1 00:00:00 --> 00:00:00,171 2 00:00:00,171 --> 00:00:04,61 You have ten days left for your motor, so that's a nice project 3 00:00:04,61 --> 00:00:07,617 for Spring Break. I'll give you some hints. 4 00:00:07,617 --> 00:00:11,125 Keep the friction of your rotor as low as you can. 5 00:00:11,125 --> 00:00:14,849 You can't use any oil, of course; that's not allowed. 6 00:00:14,849 --> 00:00:17,569 Balance your rotor to the best you can. 7 00:00:17,569 --> 00:00:21,006 And try to avoid that the rotor begins to bounce, 8 00:00:21,006 --> 00:00:24,228 begins to vibrate, because when it vibrates it 9 00:00:24,228 --> 00:00:28,524 loses contact with the current when it needs it so there's no 10 00:00:28,524 --> 00:00:32,98 torque. How will we test your motors? 11 00:00:32,98 --> 00:00:38,549 We do it with a stroboscope, and I've decided to demonstrate 12 00:00:38,549 --> 00:00:41,664 to you how we're going to do that. 13 00:00:41,664 --> 00:00:45,062 That's probably the best thing to do. 14 00:00:45,062 --> 00:00:49,215 We have a disk, and we're going to rotate the 15 00:00:49,215 --> 00:00:53,557 disk at thousand RPM. Let's assume that is your 16 00:00:53,557 --> 00:00:56,767 motor. And we're going to strobe it 17 00:00:56,767 --> 00:01:02,052 with a strobe light until it stands still. 18 00:01:02,052 --> 00:01:05,748 In this case, I have set the strobe so that 19 00:01:05,748 --> 00:01:10,147 it will stand still, roughly, and the strobe is now 20 00:01:10,147 --> 00:01:14,635 going at five hundred RPM, and the motor is going at 21 00:01:14,635 --> 00:01:18,066 thousand RPM. So this clearly is not the 22 00:01:18,066 --> 00:01:23,169 rotation rate of your motor. In fact, your motor goes twice 23 00:01:23,169 --> 00:01:28,185 around between the blinks. And we'd have no way of knowing 24 00:01:28,185 --> 00:01:31 that, so we double the frequency. 25 00:01:31 --> 00:01:37,141 I'm trying to double it now, double the frequency of the 26 00:01:37,141 --> 00:01:42,294 blinking of the strobe light. And now it stands still again. 27 00:01:42,294 --> 00:01:47,272 So now we may think that your motor is going thousand RPM, 28 00:01:47,272 --> 00:01:51,901 but we don't know yet. Maybe it's going three thousand 29 00:01:51,901 --> 00:01:55,831 R- two thousand RPM. Maybe three thousand RPM. 30 00:01:55,831 --> 00:02:00,721 So what are we going to do now, we're going to double the 31 00:02:00,721 --> 00:02:03,778 frequency. And so we go now with the 32 00:02:03,778 --> 00:02:06,573 strobe light to two thousand RPM. 33 00:02:06,573 --> 00:02:11,192 And what we see now is we see a double image. 34 00:02:11,192 --> 00:02:14,148 So two thousand RPM is out, and any multiple of two 35 00:02:14,148 --> 00:02:16,985 thousand RPM is out. So four thousand RPM is out, 36 00:02:16,985 --> 00:02:19,291 six thousand, and eight thousand is out. 37 00:02:19,291 --> 00:02:22,72 But what is not yet out is three thousand and five thousand 38 00:02:22,72 --> 00:02:25,498 and seven thousand. So we would have to test for 39 00:02:25,498 --> 00:02:26,858 that. On the other hand, 40 00:02:26,858 --> 00:02:30,168 I told you already that this motor is going thousand RPM, 41 00:02:30,168 --> 00:02:32,474 so there's no sense us testing that now. 42 00:02:32,474 --> 00:02:36,08 But during the actual contest, of course, we will continue all 43 00:02:36,08 --> 00:02:39,627 the way until we are convinced that we have the right RPM for 44 00:02:39,627 --> 00:02:44,055 your motor. And so that's the way we will 45 00:02:44,055 --> 00:02:46,873 do it. We will put a little bit of 46 00:02:46,873 --> 00:02:52,255 white paint on one side of your rotor, so that's the way it will 47 00:02:52,255 --> 00:02:55,244 be done. Of course, if your motor is 48 00:02:55,244 --> 00:02:58,832 highly unstable in terms of rotation rates, 49 00:02:58,832 --> 00:03:03,018 it will not be easy to get a right correct number. 50 00:03:03,018 --> 00:03:06,349 I want to talk with you about the heart. 51 00:03:06,349 --> 00:03:09,595 The heart, our heart has four chambers. 52 00:03:09,595 --> 00:03:15,486 Looks sort of like this. The left atrium and right 53 00:03:15,486 --> 00:03:19,148 atrium. Maybe this is why it's -- this 54 00:03:19,148 --> 00:03:24,79 is why it's called the heart. And here is the left and the 55 00:03:24,79 --> 00:03:29,74 ri- and the right ventricle. And here is the aorta. 56 00:03:29,74 --> 00:03:34,293 The sole purpose of the heart is to pump blood. 57 00:03:34,293 --> 00:03:40,034 About five quarts per minute, which is seventy-five gallons 58 00:03:40,034 --> 00:03:44,191 per hour, which is seventy barrels per day, 59 00:03:44,191 --> 00:03:50,575 which is about two million barrels in seventy-five years. 60 00:03:50,575 --> 00:03:54,05 And it pumps about seventy times per minute. 61 00:03:54,05 --> 00:03:58,495 If the blood to your brain stops for about five seconds, 62 00:03:58,495 --> 00:04:02,374 you lose consciousness. So it's five skips of the 63 00:04:02,374 --> 00:04:05,525 heartbeat, and you're down on the floor. 64 00:04:05,525 --> 00:04:09,243 And four minutes later, permanent brain damage. 65 00:04:09,243 --> 00:04:13,364 The way the heart works is absolutely mind-boggling. 66 00:04:13,364 --> 00:04:18,86 Extremely complicated. Nature had one billion years to 67 00:04:18,86 --> 00:04:22,853 design it, but nevertheless it's impressive. 68 00:04:22,853 --> 00:04:26,754 Each heart cell is a mini chemical battery, 69 00:04:26,754 --> 00:04:30,562 and it pumps ions in or out as it pleases. 70 00:04:30,562 --> 00:04:35,298 In the normal state, each heart cell is minus eighty 71 00:04:35,298 --> 00:04:39,756 millivolts on the inside relative to the outside. 72 00:04:39,756 --> 00:04:44,678 There are some cells which are called pacemaker cells. 73 00:04:44,678 --> 00:04:48,858 They are located in a very small 74 00:04:48,858 --> 00:04:52,671 area, about one square millimeter, near the atrium, 75 00:04:52,671 --> 00:04:56,332 the right atrium, and they change their potential 76 00:04:56,332 --> 00:05:00,451 from minus eighty millivolts to plus twenty millivolts. 77 00:05:00,451 --> 00:05:03,578 Now why they do that is a different story, 78 00:05:03,578 --> 00:05:07,545 which I will not address. Once they go to plus twenty 79 00:05:07,545 --> 00:05:10,595 millivolts, the neighboring cells follow, 80 00:05:10,595 --> 00:05:13,341 and a wave propagates over the heart. 81 00:05:13,341 --> 00:05:15,706 I'll make you a drawing shortly. 82 00:05:15,706 --> 00:05:19,977 So the wave first moves over the atrial chambers and then 83 00:05:19,977 --> 00:05:23,656 over the ventricle chambers. 84 00:05:23,656 --> 00:05:28,259 And when the cells are at plus twenty millivolts inside 85 00:05:28,259 --> 00:05:31,498 relative to the outside, they contract. 86 00:05:31,498 --> 00:05:35,589 So they form a muscle. The whole heart is one big 87 00:05:35,589 --> 00:05:38,828 muscle. And after about two tenths of a 88 00:05:38,828 --> 00:05:43,175 second, the cells return to minus eighty millivolts, 89 00:05:43,175 --> 00:05:46,414 and this wave goes from below to above. 