take one electron off you get a positive ion. 47 00:03:02,853 --> 00:03:07,14 If you add an electron then you get a negative ion. 48 00:03:07,14 --> 00:03:11,085 The charge of the electron is the 49 00:03:11,085 --> 00:03:13,45 same as the charge of the proton. 50 00:03:13,45 --> 00:03:17,221 That's why the number is the same for neutral atoms. 51 00:03:17,221 --> 00:03:21,361 The mass of the electron is about eighteen hundred thirty 52 00:03:21,361 --> 00:03:24,392 times smaller than the mass of the proton. 53 00:03:24,392 --> 00:03:27,718 It's therefore negligibly small in most cases. 54 00:03:27,718 --> 00:03:30,75 All the mass of an atom is in the nucleus. 55 00:03:30,75 --> 00:03:34,52 If I take six billion atoms lined up touching other, 56 00:03:34,52 --> 00:03:38,808 I take six billion because that's about about the number of 57 00:03:38,808 --> 00:03:43,244 people on earth. Then you would only have a 58 00:03:43,244 --> 00:03:48,797 length of sixty centimeters. Gives you an idea of how small 59 00:03:48,797 --> 00:03:53,106 the atoms are. The nucleus has a size of about 60 00:03:53,106 --> 00:03:56,553 ten to the minus twelfth centimeters. 61 00:03:56,553 --> 00:04:01,724 And the atom itself is about ten thousand times larger. 62 00:04:01,724 --> 00:04:06,894 The cloud of electrons. Which is about ten to the minus 63 00:04:06,894 --> 00:04:11,491 eight centimeters. And if you line six billion of 64 00:04:11,491 --> 00:04:17,989 those up you only get this much. Already in six hundred BC, 65 00:04:17,989 --> 00:04:23,043 it was known that if you rub amber that it can attract pieces 66 00:04:23,043 --> 00:04:26,833 of dry leaves. And the Greek word for amber is 67 00:04:26,833 --> 00:04:30,201 electron. So that's where electricity got 68 00:04:30,201 --> 00:04:33,823 its name from. In the sev- sixteenth century 69 00:04:33,823 --> 00:04:37,444 there were more substances known to do this. 70 00:04:37,444 --> 00:04:42,077 For instance glass and sulfur. And it was also known and 71 00:04:42,077 --> 00:04:47,13 written that when people were bored at parties that the women 72 00:04:47,13 --> 00:04:51,462 would rub their amber jewelry and would 73 00:04:51,462 --> 00:04:55,762 touch frogs which then would start jumping of desperation 74 00:04:55,762 --> 00:05:00,062 which people considered to be fun, not understanding what 75 00:05:00,062 --> 00:05:04,668 actually was happening to the amber nor what was happening to 76 00:05:04,668 --> 00:05:07,586 the frogs. In the eighteenth century it 77 00:05:07,586 --> 00:05:11,732 was discovered that there are two types of electricity. 78 00:05:11,732 --> 00:05:15,725 One if you rub glass and another if you rub rubber or 79 00:05:15,725 --> 00:05:19,717 amber for that matter. Let's call one A and the other 80 00:05:19,717 --> 00:05:23,454 B. It was known that A repels A 81 00:05:23,454 --> 00:05:28,498 and B repels B but A attracts B. And it was Benjamin Franklin 82 00:05:28,498 --> 00:05:32,618 without any knowledge of electrons and protons who 83 00:05:32,618 --> 00:05:37,495 introduced the idea that all substances are penetrated with 84 00:05:37,495 --> 00:05:41,194 what he called electric fluid, electric fire. 85 00:05:41,194 --> 00:05:45,903 And he stated if you get too much of the fire then you're 86 00:05:45,903 --> 00:05:50,864 positively charged and if you have a deficiency of that fire 87 00:05:50,864 --> 00:05:54,288 then you're negatively charged. 88 00:05:54,288 --> 00:05:57,907 He introduced the sign convention and he decided that 89 00:05:57,907 --> 00:06:02,083 if you rub glass that that is an excess of fire and he called 90 00:06:02,083 --> 00:06:05,563 that therefore positive. You will see later in this 91 00:06:05,563 --> 00:06:09,182 course why this choice he had fifty percent chance is 92 00:06:09,182 --> 00:06:12,592 extremely unfortunate but we have to live with it. 93 00:06:12,592 --> 00:06:16,42 So if you take this fluid according to Benjamin Franklin 94 00:06:16,42 --> 00:06:20,666 and bring it from one substance to the other then the one that 95 00:06:20,666 --> 00:06:23,728 gets an excess becomes positively charged but 96 00:06:23,728 --> 00:06:27,881 automatically as a consequence of that the 97 00:06:27,881 --> 00:06:31,515 one from which you take the fluid becomes negatively 98 00:06:31,515 --> 00:06:34,081 charged. And so that's the whole idea 99 00:06:34,081 --> 00:06:36,432 behind the conservation of charge. 100 00:06:36,432 --> 00:06:40,138 You cannot create charge. If you create plus then you 101 00:06:40,138 --> 00:06:44,271 automatically create minus. Plus and plus repel each other. 102 00:06:44,271 --> 00:06:46,552 Minus and minus repel each other. 103 00:06:46,552 --> 00:06:50,542 And plus and minus attract. And Benjamin Franklin who did 104 00:06:50,542 --> 00:06:54,462 experiments also noticed that the more fire you have the 105 00:06:54,462 --> 00:06:57,274 stronger the forces. 106 00:06:57,274 --> 00:07:02,863 The closer these objects are to each other the stronger the 107 00:07:02,863 --> 00:07:06,332 forces. And there are some substances 108 00:07:06,332 --> 00:07:12,018 that he noticed which conduct this fluid, which conduct this 109 00:07:12,018 --> 00:07:15,487 fire, and they are called conductors. 110 00:07:15,487 --> 00:07:21,365 If I have a glass rod as I have here and I rub it then it gets 111 00:07:21,365 --> 00:07:25,509 this positive charge that we just discussed. 112 00:07:25,509 --> 00:07:29,942 So here is this rod and I rub it 113 00:07:29,942 --> 00:07:34,026 with some silk and it will get positively charged. 114 00:07:34,026 --> 00:07:38,943 What happens now to an object that I bring close to this rod 115 00:07:38,943 --> 00:07:42,611 and I will start off with taking a conductor. 116 00:07:42,611 --> 00:07:47,362 And the reason why I choose a conductor is that conductors 117 00:07:47,362 --> 00:07:52,529 have a small fraction of their electrons which are not bound to 118 00:07:52,529 --> 00:07:57,113 atoms but which can freely move around in the conductor. 119 00:07:57,113 --> 00:08:02,365 That's characteristic for a conductor, for metals. 120 00:08:02,365 --> 00:08:05,016 That's not the case with nonconductors. 121 00:08:05,016 --> 00:08:08,714 There the all electrons are fixed to individual atoms. 122 00:08:08,714 --> 00:08:12,551 So here we have a certain fraction of electrons that can 123 00:08:12,551 --> 00:08:15,412 wander around. What's going to happen that 124 00:08:15,412 --> 00:08:19,319 electrons want to be attracted by these positive charges. 125 00:08:19,319 --> 00:08:21,622 Plus and minus attract each other. 126 00:08:21,622 --> 00:08:25,878 And so some of these electrons which can freely move will move 127 00:08:25,878 --> 00:08:29,018 in this direction and so the plus stay behind. 128 00:08:29,018 --> 00:08:34,6 This process we call induction. You get sort of a polarization. 129 00:08:34,6 --> 00:08:38,063 You get a charge division. It's a very small effect, 130 00:08:38,063 --> 00:08:41,933 perhaps only one in ten to the thirteen electrons that was 131 00:08:41,933 --> 00:08:45,736 originally here will end up here but that's all it takes. 132 00:08:45,736 --> 00:08:49,878 So we get a polarization and we get a little bit more negative 133 00:08:49,878 --> 00:08:53,545 charge on the right side than we have on the left side. 134 00:08:53,545 --> 00:08:57,619 And so what's going to happen is since the attraction between 135 00:08:57,619 --> 00:09:01,557 these two will be stronger than the repelling force between 136 00:09:01,557 --> 00:09:05,428 these two because the distance is smaller and Franklin had 137 00:09:05,428 --> 00:09:09,695 already noticed the shorter the distance the 138 00:09:09,695 --> 00:09:13,189 stronger the force. What will happen is that if 139 00:09:13,189 --> 00:09:17,518 this object is free to move it will move towards this rod. 140 00:09:17,518 --> 00:09:21,772 And this is the first thing that I would like you to see. 141 00:09:21,772 --> 00:09:25,949 I have here a conductor that is a balloon, helium-filled 142 00:09:25,949 --> 00:09:28,683 balloon. And I will rub this rod with 143 00:09:28,683 --> 00:09:31,341 silk. And as I approach that balloon 144 00:09:31,341 --> 00:09:34,835 you will see that the balloon comes to the rod. 145 00:09:34,835 --> 00:09:40,456 I will then try to rub with that rod several times on that 146 00:09:40,456 --> 00:09:43,467 balloon. It will take a while perhaps 147 00:09:43,467 --> 00:09:47,65 because the rod itself is a very good nonconductor. 148 00:09:47,65 --> 00:09:52,25 It's not so easy to get charge exchange between the two. 149 00:09:52,25 --> 00:09:57,186 But if I do it long enough I can certainly make that balloon 150 00:09:57,186 --> 00:10:00,197 positive. Then they're both positive. 151 00:10:00,197 --> 00:10:03,125 And then they will repel each other. 152 00:10:03,125 --> 00:10:08,144 But first the induction part whereby you will see the balloon 153 00:10:08,144 --> 00:10:13,673 come to the glass rod. These experiments work best 154 00:10:13,673 --> 00:10:16,287 when it is dry. In the winter. 155 00:10:16,287 --> 00:10:21,875 They don't work so well when it is humid so it's a good time to 156 00:10:21,875 --> 00:10:24,759 teach eight oh two in the winter. 157 00:10:24,759 --> 00:10:29,446 OK there we go this should be positively charged now. 158 00:10:29,446 --> 00:10:33,232 And the balloon wants to come to the glass. 159 00:10:33,232 --> 00:10:35,575 You see that? Very clearly. 160 00:10:35,575 --> 00:10:37,018 Come on baby. OK. 161 00:10:37,018 --> 00:10:43,528 So now I will try to get this balloon charged a little so 162 00:10:43,528 --> 00:10:49,073 there is a change of electrons that go from the balloon to the 163 00:10:49,073 --> 00:10:52,346 glass. And the glass doesn't it's not 164 00:10:52,346 --> 00:10:57,709 a conductor itself so it is not always so easy to get charge 165 00:10:57,709 --> 00:11:01,073 exchanges. OK let's see whether I have 166 00:11:01,073 --> 00:11:06,163 succeeded now in making the balloon positively charged as 167 00:11:06,163 --> 00:11:10,618 well as the glass rod. If that's the case then the 168 00:11:10,618 --> 00:11:16,269 balloon is not going to like me. The balloon will now be 169 00:11:16,269 --> 00:11:18,762 repelled. And you see that very clearly. 170 00:11:18,762 --> 00:11:22,533 To show you now that there are indeed two different kinds of 171 00:11:22,533 --> 00:11:26,368 electricity if I now rub with cat fur by tradition we do that 172 00:11:26,368 --> 00:11:30,266 with cat fur I don't know why by tradition we use silk for the 173 00:11:30,266 --> 00:11:32,503 glass. So if we do this with cat fur 174 00:11:32,503 --> 00:11:35,06 now then this becomes negatively charged. 175 00:11:35,06 --> 00:11:37,872 Remember there were two types of electricity. 176 00:11:37,872 --> 00:11:41,132 And since that balloon is positively charged now the 177 00:11:41,132 --> 00:11:43,688 balloon will come to me. And there it is. 178 00:11:43,688 --> 00:11:47,139 Now it comes to me. So you've seen 179 00:11:47,139 --> 00:11:52,24 for the first time now clearly that there are two different 180 00:11:52,24 --> 00:11:56,636 kinds of electricity. The positive charge is chosen 181 00:11:56,636 --> 00:12:01,737 by Franklin on the glass rod and the negative charge on the 182 00:12:01,737 --> 00:12:04,99 rubber. So now you may think that if I 183 00:12:04,99 --> 00:12:10,354 approach a nonconducting balloon with a glass rod and I have a 184 00:12:10,354 --> 00:12:15,718 nonconducting balloon here you may think now that this balloon 185 00:12:15,718 --> 00:12:21,652 will not come to the glass rod because there are no free 186 00:12:21,652 --> 00:12:24,666 electrons. So these electrons cannot 187 00:12:24,666 --> 00:12:28,971 freely move and so you don't get this polarization. 188 00:12:28,971 --> 00:12:33,621 You don't get this induction. But that is not the case. 189 00:12:33,621 --> 00:12:36,463 And this is actually quite subtle. 190 00:12:36,463 --> 00:12:39,907 You have to look now at the atomic scale. 191 00:12:39,907 --> 00:12:43,007 If I take an atom like you have here. 192 00:12:43,007 --> 00:12:48,173 You have positive charge and you have the electrons here in a 193 00:12:48,173 --> 00:12:52,25 cloud around the positive nucleus. 