90 00:05:46,414 --> 00:05:51,528 And then the whole thing waits again for another message from 91 00:05:51,528 --> 00:05:57,093 the pacemaker cells. Takes about one second, 92 00:05:57,093 --> 00:06:01,652 and then the whole process starts all over. 93 00:06:01,652 --> 00:06:07,404 Now I want to be more precise. Here is one heart cell. 94 00:06:07,404 --> 00:06:11,312 So this is about ten microns in size. 95 00:06:11,312 --> 00:06:17,173 And this cell has eighty millivolts with respect to the 96 00:06:17,173 --> 00:06:21,189 outside. So that means it has repelled 97 00:06:21,189 --> 00:06:27,375 positive ions, and so the inside is negative. 98 00:06:27,375 --> 00:06:31,805 And there is no E field here outside, because if you put a 99 00:06:31,805 --> 00:06:36,39 Gaussian surface around here, there is no net charge inside. 100 00:06:36,39 --> 00:06:39,498 But there is, of course, a electric field 101 00:06:39,498 --> 00:06:42,684 across the walls here, from plus to minus. 102 00:06:42,684 --> 00:06:46,88 Now the depolarization, which is the change to the plus 103 00:06:46,88 --> 00:06:51,154 twenty millivolt state starts, and it starts from above. 104 00:06:51,154 --> 00:06:55,506 And I will assume now that it is not plus twenty but zero 105 00:06:55,506 --> 00:06:59,391 millivolts, and it's easier to see. 106 00:06:59,391 --> 00:07:02,237 If we have this cell, and the wave is, 107 00:07:02,237 --> 00:07:05,159 say, halfway down, and this is now zero 108 00:07:05,159 --> 00:07:09,695 millivolts, then there is no longer minus charge here and no 109 00:07:09,695 --> 00:07:13,309 longer plus charge here, because zero millivolts 110 00:07:13,309 --> 00:07:17,846 relative to the outside world. So there is no electric field 111 00:07:17,846 --> 00:07:20,691 across here anymore. [unintelligible]. 112 00:07:20,691 --> 00:07:23,613 In other words, what the cell has done, 113 00:07:23,613 --> 00:07:26,227 it has moved positive ions back in. 114 00:07:26,227 --> 00:07:31,302 But here the situation is still as it was before, 115 00:07:31,302 --> 00:07:35,883 so this is still at your minus eighty millivolts. 116 00:07:35,883 --> 00:07:40,653 And if you look now, you have here a minus layer on 117 00:07:40,653 --> 00:07:44,279 top of a positive layer. Positive here, 118 00:07:44,279 --> 00:07:48,192 minus on top. And that creates an electric 119 00:07:48,192 --> 00:07:53,44 field, which has roughly the shape of a electric dipole. 120 00:07:53,44 --> 00:07:58,115 It has this shape. So as the wave goes through the 121 00:07:58,115 --> 00:08:02,996 cells, only then do they create a dipole. 122 00:08:02,996 --> 00:08:05,95 And we call this the depolarization. 123 00:08:05,95 --> 00:08:10,085 A little later in time, when this wave has passed, 124 00:08:10,085 --> 00:08:13,545 the whole thing is plus twenty millivolts. 125 00:08:13,545 --> 00:08:17,258 I chose zero here, but it really goes to plus 126 00:08:17,258 --> 00:08:20,465 twenty. This is just easier to explain. 127 00:08:20,465 --> 00:08:23,926 So that means that now the inside is plus, 128 00:08:23,926 --> 00:08:28,061 so positive ions are now inside, negative ions are 129 00:08:28,061 --> 00:08:31,69 outside, and the E field here is again zero. 130 00:08:31,69 --> 00:08:36,331 Now there is the repolarization wave, 131 00:08:36,331 --> 00:08:40,34 which comes from below, when it goes back to minus 132 00:08:40,34 --> 00:08:44,103 eighty millivolts. And I will again do the same 133 00:08:44,103 --> 00:08:48,521 trick that I did before; I will just assume the wave is 134 00:08:48,521 --> 00:08:53,02 halfway, that it is not minus eighty but that it is zero 135 00:08:53,02 --> 00:08:56,292 millivolts. So there are no charges here, 136 00:08:56,292 --> 00:08:59,074 but the charges here are unchanged. 137 00:08:59,074 --> 00:09:03,818 So what do you have now here? You have a minus layer on top 138 00:09:03,818 --> 00:09:08,399 of a plus layer. So you have exactly what 139 00:09:08,399 --> 00:09:12,562 you had before. So again you get an electric 140 00:09:12,562 --> 00:09:16,919 field, [unintelligible] electric dipole field, 141 00:09:16,919 --> 00:09:22,63 which has again the same shape. So what's going to happen is 142 00:09:22,63 --> 00:09:28,729 the depolarization wave is going to run down, leaves behind here 143 00:09:28,729 --> 00:09:33,473 the cells at plus twenty millivolts, when they are 144 00:09:33,473 --> 00:09:38,894 contracted, so this part of the heart has already pumped, 145 00:09:38,894 --> 00:09:43,993 and it moves down. And only the cells where the 146 00:09:43,993 --> 00:09:47,665 depolarization occurs, that's only the ones on the 147 00:09:47,665 --> 00:09:51,111 ring, contribute to that electric dipole field. 148 00:09:51,111 --> 00:09:54,783 If there is no wave, which is sizeable fraction of 149 00:09:54,783 --> 00:09:57,855 the heart, of the cycle, there is no wave, 150 00:09:57,855 --> 00:10:00,702 then there is no electric dipole field. 151 00:10:00,702 --> 00:10:04,823 And when the repolarization goes in the other direction, 152 00:10:04,823 --> 00:10:09,018 when the heart relaxes because the cells go back to minus 153 00:10:09,018 --> 00:10:13,964 eighty millivolts, then again there is an electric 154 00:10:13,964 --> 00:10:18,081 dipole field, but only from the cells through 155 00:10:18,081 --> 00:10:21,356 which the repolarization wave moves. 156 00:10:21,356 --> 00:10:26,783 And you can very easily see that the electric dipole fields 157 00:10:26,783 --> 00:10:30,713 of all these cells here support each other. 158 00:10:30,713 --> 00:10:34,456 So you get a dipole field from the heart. 159 00:10:34,456 --> 00:10:39,789 And so if I make you look at your heart -- so this is you, 160 00:10:39,789 --> 00:10:44 this is your body, your legs, and this is your 161 00:10:44 --> 00:10:49,685 arms, and here is your heart, and there goes this 162 00:10:49,685 --> 00:10:52,549 wave. And so here is your electric 163 00:10:52,549 --> 00:10:56,628 field that is generated while the wave is going, 164 00:10:56,628 --> 00:11:00,707 either depolarization down or repolarization up. 165 00:11:00,707 --> 00:11:05,48 But if there is an electric field, there's going to be a 166 00:11:05,48 --> 00:11:10,427 potential difference between different parts of your body. 167 00:11:10,427 --> 00:11:15,113 You look here at your belly button, and you follow this 168 00:11:15,113 --> 00:11:18,78 electric field line at your head, 169 00:11:18,78 --> 00:11:22,079 there is an E field. The integral E dot D L gives 170 00:11:22,079 --> 00:11:25,514 you a potential difference. And so now you see that 171 00:11:25,514 --> 00:11:29,706 there're going to be potential differences between the various 172 00:11:29,706 --> 00:11:33,004 parts of your body. And that's the idea behind an 173 00:11:33,004 --> 00:11:36,027 electrocardiogram. Typically there are twelve 174 00:11:36,027 --> 00:11:40,081 electrodes attached to arms, legs, head, and chest to get as 175 00:11:40,081 --> 00:11:42,967 much information about the heart as we can. 176 00:11:42,967 --> 00:11:46,197 And the maximum potential difference between two 177 00:11:46,197 --> 00:11:50,595 electrodes, in general, is not more than about 178 00:11:50,595 --> 00:11:58,571 two to three millivolts. I'd like to show you a healthy 179 00:11:58,571 --> 00:12:04,037 heart cardiogram, of a healthy person. 