194 00:12:52,25 --> 00:12:56,463 If I bring a glass rod positively charged nearby then 195 00:12:56,463 --> 00:13:00,028 these electrons which are stuck to the atoms, 196 00:13:00,028 --> 00:13:05,133 they cannot freely move like in conductors, however will spend a 197 00:13:05,133 --> 00:13:10,156 little bit more time on the side where the glass rod is because 198 00:13:10,156 --> 00:13:14,855 they feel attracted by the glass rod, whereas the nuclei if 199 00:13:14,855 --> 00:13:18,339 anything want to go away from the glass rod, 200 00:13:18,339 --> 00:13:22,309 so what you're going to see is that 201 00:13:22,309 --> 00:13:26,438 in a way if I started off with a spherical atom let's suppose 202 00:13:26,438 --> 00:13:30,499 this were a spherical atom or a spherical molecule then what 203 00:13:30,499 --> 00:13:34,628 will happen is that you get sort of a shape like this and the 204 00:13:34,628 --> 00:13:38,619 electrons spend a little bit more time here than they spend 205 00:13:38,619 --> 00:13:42,748 here and that means that I have actually polarized that atom. 206 00:13:42,748 --> 00:13:46,602 If the electrons spend more time on this side of the atom 207 00:13:46,602 --> 00:13:50,318 than on this side I have also created the phenomenon of 208 00:13:50,318 --> 00:13:54,585 induction and I therefore expect that this side 209 00:13:54,585 --> 00:13:59,144 becomes more negative than that side. 210 00:13:59,144 --> 00:14:06,238 And I can show you that in a nice way with a transparency 211 00:14:06,238 --> 00:14:13,965 whereby I have plus and minus signs and I have equal number of 212 00:14:13,965 --> 00:14:19,791 plus and minus signs. So they represent neutral 213 00:14:19,791 --> 00:14:22,958 atoms. There you see them. 214 00:14:22,958 --> 00:14:27,138 Boy. It's a little dirty but maybe 215 00:14:27,138 --> 00:14:31,325 see I can clean it a little. 216 00:14:31,325 --> 00:14:31,792 OK. OK. 217 00:14:31,792 --> 00:14:34,827 So here we go. So notice there are equal 218 00:14:34,827 --> 00:14:39,262 amount of pluses and minuses, so think of the plus and the 219 00:14:39,262 --> 00:14:43,152 minuses as one neutral atom. Just a representation. 220 00:14:43,152 --> 00:14:47,743 Now I'm holding a glass rod on this side which is positively 221 00:14:47,743 --> 00:14:50,7 charged. And so each atom the electrons 222 00:14:50,7 --> 00:14:55,369 want to go a little bit to this side and so the nucleus stays 223 00:14:55,369 --> 00:14:58,326 behind. And if each atom does that this 224 00:14:58,326 --> 00:15:03,914 is what's going to happen. And now notice what you end up 225 00:15:03,914 --> 00:15:06,439 with. In the middle of the substance 226 00:15:06,439 --> 00:15:09,614 plus and minuses cancel each other out again. 227 00:15:09,614 --> 00:15:13,799 But on the right side you have created a negatively charged 228 00:15:13,799 --> 00:15:17,767 layer and on the left side you have created a positively 229 00:15:17,767 --> 00:15:20,942 charged layer. And so in a way you have again 230 00:15:20,942 --> 00:15:23,683 induction. So even in the nonconducting 231 00:15:23,683 --> 00:15:27,94 objects this side will turn negative and this side will turn 232 00:15:27,94 --> 00:15:32,197 positive and therefore if I approach a nonconducting balloon 233 00:15:32,197 --> 00:15:36,784 with a glass rod I will also see the balloon 234 00:15:36,784 --> 00:15:39,832 come to me. And so I can easily show you 235 00:15:39,832 --> 00:15:42,567 that. It doesn't make any difference 236 00:15:42,567 --> 00:15:46,396 whether I choose glass or whether I choose rubber. 237 00:15:46,396 --> 00:15:50,382 I can do it with both. Nonconducting balloons always 238 00:15:50,382 --> 00:15:54,602 have a potential problem. The potential problem is that 239 00:15:54,602 --> 00:15:58,665 they can be charged by themselves just like the metal 240 00:15:58,665 --> 00:16:01,557 balloons can be charged by themselves. 241 00:16:01,557 --> 00:16:07,316 However, if I touch the metal balloon then any charges there 242 00:16:07,316 --> 00:16:12,026 will immediately flow through me to the earth we will understand 243 00:16:12,026 --> 00:16:14,942 that later. Because this is a conductor. 244 00:16:14,942 --> 00:16:19,053 That remember the electric fluid is conducted by a metal 245 00:16:19,053 --> 00:16:22,642 but not by a nonconductor. So with this it's more 246 00:16:22,642 --> 00:16:25,632 difficult. Even if I kiss it and touch it 247 00:16:25,632 --> 00:16:29,295 it's not clear that I can take all the charge off. 248 00:16:29,295 --> 00:16:32,734 In fact by doing that I may even make it worse. 249 00:16:32,734 --> 00:16:36,398 Let's hope that it is not charged 250 00:16:36,398 --> 00:16:41,899 too much and let's approach it with this glass rod and see 251 00:16:41,899 --> 00:16:47,69 whether I can convince you that indeed it's coming to the rod 252 00:16:47,69 --> 00:16:52,709 not because of the free electrons but because of that 253 00:16:52,709 --> 00:16:54,157 process. Oh boy. 254 00:16:54,157 --> 00:16:57,342 Ho. And it should also do the same 255 00:16:57,342 --> 00:17:01,975 with rubber I hope. If it were negatively it'd go 256 00:17:01,975 --> 00:17:05,063 away. Ha it does go away so it is 257 00:17:05,063 --> 00:17:12,349 negatively charged you see that. By touching it I actually 258 00:17:12,349 --> 00:17:18,73 probably charged it and there's not much I can do about it. 259 00:17:18,73 --> 00:17:22,25 Very difficult to get charge off. 260 00:17:22,25 --> 00:17:28,849 I already had a suspicion when I approached it with the glass 261 00:17:28,849 --> 00:17:32,92 it was too eager to come to the glass. 262 00:17:32,92 --> 00:17:38,2 Still negatively charged. That's the way it goes. 263 00:17:38,2 --> 00:17:43,259 It's not because the demonstration 264 00:17:43,259 --> 00:17:47,962 failed but it's because the balloon is charged and doesn't 265 00:17:47,962 --> 00:17:52,417 want to give it up because it's a it is a nonconductor. 266 00:17:52,417 --> 00:17:57,12 Friction can cause electric charge and that's exactly what 267 00:17:57,12 --> 00:18:01,905 happened when I touched this balloon and tried to discharge 268 00:18:01,905 --> 00:18:04,71 it. Through friction I may actually 269 00:18:04,71 --> 00:18:08,505 have charged it. If I take these party balloons 270 00:18:08,505 --> 00:18:13,29 that all of you may have seen and you just rub them on your 271 00:18:13,29 --> 00:18:18,24 shirt on your trousers they stick to my hand. 272 00:18:18,24 --> 00:18:21,577 They have charge on them. Whether it's positive or 273 00:18:21,577 --> 00:18:24,572 negative I don't know, I don't even remember. 274 00:18:24,572 --> 00:18:27,908 It's not important. And so when I bring them to my 275 00:18:27,908 --> 00:18:31,925 hand, my hand is not a good conductor but you get induction, 276 00:18:31,925 --> 00:18:36,011 this phenomenon that we just discussed and so the two attract 277 00:18:36,011 --> 00:18:38,734 each other. The positive and the negative 278 00:18:38,734 --> 00:18:42,342 side attract each other. And you can stick them on the 279 00:18:42,342 --> 00:18:44,793 ceiling. Or you can stick them on the 280 00:18:44,793 --> 00:18:47,313 board. You can decorate your room that 281 00:18:47,313 --> 00:18:50,932 way. Very pretty isn't it. 282 00:18:50,932 --> 00:18:55,528 All that you can do now because of eight oh two. 283 00:18:55,528 --> 00:19:01,297 Now these heavy balloons may be a little bit more difficult. 284 00:19:01,297 --> 00:19:06,577 Also I'm wearing cotton. If you wear nylon or polyester 285 00:19:06,577 --> 00:19:10,781 it's much better. It's much easier to get oh 286 00:19:10,781 --> 00:19:16,453 that's good, that's a nice one, I think we need a blue one. 287 00:19:16,453 --> 00:19:20,702 There we go. So you see friction causes 288 00:19:20,702 --> 00:19:23,702 electricity. That's of course why the silk 289 00:19:23,702 --> 00:19:28,238 when we rubbed the glass and the cat fur we rub the rubber then 290 00:19:28,238 --> 00:19:32,263 we create charge on one. Of course if you make the glass 291 00:19:32,263 --> 00:19:36,653 positively charged your silk will be automatically negatively 292 00:19:36,653 --> 00:19:39,507 charged. When you comb your hair you may 293 00:19:39,507 --> 00:19:43,604 have noticed with dry weather that you hear some cracking 294 00:19:43,604 --> 00:19:46,092 noise. Cracking noise means sparks. 295 00:19:46,092 --> 00:19:49,824 And you will learn all about sparks 296 00:19:49,824 --> 00:19:53,568 in this course though not today. 297 00:19:53,568 --> 00:19:58,4 But you can hear it if you're very quiet. 298 00:19:58,4 --> 00:20:02,991 And as you do that you charge the comb. 299 00:20:02,991 --> 00:20:07,34 I can hear the cracking. Interesting. 300 00:20:07,34 --> 00:20:14,105 So the comb is now charged. Probably so am I and there it 301 00:20:14,105 --> 00:20:15,313 comes. See. 302 00:20:15,313 --> 00:20:22,199 It's not as good as the glass but same idea. 303 00:20:22,199 --> 00:20:27,98 If you take your shirt off and you make it and you make it dark 304 00:20:27,98 --> 00:20:33,762 in your dormitory and you stand in front of a mirror an amazing 305 00:20:33,762 --> 00:20:37,493 experience. And I'd be happy to do it for 306 00:20:37,493 --> 00:20:42,435 you because but I told you I really wear cotton and it 307 00:20:42,435 --> 00:20:45,419 doesn't work with cotton so well. 308 00:20:45,419 --> 00:20:49,429 You really have to do it with a nylon shirt. 309 00:20:49,429 --> 00:20:53,813 And when you take that nylon shirt 310 00:20:53,813 --> 00:20:58,667 off not only do you hear the cracking but you actually see 311 00:20:58,667 --> 00:21:02,33 the glow of these teeny weeny little sparks. 312 00:21:02,33 --> 00:21:05,225 You actually are like a light bulb. 313 00:21:05,225 --> 00:21:08,632 It is an experiment that you cannot miss. 314 00:21:08,632 --> 00:21:12,465 And I would suggest you try that this weekend. 315 00:21:12,465 --> 00:21:15,957 Do it with a friend. That's even more fun. 316 00:21:15,957 --> 00:21:20,386 We'll all perhaps remember when you just walk around. 317 00:21:20,386 --> 00:21:23,452 Do your normal things during the day. 318 00:21:23,452 --> 00:21:29,531 There are rugs in rooms and you want to leave the room and 319 00:21:29,531 --> 00:21:32,796 you touch the doorknob and you get a shock. 320 00:21:32,796 --> 00:21:36,371 It's a spark that flies over. It's electricity. 321 00:21:36,371 --> 00:21:40,879 Even when you touch a person you sometimes feel this shock. 322 00:21:40,879 --> 00:21:45,232 When you cook and you take saran wrap off these rolls the 323 00:21:45,232 --> 00:21:50,051 damn stuff just doesn't want to come off because as you roll it 324 00:21:50,051 --> 00:21:54,559 off there is friction and it gets charged and it often gets 325 00:21:54,559 --> 00:22:00,232 crumpled up and it's very bad, very difficult to handle it. 326 00:22:00,232 --> 00:22:05,017 You've all experienced that. Also cellophane around boxes 327 00:22:05,017 --> 00:22:08,178 with chocolate the same thing happens. 328 00:22:08,178 --> 00:22:13,305 As you take it off you charge it, whether you like it or not. 329 00:22:13,305 --> 00:22:17,833 I now want to do an experiment and I need a volunteer. 330 00:22:17,833 --> 00:22:22,789 I need a student who actually is wearing preferably not all 331 00:22:22,789 --> 00:22:27,915 cotton but I think Simon you have a beautiful wonderful nylon 332 00:22:27,915 --> 00:22:30,478 parka. So if you are willing to 333 00:22:30,478 --> 00:22:35,922 sacrifice a little bit for the sake of 334 00:22:35,922 --> 00:22:41,376 science and come over here and sit down here. 335 00:22:41,376 --> 00:22:46,21 Just relax. Make sure that your feet are 336 00:22:46,21 --> 00:22:48,442 off the ground. OK. 337 00:22:48,442 --> 00:22:55,755 So what I'm going to do now Simon I'm going to beat you with 338 00:22:55,755 --> 00:23:00,465 cat fur. And as I beat you with cat fur 339 00:23:00,465 --> 00:23:04,721 you will get charged and since I 340 00:23:04,721 --> 00:23:09,684 don't want you to be the only person who suffers under this 341 00:23:09,684 --> 00:23:14,733 experiment I will also stand on an insulated stool so if you 342 00:23:14,733 --> 00:23:19,697 become for instance positively charged I don't know whether 343 00:23:19,697 --> 00:23:24,489 it's positive or negative I would get the other amount of 344 00:23:24,489 --> 00:23:27,313 charge. So we share in the charge. 