180 00:12:04,037 --> 00:12:10,389 I have that here. The time here is about one 181 00:12:10,389 --> 00:12:18,071 second, and from here to here is about one millivolt. 182 00:12:18,071 --> 00:12:23,533 The P wave -- we call this the P wave -- that 183 00:12:23,533 --> 00:12:26,96 is observed when the atrium is being depolarized, 184 00:12:26,96 --> 00:12:30,672 so when the depolarization wave goes over the atrium. 185 00:12:30,672 --> 00:12:35,098 A little later it goes over the ventricle, and you get a larger 186 00:12:35,098 --> 00:12:39,024 potential difference because there is more muscle in the 187 00:12:39,024 --> 00:12:41,737 ventricle. So that's why this R wave is 188 00:12:41,737 --> 00:12:43,45 higher. The T wave is the 189 00:12:43,45 --> 00:12:46,52 repolarization, when the wave goes back over 190 00:12:46,52 --> 00:12:49,804 the ventricles. The dipole field is in the same 191 00:12:49,804 --> 00:12:53,759 direction, remember. That's the T wave. 192 00:12:53,759 --> 00:12:56,452 It's not known, at least it wasn't known 193 00:12:56,452 --> 00:12:59,628 recent- until recently, what causes the U wave. 194 00:12:59,628 --> 00:13:03,495 I talked to a heart expert about this, Professor Cohen at 195 00:13:03,495 --> 00:13:07,43 MIT, and I was surprised to learn that it's not known what 196 00:13:07,43 --> 00:13:10,952 the U wave is about. Not everyone's cardiogram looks 197 00:13:10,952 --> 00:13:14,473 as healthy as this one. There is a terrible disease, 198 00:13:14,473 --> 00:13:18,34 which four hundred thousand people die of per year in the 199 00:13:18,34 --> 00:13:22,482 United States, which is known as ventricular 200 00:13:22,482 --> 00:13:25,719 fibrillation, also known as sudden death. 201 00:13:25,719 --> 00:13:30,411 The ventricles fire without any message from the pace wave, 202 00:13:30,411 --> 00:13:33,242 pacemaker wave, and there is random, 203 00:13:33,242 --> 00:13:37,773 non-synchronous depolarization. So the heart doesn't pump 204 00:13:37,773 --> 00:13:41,575 anymore, in five seconds you lose consciousness, 205 00:13:41,575 --> 00:13:44,569 on the floor, and in four minutes you, 206 00:13:44,569 --> 00:13:47,077 um, have permanent brain damage. 207 00:13:47,077 --> 00:13:50,07 In hospitals, heart patients are being 208 00:13:50,07 --> 00:13:54,029 monitored, and as soon as it's noticed 209 00:13:54,029 --> 00:13:57,708 that there is something wrong like this, so severe as the 210 00:13:57,708 --> 00:14:00,205 fibrillation, ventricular fibrillation, 211 00:14:00,205 --> 00:14:02,834 then they apply electric shock treatment. 212 00:14:02,834 --> 00:14:06,119 So you have to be fast, you only have a few minutes 213 00:14:06,119 --> 00:14:09,733 before you get brain damage. And three thousand volts is 214 00:14:09,733 --> 00:14:13,084 applied, one amperes, for about a tenth of a second. 215 00:14:13,084 --> 00:14:16,567 Large plates are being used on each side of the chest. 216 00:14:16,567 --> 00:14:19,852 And this, of course, is enough to kill the patient. 217 00:14:19,852 --> 00:14:23,86 But it makes little difference, because the patient would have 218 00:14:23,86 --> 00:14:26,061 died anyhow. 219 00:14:26,061 --> 00:14:30,957 Heart patients can also get synchronization problems and 220 00:14:30,957 --> 00:14:35,053 then they implant a pacemaker that's a circuit. 221 00:14:35,053 --> 00:14:40,038 And this pacemaker takes over the role from the pacemaker 222 00:14:40,038 --> 00:14:43,154 cells. When the heartbeat rate falls 223 00:14:43,154 --> 00:14:47,695 below a certain rate, the artificial pacemaker takes 224 00:14:47,695 --> 00:14:50,9 over. About ten milliamperes for half 225 00:14:50,9 --> 00:14:54,817 a millisecond, and it does it sixty times per 226 00:14:54,817 --> 00:14:58,423 minute. And so it triggers, 227 00:14:58,423 --> 00:15:02,523 then, the depolarization wave. These pacemakers are 228 00:15:02,523 --> 00:15:06,46 susceptible to influences from the outside world, 229 00:15:06,46 --> 00:15:10,397 and one person's pacemaker stopped, for instance, 230 00:15:10,397 --> 00:15:14,989 every ten seconds due to a radar sweep from a police car. 231 00:15:14,989 --> 00:15:19,582 It's also possible that you get a built-in defibrillator, 232 00:15:19,582 --> 00:15:22,699 in other words, a system that gives you 233 00:15:22,699 --> 00:15:28,44 electric shocks when sudden death might otherwise occur. 234 00:15:28,44 --> 00:15:32,527 So it senses that something is wrong, that the ventricle is 235 00:15:32,527 --> 00:15:35,628 going into fibrillation, and then it applies, 236 00:15:35,628 --> 00:15:38,306 all by itself, six hundred fifty volts, 237 00:15:38,306 --> 00:15:40,702 about five and a half milliseconds, 238 00:15:40,702 --> 00:15:44,579 up to five to ten amperes. And that's not enough to kill 239 00:15:44,579 --> 00:15:49,019 the patient, and the whole idea is sort of a wake-up call to the 240 00:15:49,019 --> 00:15:51,908 heart to get it back into synchronization, 241 00:15:51,908 --> 00:15:55,855 to get this depolarization wave being synchronized again. 242 00:15:55,855 --> 00:15:59,943 So clearly I would like to show now a heart cardiogram of a 243 00:15:59,943 --> 00:16:03,975 student, and I prefer to have a healthy 244 00:16:03,975 --> 00:16:07,31 one to avoid some difficulties. You feel strong? 245 00:16:07,31 --> 00:16:10,788 You a healthy person? You don't mind volunteering? 246 00:16:10,788 --> 00:16:13,768 Tight pants, we have to do something about. 247 00:16:13,768 --> 00:16:16,465 OK, why don't you sit down. [laughter]. 248 00:16:16,465 --> 00:16:18,878 Well, there's nothing I -- come in. 249 00:16:18,878 --> 00:16:20,865 We'll, we'll, we will, we'll, 250 00:16:20,865 --> 00:16:24,272 we'll find a way. All right, so we have to attach 251 00:16:24,272 --> 00:16:28,033 -- we don't have twelve electrodes, we only use three. 252 00:16:28,033 --> 00:16:33,284 And the first one -- that's why I was worried about your 253 00:16:33,284 --> 00:16:36,829 tight pants. Can you roll them up a little? 254 00:16:36,829 --> 00:16:39,023 OK. Oh, this one goes here. 255 00:16:39,023 --> 00:16:42,146 Let's hope that it makes good contact. 256 00:16:42,146 --> 00:16:46,45 Now the others go on your arm, and we need very good 257 00:16:46,45 --> 00:16:50,163 electrical contact, and therefore we put some 258 00:16:50,163 --> 00:16:54,973 conducting grease on there. It will make it a little -- it 259 00:16:54,973 --> 00:16:59,699 will make it a bit of a mess, but we'll give you a chance 260 00:16:59,699 --> 00:17:05,101 later to clean up. So let's first put this one -- 261 00:17:05,101 --> 00:17:07,099 you're relaxed, right? 