345 00:23:27,313 --> 00:23:32,02 And as I beat you you will charge up more and more and I 346 00:23:32,02 --> 00:23:37,373 will charge up more and more and then we 347 00:23:37,373 --> 00:23:44,375 will have to convince the class that that we are both charged. 348 00:23:44,375 --> 00:23:50,917 And we will do that in a way that will be hopefully rather 349 00:23:50,917 --> 00:23:55,738 convincing. I let me just start beating you 350 00:23:55,738 --> 00:24:00,099 a little bit. To make you feel at home. 351 00:24:00,099 --> 00:24:03,313 We know each other right. OK. 352 00:24:03,313 --> 00:24:12,194 Now of course as I mentioned to you these experiments work well 353 00:24:12,194 --> 00:24:19,066 when it is dry and so if you are too wet it won't work. 354 00:24:19,066 --> 00:24:26,066 But let's see if you sweat a little bit too much then it 355 00:24:26,066 --> 00:24:30,393 doesn't work too well. So we ready? 356 00:24:30,393 --> 00:24:35,484 I have here in my hand a neon flash tube. 357 00:24:35,484 --> 00:24:42,582 And although we don't know yet what voltage is because we 358 00:24:42,582 --> 00:24:47,614 will learn about that in this course, to get a good flash out 359 00:24:47,614 --> 00:24:51,304 of these you need about a few thousand volts. 360 00:24:51,304 --> 00:24:56,253 And so we will see and we'll make it dark shortly and I will 361 00:24:56,253 --> 00:25:00,195 hold the flashlight, the flashlight in one hand, 362 00:25:00,195 --> 00:25:04,808 the neon discharge tube, and then Simon will touch it on 363 00:25:04,808 --> 00:25:08,666 the other side. And if we've succeeded then you 364 00:25:08,666 --> 00:25:13,699 may see some light. So Simon look at me first, 365 00:25:13,699 --> 00:25:19,691 don't touch it yet, because we're going to make it 366 00:25:19,691 --> 00:25:24,339 all the way dark. You know where it is, 367 00:25:24,339 --> 00:25:28,864 it's there, OK, make it darker Marcos. 368 00:25:28,864 --> 00:25:31,066 Touch it. Touch it. 369 00:25:31,066 --> 00:25:34,979 OK, try it again, touch it again. 370 00:25:34,979 --> 00:25:36,569 OK. Thank you. 371 00:25:36,569 --> 00:25:43,174 Can we have some light. [applause] Thank you very much. 372 00:25:43,174 --> 00:25:49,981 Equal charges repel each other. I've shown that, 373 00:25:49,981 --> 00:25:52,566 the demonstration with the balloons. 374 00:25:52,566 --> 00:25:56,85 Here we have an instrument which is called the Vandegraaff. 375 00:25:56,85 --> 00:26:00,913 It's named after Professor Vandegraaff, who invented it. 376 00:26:00,913 --> 00:26:04,162 It was an MIT professor. And this instrument, 377 00:26:04,162 --> 00:26:08,372 which I will not discuss in any detail though but you will 378 00:26:08,372 --> 00:26:12,804 understand it later on in the course, I'll tell you all about 379 00:26:12,804 --> 00:26:15,611 it later. Just think of this instrument 380 00:26:15,611 --> 00:26:20,912 as a super amber rod. And although we don't know yet 381 00:26:20,912 --> 00:26:24,851 what voltage is, I mentioned already the twenty 382 00:26:24,851 --> 00:26:30,245 thousand volts between Simon and me, in this instrument you have 383 00:26:30,245 --> 00:26:34,612 to think in terms of several hundred thousand volts. 384 00:26:34,612 --> 00:26:38,037 So this instrument is not without danger. 385 00:26:38,037 --> 00:26:42,918 But that of course makes it more exciting to work with it. 386 00:26:42,918 --> 00:26:49,083 So it's a super amber rod and what I will do first now is to 387 00:26:49,083 --> 00:26:55,946 put some confetti on top and when we turn on the Vandegraaff 388 00:26:55,946 --> 00:27:01,529 the confetti may at first go to the charged dome, 389 00:27:01,529 --> 00:27:07,927 it is already on top of it, and when it picks up some of 390 00:27:07,927 --> 00:27:14,79 the charge it will then spread out because it it will repel. 391 00:27:14,79 --> 00:27:21,42 So let's get some some light on there which will make it a 392 00:27:21,42 --> 00:27:28,4 little bit better to see. Let me put some of this 393 00:27:28,4 --> 00:27:37,95 on top. It's just regular confetti, 394 00:27:37,95 --> 00:27:50,029 pieces of paper. All right now all I have to 395 00:27:50,029 --> 00:28:06,031 remember is how to start the most of the action has already 396 00:28:06,031 --> 00:28:14,332 occurred. I will put a little bit more 397 00:28:14,332 --> 00:28:21,288 on. [laughter] If you see sparks 398 00:28:21,288 --> 00:28:31,16 don't worry yet. [laughter] Put some more on. 399 00:28:31,16 --> 00:28:41,032 More and nothing left for the second 400 00:28:41,032 --> 00:28:45,6 class. [laughter] Make it perhaps a 401 00:28:45,6 --> 00:28:50,033 little darker. Ah that's too dark. 402 00:28:50,033 --> 00:28:55,944 [laughter] OK. We'll try it once more give it 403 00:28:55,944 --> 00:29:00,78 a zap so look at the confetti on top. 404 00:29:00,78 --> 00:29:05,214 And I think it's quite convincing. 405 00:29:05,214 --> 00:29:10,05 Some of the confetti will stay there. 406 00:29:10,05 --> 00:29:19,46 Well that's the reason that it's not a good conductor and 407 00:29:19,46 --> 00:29:28,23 so it get it first sucked in and if it doesn't get charge of the 408 00:29:28,23 --> 00:29:33,66 Vandegraaff then it will not spread out. 409 00:29:33,66 --> 00:29:39,228 All right. So now let's try for the first 410 00:29:39,228 --> 00:29:44,935 time to be a little bit more quantitative. 