262 00:17:07,099 --> 00:17:11,382 Yes, of course. So can you roll up your sleeve 263 00:17:11,382 --> 00:17:12,905 there? Very good. 264 00:17:12,905 --> 00:17:16,997 And mayb- oh, oh, and maybe you can put this 265 00:17:16,997 --> 00:17:20,614 over your arm, yeah, over your -- yeah, 266 00:17:20,614 --> 00:17:22,517 that's good. High up. 267 00:17:22,517 --> 00:17:25,277 Oh man, boy, you have muscles. 268 00:17:25,277 --> 00:17:28,418 [laughter]. All right, just relax. 269 00:17:28,418 --> 00:17:33,271 If it hurts a little, oh well, that's the way it is. 270 00:17:33,271 --> 00:17:38,837 [laughter]. We need, we ne- we need good 271 00:17:38,837 --> 00:17:41,839 contact. OK, now your other arm. 272 00:17:41,839 --> 00:17:45,13 Again, a little bit of gooky stuff. 273 00:17:45,13 --> 00:17:50,746 Oh, you need another rubber band, so put it around your arm 274 00:17:50,746 --> 00:17:52,682 high up. That's good, 275 00:17:52,682 --> 00:17:54,134 yeah. All right. 276 00:17:54,134 --> 00:17:59,749 So now it's very important that you relax, because when you 277 00:17:59,749 --> 00:18:03,622 start moving, the other muscle cells will 278 00:18:03,622 --> 00:18:08,656 also produce electric dipole fields. 279 00:18:08,656 --> 00:18:14,273 And that we don't want, because then that will be 280 00:18:14,273 --> 00:18:16,964 overpowering. All right. 281 00:18:16,964 --> 00:18:22,697 I'm going to take -- make you take a look at this. 282 00:18:22,697 --> 00:18:24,92 Yeah? Everything OK? 283 00:18:24,92 --> 00:18:31,589 So I'm going to change the light situation so that you can 284 00:18:31,589 --> 00:18:35,567 see shortly there what's coming up. 285 00:18:35,567 --> 00:18:38,609 Oh, man. What a da- oh gee, 286 00:18:38,609 --> 00:18:42,939 look at this. Oh, gee. 287 00:18:42,939 --> 00:18:45,813 Oh, look! I see your -- I don't see your 288 00:18:45,813 --> 00:18:47,139 P wave. [laughter]. 289 00:18:47,139 --> 00:18:50,308 But you have an amazing T wave, right -- no, 290 00:18:50,308 --> 00:18:54,878 no, you have an amazing R wave! It looks like your R wave is in 291 00:18:54,878 --> 00:18:57,162 the wrong direction. [laughter]. 292 00:18:57,162 --> 00:18:59,005 You feel OK though, right? 293 00:18:59,005 --> 00:19:03,5 Yeah, I can't see your P wave. Well, maybe some people survive 294 00:19:03,5 --> 00:19:05,49 without P waves. [laughter]. 295 00:19:05,49 --> 00:19:08,806 It's a certainly an unusual, an unusual heart, 296 00:19:08,806 --> 00:19:13,154 but if you tell me that you are healthy, 297 00:19:13,154 --> 00:19:15,254 then I'll take your word for that. 298 00:19:15,254 --> 00:19:18,181 I think it would be nice if you show class now, 299 00:19:18,181 --> 00:19:22,125 why don't you just stand up for a while, let your other muscles 300 00:19:22,125 --> 00:19:24,734 begin to act, and y- they will see then -- 301 00:19:24,734 --> 00:19:27,469 just stand up. You will see -- move a little 302 00:19:27,469 --> 00:19:29,314 bit your arms. Move your arms. 303 00:19:29,314 --> 00:19:31,796 You see? Now you get the electric dipole 304 00:19:31,796 --> 00:19:35,358 field from the other muscles in the body [laughter] which 305 00:19:35,358 --> 00:19:37,776 contract. This is even more interesting 306 00:19:37,776 --> 00:19:39,048 than your heart, man. 307 00:19:39,048 --> 00:19:40,384 [laughter]. All right. 308 00:19:40,384 --> 00:19:42,675 Sit down again and let's take it off. 309 00:19:42,675 --> 00:19:47,073 And then you can clean up. Looks good. 310 00:19:47,073 --> 00:19:50,534 Yeah. Heart cardiograms are not so 311 00:19:50,534 --> 00:19:55,149 easy to interpret. But I think you're looking 312 00:19:55,149 --> 00:19:58,085 fine. And you feel all right, 313 00:19:58,085 --> 00:20:00,497 right? That's important. 314 00:20:00,497 --> 00:20:04,588 So thank you very much for volunteering, 315 00:20:04,588 --> 00:20:07,419 very courageous. [applause]. 316 00:20:07,419 --> 00:20:12,139 Yeah, make sure you clean that stuff off, huh. 317 00:20:12,139 --> 00:20:16,124 It's water-soluble, so it's not so bad. 318 00:20:16,124 --> 00:20:19,166 What is your name, by the way? 319 00:20:19,166 --> 00:20:23,57 Danny. You were great, 320 00:20:23,57 --> 00:20:26,135 Danny. [laughter]. 321 00:20:26,135 --> 0. 322 0. --> 00:20:30,509 And now I want to talk about Now I want to talk about 323 00:20:30,509 --> 00:20:37,599 Aurora Borealis. If we have a magnetic field and 324 00:20:37,599 --> 00:20:45,141 we have a charged particle, let's say make it plus, 325 00:20:45,141 --> 00:20:52,834 and the velocity of that charged particle is in this 326 00:20:52,834 --> 00:21:00,051 direction, then the force on that charged 327 00:21:00,051 --> 00:21:05,239 particle, the Lorenz force, equals Q times V cross V. 328 00:21:05,239 --> 00:21:11,025 I'm going to decompose this velocity now into one component 329 00:21:11,025 --> 00:21:16,014 parallel to the magnetic field and into a component 330 00:21:16,014 --> 00:21:21,9 perpendicular to the magnetic field, so the vectorial sum of 331 00:21:21,9 --> 00:21:26,39 these two is V. And so I can rewrite this as Q 332 00:21:26,39 --> 00:21:31,278 times V parallel plus V perpendicular 333 00:21:31,278 --> 00:21:35,273 crossed with V. But V parallel crossed with V 334 00:21:35,273 --> 00:21:40,63 is zero, because the angle is either zero degrees or hundred 335 00:21:40,63 --> 00:21:44,444 eighty degrees, so the sine of the angle is 336 00:21:44,444 --> 00:21:47,712 zero. And so the force is exclusively 337 00:21:47,712 --> 00:21:52,161 determined by this term. It's by the perpendicular 338 00:21:52,161 --> 00:21:55,884 component. And so what is going to happen, 339 00:21:55,884 --> 00:22:01,423 this charged particle is going to circle around. 340 00:22:01,423 --> 00:22:05,971 But then it continues to go in this direction with that 341 00:22:05,971 --> 00:22:10,099 velocity V parallel. And so you are going to see a 342 00:22:10,099 --> 00:22:13,89 path like this. Well, by this radius R of that 343 00:22:13,89 --> 00:22:18,775 circle, that radius i- I still remember it from the lecture 344 00:22:18,775 --> 00:22:23,071 when we discussed that, that was M V divided by Q B, 345 00:22:23,071 --> 00:22:28,21 but the V now is of course the perpendicular component divided 346 00:22:28,21 --> 00:22:32,927 by Q B, and then in this direction it continues unaltered 347 00:22:32,927 --> 00:22:37,97 with the velocity which is the parallel 348 00:22:37,97 --> 00:22:41,904 component. Magnetic field of the earth, 349 00:22:41,904 --> 00:22:48,426 it's not a straight line but is curved, and so charged particles 350 00:22:48,426 --> 00:22:54,534 can spiral around the magnetic field and follow the magnetic 351 00:22:54,534 --> 00:23:00,538 field lines, and they come down on Earth where the magnetic 352 00:23:00,538 --> 00:23:05,093 field lines enter, which is near the magnetic 353 00:23:05,093 --> 00:23:08,886 poles. The sun emits a plasma. 354 00:23:08,886 --> 00:23:12,746 Plasma is highly ionized electrons and protons. 355 00:23:12,746 --> 00:23:16,941 We call that the solar wind. Sometimes it's strong, 356 00:23:16,941 --> 00:23:21,136 sometimes it's weak. And when it reaches the Earth, 357 00:23:21,136 --> 00:23:24,827 it ionizes the upper atmosphere of the Earth, 358 00:23:24,827 --> 00:23:29,106 and then it produces light. The light is very faint, 359 00:23:29,106 --> 00:23:33,301 can only be seen at night. And that light is called 360 00:23:33,301 --> 00:23:38,167 Aurora, and we here call it Northern Lights, 361 00:23:38,167 --> 00:23:41,793 but I'm sure that people in the southern hemisphere, 362 00:23:41,793 --> 00:23:45,846 they call it Southern Lights. When the sun is very active, 363 00:23:45,846 --> 00:23:49,401 it can be breathtaking, really absolutely fabulous. 364 00:23:49,401 --> 00:23:53,453 The Aurora can change very fast, on time scales of seconds 365 00:23:53,453 --> 00:23:55,728 to minutes. And it is, of course, 366 00:23:55,728 --> 00:23:59,923 strongest near the magnetic poles, and so you r- rarely ever 367 00:23:59,923 --> 00:24:03,335 see it in Boston. It can have very bright colors, 368 00:24:03,335 --> 00:24:05,895 red, green, white is the most common. 369 00:24:05,895 --> 00:24:11,004 And the color that you will see depends on the energy of the 370 00:24:11,004 --> 00:24:13,003 charged particles as they come in. 371 00:24:13,003 --> 00:24:16,577 But it also depends on whether the nitrogen molecules in the 372 00:24:16,577 --> 00:24:19,727 atmosphere or the oxygen molecules are being excited. 373 00:24:19,727 --> 00:24:23,362 Also depends on at what height, what height in the atmosphere 374 00:24:23,362 --> 00:24:26,512 the ionization occurs. I've seen it quite a few times 375 00:24:26,512 --> 00:24:28,996 in my life. I did hiking in the Adirondack 376 00:24:28,996 --> 00:24:31,601 where I saw it. I've seen it from Calgary in 377 00:24:31,601 --> 00:24:33,782 Canada. But whenever I fly to Europe, 378 00:24:33,782 --> 00:24:37,235 and flights are always at night, I always ask for a window 379 00:24:37,235 --> 00:24:40,567 seat on the left side of the plane, 380 00:24:40,567 --> 00:24:45,105 and that's the reason. So that I can look to the 381 00:24:45,105 --> 00:24:50,995 north, and a few times have I seen spectacular Aurora from the 382 00:24:50,995 --> 00:24:54,567 plane. I want to show you some slides. 383 00:24:54,567 --> 00:25:00,168 If you visit the eight oh two website I made some links for 384 00:25:00,168 --> 00:25:03,741 you to some fabulous slides of Aurora. 385 00:25:03,741 --> 00:25:08,472 But now I want to show you a few that I have here. 386 00:25:08,472 --> 00:25:13,589 Can I have the first slide? This is, um, 387 00:25:13,589 --> 00:25:17,131 white Aurora, and like a nice curtain. 388 00:25:17,131 --> 00:25:21,342 Not so uncommon. Can change on time scales of 389 00:25:21,342 --> 00:25:26,032 minutes to seconds. As I said, you can only see it 390 00:25:26,032 --> 00:25:28,424 at night. It's very faint. 391 00:25:28,424 --> 00:25:32,348 The next slide? You see another remarkable 392 00:25:32,348 --> 00:25:35,315 example of Aurora, white Aurora. 393 00:25:35,315 --> 00:25:38,57 Strange shapes, very unpredictable. 394 00:25:38,57 --> 00:25:42,685 And the moving, it's like looking at a movie 395 00:25:42,685 --> 00:25:49,768 when you actually see Aurora. The next one is red Aurora, 396 00:25:49,768 --> 00:25:52,506 like a wonderful curtain coming down. 397 00:25:52,506 --> 00:25:56,004 For reasons that are not so easy to understand, 398 00:25:56,004 --> 00:26:00,72 maximum light comes from a ring which is, which has a radius of 399 00:26:00,72 --> 00:26:04,675 about five hundred kilometers from the magnetic pole. 400 00:26:04,675 --> 00:26:09,238 And the next slide shows you a picture taken from a satellite 401 00:26:09,238 --> 00:26:12,737 which is three Earth radii away from the Earth. 402 00:26:12,737 --> 00:26:15,323 This is taken in ultraviolet light. 403 00:26:15,323 --> 00:26:19,354 And you see beautiful this, this ring. 404 00:26:19,354 --> 00:26:22,944 So this radius is roughly five hundred kilometers, 405 00:26:22,944 --> 00:26:25,654 and here is one of the magnetic poles. 406 00:26:25,654 --> 00:26:30,124 I don't know whether it's the North Pole or the South Magnetic 407 00:26:30,124 --> 00:26:32,542 Pole. And the next slide shows you 408 00:26:32,542 --> 00:26:36,132 something similar. If you look only at these four, 409 00:26:36,132 --> 00:26:39,648 they are taken twelve minutes apart, again in UV, 410 00:26:39,648 --> 00:26:43,678 and you see here a crossbar. I don't know how to explain 411 00:26:43,678 --> 00:26:46,536 that crossbar. It's called Theta Aurora. 412 00:26:46,536 --> 00:26:51,142 Obviously why theta goes without saying, 413 00:26:51,142 --> 00:26:52,638 right. Amazing. 414 00:26:52,638 --> 00:26:58,302 And this is sort of -- these are twelve minutes apart, 415 00:26:58,302 --> 00:27:04,179 so that gives you also an idea how fast this can change. 416 00:27:04,179 --> 00:27:09,522 Over here it's very dark and here it's very bright. 417 00:27:09,522 --> 00:27:13,048 So the changes are quite dramatic. 418 00:27:13,048 --> 0. 419 0. --> 00:27:15,719 And now I want to talk about Now I want to talk about 420 00:27:15,719 --> 00:27:21,699 superconductivity. Superconductivity was 421 00:27:21,699 --> 00:27:27,051 discovered by a Dutch physicist. His name was Kamerlingh Onnes. 422 00:27:27,051 --> 00:27:32,144 And he discovered that if you cool mercury to something like 423 00:27:32,144 --> 00:27:36,719 four degrees Kelvin -- he used liquid helium for that, 424 00:27:36,719 --> 00:27:41,553 in fact he actually discovered how to make liquid helium. 425 00:27:41,553 --> 00:27:46,473 That was the incredible thing. And then he used the liquid 426 00:27:46,473 --> 00:27:51,684 helium to cool down substances, among them mercury, 427 00:27:51,684 --> 00:27:56,342 and he discovered that mercury would lose completely all its 428 00:27:56,342 --> 00:27:59,499 resistivity. So the electrical resistance 429 00:27:59,499 --> 00:28:03,604 would go down to zero. And he got the Nobel Prize for 430 00:28:03,604 --> 00:28:07,472 that in nineteen thirteen. You can only understand 431 00:28:07,472 --> 00:28:11,971 superconductivity with quantum mechanics, and even quantum 432 00:28:11,971 --> 00:28:16,628 mechanics has a major problem nowadays to understand all the 433 00:28:16,628 --> 00:28:19,312 phenomenon about superconductivity. 434 00:28:19,312 --> 00:28:23,97 The problem started in nineteen eighty-six, 435 00:28:23,97 --> 00:28:27,307 when two scientists in Zurich, Muller and Bednorz, 436 00:28:27,307 --> 00:28:31,529 discovered that certain alloys can be made superconducting at a 437 00:28:31,529 --> 00:28:34,866 temperature as high as thirty-five degrees Kelvin. 438 00:28:34,866 --> 00:28:38,749 And theorists earlier had proven that it was impossible to 439 00:28:38,749 --> 00:28:42,562 ever get superconductivity at thirty-five degrees Kelvin. 440 00:28:42,562 --> 00:28:46,717 And so this was such a splash in the community that these guys 441 00:28:46,717 --> 00:28:49,101 got the Nobel Prize within one year. 442 00:28:49,101 --> 00:28:53,528 In nineteen eighty-seven they got the Nobel Prize. 