411 00:29:44,935 --> 00:29:51,2 If I take two charges and we use in 412 00:29:51,2 --> 00:29:56,646 general we use for charge the symbol Q. 413 00:29:56,646 --> 00:30:03,095 So here we have Q one. And here we have Q two. 414 00:30:03,095 --> 00:30:09,831 And let's say they're separated by a distance R. 415 00:30:09,831 --> 00:30:18,143 And the unit vector in the direction from one to two I call 416 00:30:18,143 --> 00:30:24,604 that R roof one-two. The roof stands for unit 417 00:30:24,604 --> 00:30:27,58 vector. These charges are equal, 418 00:30:27,58 --> 00:30:32,861 both minus or both plus, then they will repel each other 419 00:30:32,861 --> 00:30:37,757 and so here there is a force F which I call one-two. 420 00:30:37,757 --> 00:30:43,134 It is the force on two due to number one and since action 421 00:30:43,134 --> 00:30:49,279 equals minus reaction force here is to one equal in magnitude but 422 00:30:49,279 --> 00:30:53,695 a hundred eighty degrees in opposite direction. 423 00:30:53,695 --> 00:31:00,748 Coulomb, the French physicist, who did a lot of research on 424 00:31:00,748 --> 00:31:05,798 this in the nineteenth eighteenth century actually. 425 00:31:05,798 --> 00:31:09,838 Coulomb found the following relationship. 426 00:31:09,838 --> 00:31:15,394 That the force is proportional to the product of the two 427 00:31:15,394 --> 00:31:18,828 charges. So it's Q one times Q two. 428 00:31:18,828 --> 00:31:24,686 Times a constant which nowadays we call Coulomb's constant, 429 00:31:24,686 --> 00:31:29,333 K. Divided by the distance between 430 00:31:29,333 --> 00:31:33,218 these charges squared. And it is in direction of the 431 00:31:33,218 --> 00:31:36,036 unit vector that goes from one to two. 432 00:31:36,036 --> 00:31:39,236 This is the force on number two due to one. 433 00:31:39,236 --> 00:31:42,816 And notice that this equation is sign sensitive. 434 00:31:42,816 --> 00:31:47,387 Because if Q one and Q two are both negative the source is in 435 00:31:47,387 --> 00:31:51,5 the the force is in this direction and if they are both 436 00:31:51,5 --> 00:31:55,233 positive it's also in this direction as I have it. 437 00:31:55,233 --> 00:31:59,499 However if the if one is positive and one is negative you 438 00:31:59,499 --> 00:32:04,102 get minus this direction so this force 439 00:32:04,102 --> 00:32:09,16 flips over and that one then obviously also flips over. 440 00:32:09,16 --> 00:32:14,499 In the SI units in this course we will use for the unit of 441 00:32:14,499 --> 00:32:18,714 charge the coulomb named after this great man. 442 00:32:18,714 --> 00:32:23,491 One coulomb charge is a horrendous amount of charge. 443 00:32:23,491 --> 00:32:27,612 More than you will ever see in your lifetime. 444 00:32:27,612 --> 00:32:30,89 We normally work with microcoulombs, 445 00:32:30,89 --> 00:32:37,428 sometimes even less than that. The charge of one proton, 446 00:32:37,428 --> 00:32:42,745 which is exactly the same as the charge of one electron, 447 00:32:42,745 --> 00:32:48,642 is approximately one point six times ten to the minus nineteen 448 00:32:48,642 --> 00:32:52,025 coulomb. So one coulomb is something 449 00:32:52,025 --> 00:32:57,535 like six times ten to the eighteen protons or electrons if 450 00:32:57,535 --> 00:33:02,658 the charge is negative. This constant K in SI units is 451 00:33:02,658 --> 00:33:06,428 nine times ten to the ninth. 452 00:33:06,428 --> 00:33:12,034 And the unit you can find out because you know that this is 453 00:33:12,034 --> 00:33:17,64 newtons, this is coulomb squared and this is square meters. 454 00:33:17,64 --> 00:33:23,052 So the unit is newton square meters newtons square meters 455 00:33:23,052 --> 00:33:28,368 divided by square coulombs. But that's not so important. 456 00:33:28,368 --> 00:33:31,557 No one ever thinks of it that way. 457 00:33:31,557 --> 00:33:36,099 For historical reasons which may at 458 00:33:36,099 --> 00:33:41,506 times be a pain in the neck for you we write for K one divided 459 00:33:41,506 --> 00:33:46,115 by four pi epsilon zero. There is nothing magic about 460 00:33:46,115 --> 00:33:49,217 that. It's just a historical reason. 461 00:33:49,217 --> 00:33:54,446 And so one divided by four pi epsilon zero is nine times ten 462 00:33:54,446 --> 00:33:57,725 to the ninth. That's all that matters. 463 00:33:57,725 --> 00:34:02,955 This epsilon zero has a name it's called the permittivity of 464 00:34:02,955 --> 00:34:06,588 free space. But you can forget about that. 465 00:34:06,588 --> 00:34:12,818 It's not important the name. Notice that there is a clear 466 00:34:12,818 --> 00:34:17,138 parallel with gravity. Newton's law of gravity that 467 00:34:17,138 --> 00:34:21,457 the force, which in that case is always attracting, 468 00:34:21,457 --> 00:34:25,69 gravity never repels, is the product of two masses 469 00:34:25,69 --> 00:34:30,701 and then you have here the gravitational constant and again 470 00:34:30,701 --> 00:34:35,279 you have the distance squared. So there is an enormous 471 00:34:35,279 --> 00:34:39,771 parallel between the two. There's a great beauty that 472 00:34:39,771 --> 00:34:43,831 electricity acts in a way that is 473 00:34:43,831 --> 00:34:47,983 very parallel to the way that gravity works. 474 00:34:47,983 --> 00:34:52,23 If I added a third charge, for instance here, 475 00:34:52,23 --> 00:34:58,119 Q three, and if now I want to know what the force is on Q two, 476 00:34:58,119 --> 00:35:03,815 then I use the superposition principle which we've used many 477 00:35:03,815 --> 00:35:08,835 times in eight oh one, and we say OK the net force on 478 00:35:08,835 --> 00:35:15,786 number two is the force due to number one plus the force from 479 00:35:15,786 --> 00:35:19,619 number three. If number three if this is 480 00:35:19,619 --> 00:35:25,616 positive and this is positive and this were negative then this 481 00:35:25,616 --> 00:35:29,449 force would be in this direction, F one, 482 00:35:29,449 --> 00:35:35,347 F three two and then the net force on number two would be the 483 00:35:35,347 --> 00:35:40,164 vectorial sum of these two. Is it obvious that the 484 00:35:40,164 --> 00:35:44,194 superposition principal works? Not at all. 485 00:35:44,194 --> 00:35:49,594 It's not at all obvious. Do we believe in it? 486 00:35:49,594 --> 00:35:52,591 Yes we do. Why do we believe in it? 487 00:35:52,591 --> 00:35:57,528 Because it's consistent with all experiments that we have 488 00:35:57,528 --> 00:36:00,701 done. But the superposition principle 489 00:36:00,701 --> 00:36:05,549 which is very powerful is really not a matter of course. 