443 00:28:53,528 --> 00:28:57,202 I don't think there's any other example that I recall whereby a 444 00:28:57,202 --> 00:29:00,581 discovery was made and within one year the Nobel Prize was 445 00:29:00,581 --> 00:29:02,596 awarded. And theorists still cannot 446 00:29:02,596 --> 00:29:06,271 explain today fully why there is what's called high-temperature 447 00:29:06,271 --> 00:29:08,346 superconductivity. The record today, 448 00:29:08,346 --> 00:29:11,309 I checked that yesterday with Professor Lee at MIT, 449 00:29:11,309 --> 00:29:14,391 the record is now hundred thirty-five degrees Kelvin. 450 00:29:14,391 --> 00:29:17,651 So certain alloys can be made superconducting at hundred 451 00:29:17,651 --> 00:29:20,852 thirty-five degrees Kelvin. And since you probably know 452 00:29:20,852 --> 00:29:24,409 that liquid nitrogen has a temperature of 453 00:29:24,409 --> 00:29:28,263 seventy-seven degrees Kelvin, anyone can now play nowadays 454 00:29:28,263 --> 00:29:32,253 with superconducting subjects, materials, even high schools, 455 00:29:32,253 --> 00:29:35,431 because liquid nitrogen is very easy to come by. 456 00:29:35,431 --> 00:29:39,151 If you make power lines out of superconducting material, 457 00:29:39,151 --> 00:29:41,315 there would be no loss of energy. 458 00:29:41,315 --> 00:29:45,372 People are thinking about that. You can imagine how costly it 459 00:29:45,372 --> 00:29:49,497 might be, but in principle you could transport electric energy 460 00:29:49,497 --> 00:29:52,338 without any loss, without any omega losses. 461 00:29:52,338 --> 00:29:54,569 No I squared R, because R is zero. 462 00:29:54,569 --> 00:29:58,288 Also, if you have zero resistance in 463 00:29:58,288 --> 00:30:02,736 a material, you can run extremely high current through 464 00:30:02,736 --> 00:30:07,519 it, and you can therefore get very strong magnetic fields. 465 00:30:07,519 --> 00:30:11,798 Using superconducting coils, you can get very strong 466 00:30:11,798 --> 00:30:15,407 magnetic fields, and these colliders that we 467 00:30:15,407 --> 00:30:19,435 talked about earlier, these atom-smashers like we 468 00:30:19,435 --> 00:30:24,469 have at Fermilab and in Geneva, they are going to make use of 469 00:30:24,469 --> 00:30:29,336 superconducting coils to get magnetic fields as high as six 470 00:30:29,336 --> 00:30:32,6 tesla or so, or even higher. 471 00:30:32,6 --> 00:30:35,987 No electric field can exist in a superconductor. 472 00:30:35,987 --> 00:30:40,095 And you can very easily see that, because if there were an 473 00:30:40,095 --> 00:30:44,419 electric field -- if this is a superconductor and there is an 474 00:30:44,419 --> 00:30:47,157 electric field, say, in this direction, 475 00:30:47,157 --> 00:30:50,4 there would be a potential difference over the 476 00:30:50,4 --> 00:30:52,49 superconductor. And Ohm's Law, 477 00:30:52,49 --> 00:30:55,661 V equals I R, tells you then immediately that 478 00:30:55,661 --> 00:30:59,769 if this is not zero but if this is zero that I would go to 479 00:30:59,769 --> 00:31:04,02 infinity. So you cannot have any electric 480 00:31:04,02 --> 00:31:09,372 field in a superconductor. If I approach a superconducting 481 00:31:09,372 --> 00:31:14,911 disk or material with a magnet -- say this is the north pole 482 00:31:14,911 --> 00:31:20,074 and this is the south pole, so we have an electric field 483 00:31:20,074 --> 00:31:25,331 configur- a magnetic field configuration roughly like so. 484 00:31:25,331 --> 00:31:29,368 If I approach this superconducting material, 485 00:31:29,368 --> 00:31:34,532 then the EMF generated in here because of Faraday's Law, 486 00:31:34,532 --> 00:31:41,027 because there's a change in the magnetic flux coming off, 487 00:31:41,027 --> 00:31:45,946 that EMF must remain zero, because you cannot have an 488 00:31:45,946 --> 00:31:49,54 electric field inside a superconductor. 489 00:31:49,54 --> 00:31:52,094 And this, of course, is I R. 490 00:31:52,094 --> 00:31:56,445 So R is also zero. So I now can have any value, 491 00:31:56,445 --> 00:32:01,363 completely legitimate. So you can have a huge current 492 00:32:01,363 --> 00:32:04,863 inside the superconductor, but no EMF. 493 00:32:04,863 --> 00:32:09,213 And so as you approach with this 494 00:32:09,213 --> 00:32:15,165 magnet, eddy currents are going to run inside the superconductor 495 00:32:15,165 --> 00:32:20,172 in such a way that D phi D T, the flux change in here, 496 00:32:20,172 --> 00:32:24,518 is always zero. And so these eddy currents will 497 00:32:24,518 --> 00:32:29,997 flow to never allow any magnetic flux, because there was no 498 00:32:29,997 --> 00:32:35,193 magnetic flux to start with when the magnet was high up, 499 00:32:35,193 --> 00:32:38,216 so there can never be any change. 500 00:32:38,216 --> 00:32:42,846 And so the eddy c- currents create a 501 00:32:42,846 --> 00:32:46,594 magnetic field themselves which, if you vectorially add 502 00:32:46,594 --> 00:32:50,342 them to this magnetic field, will always make sure that 503 00:32:50,342 --> 00:32:54,228 there is no net magnetic field inside the superconductor. 504 00:32:54,228 --> 00:32:56,935 And so if you now make a drawing of the, 505 00:32:56,935 --> 00:33:00,336 of the two fields, the one that is produced by the 506 00:33:00,336 --> 00:33:04,223 eddy currents and the one that is produced by the magnet, 507 00:33:04,223 --> 00:33:08,04 when the magnet comes very close -- so here is north and 508 00:33:08,04 --> 00:33:11,164 here is south, and here is your superconductor 509 00:33:11,164 --> 00:33:14,921 -- then the superposition of those 510 00:33:14,921 --> 00:33:18,461 two fields then, effectively comes down to the 511 00:33:18,461 --> 00:33:23,418 effect that this magnetic field is completely repelled -- that's 512 00:33:23,418 --> 00:33:28,059 another way of looking at it. You get a squeezed field here. 513 00:33:28,059 --> 00:33:32,15 But that is the superposition of two magnetic fields, 514 00:33:32,15 --> 00:33:36,084 one produced by the eddy currents and one from the, 515 00:33:36,084 --> 00:33:39,388 the magnet. And whenever you have here such 516 00:33:39,388 --> 00:33:44,738 a squeezed magnetic field, there is magnetic pressure. 517 00:33:44,738 --> 00:33:48,9 We know why there is magnetic pressure, because north and 518 00:33:48,9 --> 00:33:53,286 north poles repel each other, but we never expressed that in 519 00:33:53,286 --> 00:33:56,631 terms of a quantity. And the magnetic pressure 520 00:33:56,631 --> 00:33:59,902 equals B squared, which is the magnetic field 521 00:33:59,902 --> 00:34:02,355 strength, divided by -- is it two? 522 00:34:02,355 --> 00:34:05,403 Yeah, two mu zero, divided by two mu zero. 523 00:34:05,403 --> 00:34:09,194 I'll get back to this a little later in the lecture. 524 00:34:09,194 --> 00:34:12,614 And this is pressure, so this is in newtons per 525 00:34:12,614 --> 00:34:14,808 square meter. 526 00:34:14,808 --> 00:34:18,601 This is not entirely new, this idea of pressure, 527 00:34:18,601 --> 00:34:23,445 because you may have seen at people's desks nice conversation 528 00:34:23,445 --> 00:34:25,22 piece. You have here a, 529 00:34:25,22 --> 00:34:29,337 a magnet, and you have here a magnet, and this is a, 530 00:34:29,337 --> 00:34:32,646 a wooden stick. There's a hole in here and 531 00:34:32,646 --> 00:34:36,036 there's a hole in here. This is north pole, 532 00:34:36,036 --> 00:34:40,395 south pole, north pole, south pole, and they repel each 533 00:34:40,395 --> 00:34:42,897 other. That's magnetic pressure. 534 00:34:42,897 --> 00:34:48,429 It's the same thing. And if you drew the magnetic 535 00:34:48,429 --> 00:34:51,861 field configurations here, go like this, 536 00:34:51,861 --> 00:34:57,053 that's the magnetic field from this magnet and this would be 537 00:34:57,053 --> 00:35:00,133 the magnetic field from that magnet. 538 00:35:00,133 --> 00:35:04,181 We get the same idea. You get magnetic pressure 539 00:35:04,181 --> 00:35:07,261 there. If I rotate this magnet here, 540 00:35:07,261 --> 00:35:11,045 so th- first of all, the magnet is repelled, 541 00:35:11,045 --> 00:35:16,59 which is in a way a form of levitation, and we're 542 00:35:16,59 --> 00:35:20,291 going to show you that. The magnet is just pushed up by 543 00:35:20,291 --> 00:35:23,581 the superconductor is the way you can look at it. 544 00:35:23,581 --> 00:35:26,46 But if you start rotating it, for instance, 545 00:35:26,46 --> 00:35:30,641 around with the south pole here or the south pole there or the 546 00:35:30,641 --> 00:35:33,314 north pole there, the eddy currents will 547 00:35:33,314 --> 00:35:36,741 instantaneously adjust to always repel that magnet. 548 00:35:36,741 --> 00:35:40,237 So even if you rotate it, it would still hang there, 549 00:35:40,237 --> 00:35:43,115 levitated, rotating. What is not so easy to 550 00:35:43,115 --> 00:35:46,543 understand is why the whole thing 551 00:35:46,543 --> 00:35:51,478 is so very stable. As you will shortly see, 552 00:35:51,478 --> 00:35:57,119 it's quite stable. So I'm going to show you there 553 00:35:57,119 --> 00:36:03,818 this superconducting idea. I first have to top it off with 554 00:36:03,818 --> 00:36:09,929 some liquid nitrogen, so let me do that when we still 555 00:36:09,929 --> 00:36:12,749 have full lights. Oh boy. 556 00:36:12,749 --> 00:36:16,157 Good. So I have to top it off. 557 00:36:16,157 --> 00:36:21,249 And this disk, which is about an inch in 558 00:36:21,249 --> 00:36:24,088 diameter, going to be superconducting. 559 00:36:24,088 --> 00:36:27,695 I can even tell you what kind of material it is. 560 00:36:27,695 --> 00:36:31,532 It is a copper oxide mixed with yttrium and barium, 561 00:36:31,532 --> 00:36:35,753 and it becomes superconducting at ninety degrees Kelvin. 562 00:36:35,753 --> 00:36:39,13 And liquid nitrogen is seventy-seven degrees. 563 00:36:39,13 --> 00:36:42,43 So we're going to put a small magnet on top, 564 00:36:42,43 --> 00:36:46,42 which we will levitate. For that, we're going to have 565 00:36:46,42 --> 00:36:52,197 the following light situation. And of course you want to see 566 00:36:52,197 --> 00:36:55,605 it also, don't you. And you want some lights. 567 00:36:55,605 --> 00:36:59,245 So there you see the disk, which is -- should be 568 00:36:59,245 --> 00:37:02,653 superconducting now. And here comes my little 569 00:37:02,653 --> 00:37:05,364 magnet. So there is no magnetic flux 570 00:37:05,364 --> 00:37:08,927 going through there. It itself is not a magnet. 571 00:37:08,927 --> 00:37:12,413 But now I'm going to come close with a magnet, 572 00:37:12,413 --> 00:37:17,06 and the eddy currents go nuts in there, and it just floats on 573 00:37:17,06 --> 00:37:18,377 top. It's amazing, 574 00:37:18,377 --> 00:37:22,947 isn't it. So this is magnetic levitation, 575 00:37:22,947 --> 00:37:30,914 and you can rotate it around, and the eddy currents adjust 576 00:37:30,914 --> 00:37:35,387 instantaneously. And there it is. 577 00:37:35,387 --> 00:37:40,419 Now you've all seen it, clear enough. 578 00:37:40,419 --> 00:37:45,731 OK, let me get the -- rescue my magnet. 579 00:37:45,731 --> 00:37:52,86 Imagine in the days of Kamerlingh Onnes it took four 580 00:37:52,86 --> 00:38:00,339 degrees Kelvin to have anything superconductive, 581 00:38:00,339 --> 00:38:04,737 and now you can do it as easy as that. 582 00:38:04,737 --> 00:38:09,966 There are other forms of magnetic levitation. 583 00:38:09,966 --> 00:38:15,315 One going to be very promising in our economy, 584 00:38:15,315 --> 00:38:21,851 we hope, and that is magnetic levitation can be used for 585 00:38:21,851 --> 00:38:26,011 trains. If you have a magnet and you 586 00:38:26,011 --> 00:38:30,82 move it fast over a conducting surface, 587 00:38:30,82 --> 00:38:35,428 then you also get levitation. You have to move it though, 588 00:38:35,428 --> 00:38:38,637 whereas there you don't have to move it. 589 00:38:38,637 --> 00:38:43,245 See, if you let that magnet just go, if you don't move it 590 00:38:43,245 --> 00:38:47,113 anymore, then there is an eddy current going on, 591 00:38:47,113 --> 00:38:50,898 but the eddy current never dissipates any heat. 592 00:38:50,898 --> 00:38:54,271 There is no I squared R because R is zero. 593 00:38:54,271 --> 00:38:57,069 So you never lose the eddy current. 594 00:38:57,069 --> 00:39:01,58 That's different with what is coming now. 595 00:39:01,58 --> 00:39:03,278 Now I have a, a magnet. 596 00:39:03,278 --> 00:39:05,671 Here is north and here is south. 597 00:39:05,671 --> 00:39:09,144 So we have a magnetic field sort of like this. 598 00:39:09,144 --> 00:39:13,93 And I'm going to move it over a plate, over a conducting plate, 599 00:39:13,93 --> 00:39:18,021 and I'll put the plate here. And as it comes over this 600 00:39:18,021 --> 00:39:21,649 conducting plate, the magnetic flux through that 601 00:39:21,649 --> 00:39:23,27 plate will change. Mr. 602 00:39:23,27 --> 00:39:26,28 Faralow s- Mr. Faraday says -- actually, 603 00:39:26,28 --> 00:39:27,978 it's Mr. Lenz who says, 604 00:39:27,978 --> 00:39:32,039 "I don't like that." And so they're 605 00:39:32,039 --> 00:39:36,567 going to run an eddy current in here, and the eddy current will 606 00:39:36,567 --> 00:39:40,511 undoubtedly go in this direction as it comes over here. 607 00:39:40,511 --> 00:39:45,04 And this current ring now will produce a magnetic field in this 608 00:39:45,04 --> 00:39:47,45 direction. And look what you have. 609 00:39:47,45 --> 00:39:51,614 You have again this is the north pole of this eddy current 610 00:39:51,614 --> 00:39:55,704 and this is the south pole. North pole repels north pole. 611 00:39:55,704 --> 00:39:59,795 And so if this has a high enough speed so that the change 612 00:39:59,795 --> 00:40:02,355 of the magnetic flux, 613 00:40:02,355 --> 00:40:05,693 the D phi D T is high enough, the train can float. 614 00:40:05,693 --> 00:40:08,622 Tens of tons of weight can be made to float. 615 00:40:08,622 --> 00:40:12,641 And the reason why in this case the train has to keep going, 616 00:40:12,641 --> 00:40:16,252 that if the train stops the eddy current will die out. 617 00:40:16,252 --> 00:40:20,475 There's no longer the D phi D T but there is resistance in this 618 00:40:20,475 --> 00:40:23,336 conductor, and so you get only dissipation. 