490 00:36:05,549 --> 00:36:08,546 But it works. We can always use it. 491 00:36:08,546 --> 00:36:11,983 And we will. If you compare eight oh one 492 00:36:11,983 --> 00:36:16,479 with eight oh two thereby comparing electricity with 493 00:36:16,479 --> 00:36:22,518 gravity you will see that electric forces are 494 00:36:22,518 --> 00:36:27,461 way more powerful than gravitational forces. 495 00:36:27,461 --> 00:36:34,358 And the way I can best show you that is by taking two protons 496 00:36:34,358 --> 00:36:41,14 which are a distance D apart. Here is a proton and here is a 497 00:36:41,14 --> 00:36:46,887 pro- proton and they are separated by a distance D. 498 00:36:46,887 --> 00:36:52,874 They repel each other. And the force by which they 499 00:36:52,874 --> 00:36:57,747 repel each other is of course extremely easy to calculate. 500 00:36:57,747 --> 00:37:01,68 We know Coulomb's law. That law is called after 501 00:37:01,68 --> 00:37:03,818 Coulomb. And so the force, 502 00:37:03,818 --> 00:37:08,178 the electric force with which they repel each other, 503 00:37:08,178 --> 00:37:11,854 this is just the magnitude now of the force, 504 00:37:11,854 --> 00:37:16,899 is the charge of the proton which is one point six times ten 505 00:37:16,899 --> 00:37:22,199 to the minus nineteen but I have to square that, 506 00:37:22,199 --> 00:37:25,5 I have to multiply it by Coulomb's constant, 507 00:37:25,5 --> 00:37:29,875 which is nine times ten to the ninth, and I divide it by D 508 00:37:29,875 --> 00:37:32,485 squared. That's the electric force. 509 00:37:32,485 --> 00:37:35,632 If I want to know the gravitational force, 510 00:37:35,632 --> 00:37:39,7 which is the force with which they attract each other, 511 00:37:39,7 --> 00:37:43,768 these are repelling forces, but I just want magnitudes 512 00:37:43,768 --> 00:37:47,453 here, then I have to take the mass of the proton, 513 00:37:47,453 --> 00:37:52,135 which is one point seven times ten to the minus twenty-seven I 514 00:37:52,135 --> 00:37:56,05 have to square that remember M one 515 00:37:56,05 --> 00:38:00,026 times M two times the gravitational constant. 516 00:38:00,026 --> 00:38:05,087 The gravitational constant in SI units is six point seven 517 00:38:05,087 --> 00:38:10,51 times ten to the minus eleven and I divide that by D squared. 518 00:38:10,51 --> 00:38:15,661 If now I compare the electric force with the gravitational 519 00:38:15,661 --> 00:38:20,451 force, so I divide one by the other, notice that the D 520 00:38:20,451 --> 00:38:23,343 cancels. They both have D squared 521 00:38:23,343 --> 00:38:27,048 downstairs. And so you will easily be able 522 00:38:27,048 --> 00:38:31,691 to show that this ratio is roughly 523 00:38:31,691 --> 00:38:36,3 ten to the thirty-six. So the electric force is 524 00:38:36,3 --> 00:38:41,31 thirty-six orders of magnitude more potent than the 525 00:38:41,31 --> 00:38:46,721 gravitational attraction. This teaches you some respect 526 00:38:46,721 --> 00:38:52,132 perhaps for eight oh two. If these were the only forces 527 00:38:52,132 --> 00:38:57,944 that acted on the protons and you bring them in the nucleus 528 00:38:57,944 --> 00:39:02,88 which has a size of only ten to the minus 529 00:39:02,88 --> 00:39:07,85 twelfth centimeters then the acceleration that the proton 530 00:39:07,85 --> 00:39:13,442 will experience is the electric force divided by the mass of the 531 00:39:13,442 --> 00:39:15,128 proton. F equals MA. 532 00:39:15,128 --> 00:39:19,655 Basis of eight oh one. And if you take this electric 533 00:39:19,655 --> 00:39:25,246 force when you make D ten to the minus twelfth centimeters which 534 00:39:25,246 --> 00:39:30,306 is ten to the minus fourteen meters and you calculate this 535 00:39:30,306 --> 00:39:35,431 ratio you will find that it is twenty-six orders of 536 00:39:35,431 --> 00:39:39,257 magnitude higher than the gravitational acceleration on 537 00:39:39,257 --> 00:39:41,807 earth. Twenty-six orders of magnitude 538 00:39:41,807 --> 00:39:44,216 higher. So you wonder what the hell 539 00:39:44,216 --> 00:39:48,184 holds the nucleus together. If there is such a tremendous 540 00:39:48,184 --> 00:39:51,655 force on these protons. Well, what is holding them 541 00:39:51,655 --> 00:39:55,41 together are the nuclear forces, which we do not fully 542 00:39:55,41 --> 00:39:59,732 understand, but thank goodness the nuclear forces are not part 543 00:39:59,732 --> 00:40:03,274 of eight oh two so I will leave that alone for now. 544 00:40:03,274 --> 00:40:06,887 So what holds our world together? 545 00:40:06,887 --> 00:40:11,377 Well on the nuclear scale ten to the minus twelve centimeters 546 00:40:11,377 --> 00:40:14,145 very important are the nuclear forces. 547 00:40:14,145 --> 00:40:17,736 On an atomic scale up to thousands of kilometers, 548 00:40:17,736 --> 00:40:21,926 it's really electric forces that hold our world together. 549 00:40:21,926 --> 00:40:25,817 But on a much larger scale, planets and stars and the 550 00:40:25,817 --> 00:40:29,632 galaxy, it is gravity that holds our world together. 551 00:40:29,632 --> 00:40:34,122 And now you may say ah that's very inconsistent with what you 552 00:40:34,122 --> 00:40:38,686 just told us because didn't you tell us that D 553 00:40:38,686 --> 00:40:43 cancels if you compare gravity with electricity. 554 00:40:43 --> 00:40:46,672 Yes, however, most objects are neutral or 555 00:40:46,672 --> 00:40:52,18 very close to neutral and so if you take the earth it is very 556 00:40:52,18 --> 00:40:57,78 unlikely even that the earth as a whole would have a charge of 557 00:40:57,78 --> 00:41:02,37 more than ten coulombs. That probably is already an 558 00:41:02,37 --> 00:41:06,134 exaggeration. So if I take the earth and I 559 00:41:06,134 --> 00:41:11,641 take the moon and I put on both a charge of ten 560 00:41:11,641 --> 00:41:15,446 coulombs, here's the earth and here's the moon, 561 00:41:15,446 --> 00:41:20,326 and I put say just arbitrarily ten coulombs here and that is 562 00:41:20,326 --> 00:41:23,965 put on here either minus, minus ten coulombs, 563 00:41:23,965 --> 00:41:28,679 so they will attract each other, but given their distance, 564 00:41:28,679 --> 00:41:32,814 it's almost nothing. The force is negligibly small. 565 00:41:32,814 --> 00:41:37,446 But of course the force of gravity, which is proportional 566 00:41:37,446 --> 00:41:41,002 to their masses, wins and in this particular 567 00:41:41,002 --> 00:41:45,054 case if you take the earth and the 568 00:41:45,054 --> 00:41:50,427 moon the gravitational force wins over the electric force by 569 00:41:50,427 --> 00:41:55,891 twenty-five orders of magnitude. So even though our immediate 570 00:41:55,891 --> 00:41:59,988 surroundings are dominated by electric forces, 571 00:41:59,988 --> 00:42:05,361 including your own body for that matter, the behavior of the 572 00:42:05,361 --> 00:42:09,732 universe on a large scale is dictated by gravity. 573 00:42:09,732 --> 00:42:14,558 We will use various instruments to measure charge in a 574 00:42:14,558 --> 00:42:20,906 quantitative way and one of the instruments that 575 00:42:20,906 --> 00:42:27,723 you will see we will use it often in the lectures that are 576 00:42:27,723 --> 00:42:31,789 to come, is called an electroscope. 577 00:42:31,789 --> 00:42:38,127 It's a very simple instrument. In general it is just a 578 00:42:38,127 --> 00:42:42,433 conducting rod. It could be aluminum, 579 00:42:42,433 --> 00:42:49,25 metal, and at the end are two pieces of tinsel, 580 00:42:49,25 --> 00:42:53,956 two pieces of aluminum foil, and often there is a nice knob 581 00:42:53,956 --> 00:42:57,77 here, and if I touch this with a charged object, 582 00:42:57,77 --> 00:43:02,476 then because this can conduct electricity, this can conduct 583 00:43:02,476 --> 00:43:07,588 the fire, as defined by Benjamin Franklin, if I touch it with an 584 00:43:07,588 --> 00:43:12,132 object which is positively charged, then this object will 585 00:43:12,132 --> 00:43:16,514 become positively charged. If I touch it with an object 586 00:43:16,514 --> 00:43:22,599 which is negatively charged it will become negatively charged. 587 00:43:22,599 --> 00:43:27,336 And you see now here these two very light pieces of aluminum 588 00:43:27,336 --> 00:43:31,832 foil will repel each other. And so you will see that this 589 00:43:31,832 --> 00:43:35,847 shows a certain angle, and the more charge there is 590 00:43:35,847 --> 00:43:40,102 the larger that angle. Sort of gives us a way of doing 591 00:43:40,102 --> 00:43:44,919 some quantitative measurements. There are other electroscopes 592 00:43:44,919 --> 00:43:49,334 which are not too different. There's one central rod and 593 00:43:49,334 --> 00:43:54,312 they would have one leaf hanging there and when you charge that 594 00:43:54,312 --> 00:43:59,286 one up then this leaf will go out and if 595 00:43:59,286 --> 00:44:04,028 the charge is more it will go out even further. 596 00:44:04,028 --> 00:44:07,843 I don't have an electroscope now here. 597 00:44:07,843 --> 00:44:14,338 But what I want you to see that if I charge myself up and I hold 598 00:44:14,338 --> 00:44:20,832 in my hands these Christmas tree tinsels, that in a way if I get 599 00:44:20,832 --> 00:44:26,4 enough charge on me, then these tinsels will 600 00:44:26,4 --> 00:44:29,439 spread out. It's an idea that immediately 601 00:44:29,439 --> 00:44:34,074 follows from the fact that you get a certain amount of charge, 602 00:44:34,074 --> 00:44:37,949 whether it's negative charge from me, or whether I'm 603 00:44:37,949 --> 00:44:40,988 positively charged, that doesn't make any 604 00:44:40,988 --> 00:44:44,103 difference, these tinsels will spread out. 605 00:44:44,103 --> 00:44:48,206 And of course the best way I can do that is if I charge 606 00:44:48,206 --> 00:44:51,929 myself with the Vandegraaff. And as I said earlier 607 00:44:51,929 --> 00:44:57,4 experiments of this nature are not entirely without risk. 608 00:44:57,4 --> 00:45:02,182 And so there's always the possibility of course that I 609 00:45:02,182 --> 00:45:05,069 don't survive this demonstration. 610 00:45:05,069 --> 00:45:10,212 [laughter] But don't worry because in that case there will 611 00:45:10,212 --> 00:45:15,716 be someone else who will lecture eight oh two except he is not 612 00:45:15,716 --> 00:45:19,235 likely to show this demonstration again. 613 00:45:19,235 --> 00:45:24,739 [laughter] So you might as well take a close look because this 614 00:45:24,739 --> 00:45:28,439 may be the only time you will ever see it. 615 00:45:28,439 --> 00:45:33,492 So I will give you some nice light on the Vandegraaff and 616 00:45:33,492 --> 00:45:39,121 it's always a scary moment for me, 617 00:45:39,121 --> 00:45:44,673 sleepless nights about the Vandegraaff. 618 00:45:44,673 --> 00:45:52,855 Am I going to turn it on, Marcos, or you have the courage 619 00:45:52,855 --> 00:45:57,822 to turn it on? You will turn it on? 620 00:45:57,822 --> 00:46:04,982 OK, hold it Marcos, this is too close for comfort. 621 00:46:04,982 --> 00:46:08,78 You ready? Are you nervous? 622 00:46:08,78 --> 00:46:13,164 Feel. [laughter] So look at the 623 00:46:13,164 --> 00:46:21,199 tinsels and try not to look at me please. 624 00:46:21,199 --> 00:46:28,207 Go ahead. I am now a living electroscope. 625 00:46:28,207 --> 00:46:38,018 [laughter] If the if the weather is cooperating today and 626 00:46:38,018 --> 00:46:47,654 if I had long hair you might even see that my hair would 627 00:46:47,654 --> 00:46:59,066 start to act like an electroscope. 628 00:46:59,066 --> 00:47:18,437 We can try that too. Why don't you throw it. 629 00:47:18,437 --> 00:47:36,029 [laughter] [applause] Is it working? 630 00:47:36,029 --> 00:48:01,799 OK, well, this weekend make sure you take this nylon shirt 631 00:48:01,799 --> 00:48:29,377 off in front of the mirror and enjoy your enjoy the experiment 632 00:48:29,377 --> 00:48:42,036 at home. Don't try this ever. 633 00:48:42,036 --> 48:47 See you Friday. [applause]