619 00:40:23,336 --> 00:40:26,129 You get I squared R. You get heat in here. 620 00:40:26,129 --> 00:40:29,058 And so then the train will just plunge down. 621 00:40:29,058 --> 00:40:32,465 And that's not the case with the 622 00:40:32,465 --> 00:40:36,763 superconductor because you don't dissipate any heat in the 623 00:40:36,763 --> 00:40:40,307 superconductor because the superconductor has no 624 00:40:40,307 --> 00:40:42,947 resistance. So the idea is the same, 625 00:40:42,947 --> 00:40:47,321 but you see now why you have to keep this one going and why 626 00:40:47,321 --> 00:40:50,865 there you don't. And so again you get a squeezed 627 00:40:50,865 --> 00:40:53,505 magnetic field, like you have there, 628 00:40:53,505 --> 00:40:55,918 and so you get magnetic pressure. 629 00:40:55,918 --> 00:41:00,367 Japan and Germany are really the leaders in the world in the 630 00:41:00,367 --> 00:41:03,384 technology of, uh, maglevs -- that's what 631 00:41:03,384 --> 00:41:06,478 these trains are called. 632 00:41:06,478 --> 00:41:08,977 United States is trying to catch up. 633 00:41:08,977 --> 00:41:13,117 There's an enormous reduction in friction if you can have a 634 00:41:13,117 --> 00:41:16,186 train that is not in contact with the rails. 635 00:41:16,186 --> 00:41:19,969 In fact, speeds have been recorded up to three hundred 636 00:41:19,969 --> 00:41:23,395 forty miles per hour. Both Germany and Japan have 637 00:41:23,395 --> 00:41:26,036 test trains, prototypes, in operation. 638 00:41:26,036 --> 00:41:29,748 United States has made a commitment to build a maglev 639 00:41:29,748 --> 00:41:33,246 train to go from Washington, DC, back and forth to 640 00:41:33,246 --> 00:41:37,529 Baltimore, should be ready in the year two 641 00:41:37,529 --> 00:41:40,517 thousand seven. The cost per mile at about 642 00:41:40,517 --> 00:41:44,962 thirty million dollars per mile. Now that make strike as high, 643 00:41:44,962 --> 00:41:48,825 but keep in mind that if you build a four-way highway, 644 00:41:48,825 --> 00:41:51,959 that's also thirty million dollars per mile. 645 00:41:51,959 --> 00:41:55,53 So it's no more expensive than a four-way highway. 646 00:41:55,53 --> 00:41:59,174 And again, when you visit the eight oh two website, 647 00:41:59,174 --> 00:42:01,797 I made several links to maglev sites. 648 00:42:01,797 --> 00:42:07,045 I advise you to take a look. Now there is a third form of 649 00:42:07,045 --> 00:42:12,044 magnetic levitation whereby we don't need any speed and we 650 00:42:12,044 --> 00:42:15,904 don't need any superconductors but we use AC, 651 00:42:15,904 --> 00:42:20,64 we use alternating current. And that's also easy to see 652 00:42:20,64 --> 00:42:22,745 now. Here is such a coil, 653 00:42:22,745 --> 00:42:27,57 and we run AC through it. So at one particular moment in 654 00:42:27,57 --> 00:42:31,517 time, let's say the magnetic field is like so, 655 00:42:31,517 --> 00:42:36,341 and maybe is increasing. And then of course the magnetic 656 00:42:36,341 --> 00:42:40,269 field turns around, up, down, up, 657 00:42:40,269 --> 00:42:43,905 down, because it's AC. Now I have here a conducting 658 00:42:43,905 --> 00:42:46,669 plate, and I put this above this plate. 659 00:42:46,669 --> 00:42:50,451 But now I have this continuous magnetic field change, 660 00:42:50,451 --> 00:42:54,814 so I have a continuous change of magnetic flux in that plate. 661 00:42:54,814 --> 00:42:58,232 So as this magnetic flux, as the B field down is 662 00:42:58,232 --> 00:43:02,741 increasing, you're going to get an eddy current running in this 663 00:43:02,741 --> 00:43:06,668 direction, which will create the magnetic field in that 664 00:43:06,668 --> 00:43:10,887 direction. And you're back where you were. 665 00:43:10,887 --> 00:43:14,029 You again have north pole, north pole, south pole, 666 00:43:14,029 --> 00:43:16,593 south pole. So again, the eddy current in 667 00:43:16,593 --> 00:43:19,735 the conducting plate is responsible for a magnetic 668 00:43:19,735 --> 00:43:21,98 field, and the two repel each other. 669 00:43:21,98 --> 00:43:25,378 So a little later in time, the magnetic field strength 670 00:43:25,378 --> 00:43:27,43 will decrease. When that happens, 671 00:43:27,43 --> 00:43:31,021 the eddy current will reverse direction, and the two will 672 00:43:31,021 --> 00:43:34,035 attract each other. And so you will think now it 673 00:43:34,035 --> 00:43:38,075 seems quite reasonable that half the time they will attract each 674 00:43:38,075 --> 00:43:42,256 other and the other half of the time they 675 00:43:42,256 --> 00:43:45,096 will repel each other. That, however, 676 00:43:45,096 --> 00:43:48,646 is not the case. There will be a net repelling 677 00:43:48,646 --> 00:43:50,382 force. And why that is, 678 00:43:50,382 --> 00:43:53,932 I will explain to you during the next lecture. 679 00:43:53,932 --> 00:43:56,456 But I want to demonstrate it now. 680 00:43:56,456 --> 00:44:00,401 I have here such a coil, about an area of about one 681 00:44:00,401 --> 00:44:03,556 square foot. I can simply run hundred ten 682 00:44:03,556 --> 00:44:07,343 volts, sixty hertz AC. I switch th- turn this on. 683 00:44:07,343 --> 00:44:11,84 This goes into hundred ten volt outlet, and I increase the 684 00:44:11,84 --> 00:44:15,68 current. It starts to float. 685 00:44:15,68 --> 00:44:19,457 No high-speed train, nothing superconducting. 686 00:44:19,457 --> 00:44:23,663 What do you think will happen when I turn it over? 687 00:44:23,663 --> 00:44:27,097 Excuse me? Of course it will float again. 688 00:44:27,097 --> 00:44:31,905 The ed- the eddy currents adjust themselves at any moment 689 00:44:31,905 --> 00:44:35,596 in time, and they will always make it float. 690 00:44:35,596 --> 00:44:40,575 So this is another interesting form of magnetic levitation. 691 00:44:40,575 --> 00:44:45,211 So we have the superconducting way, whereby there is no 692 00:44:45,211 --> 00:44:49,745 dissipation in the disk, so those currents 693 00:44:49,745 --> 00:44:52,864 never die out. Then you have the case of the 694 00:44:52,864 --> 00:44:56,927 train, if I call that the train, the magnetic levitation, 695 00:44:56,927 --> 00:45:00,916 whereby you have to keep the speed going, because if you 696 00:45:00,916 --> 00:45:05,269 don't have the speed then you don't have enough magnetic flux 697 00:45:05,269 --> 00:45:09,114 changes in the surface, and so you don't have the eddy 698 00:45:09,114 --> 00:45:11,798 currents. The eddy currents would die, 699 00:45:11,798 --> 00:45:15,57 you get heat dissipation. And then there is the third 700 00:45:15,57 --> 00:45:19,759 case here, whereby you simply have a 701 00:45:19,759 --> 00:45:23,558 changing magnetic field in the coil with AC, 702 00:45:23,558 --> 00:45:27,534 which then creates the changing eddy currents. 703 00:45:27,534 --> 00:45:31,333 So now we come to the levitation of a woman. 704 00:45:31,333 --> 00:45:36,103 How do we levitate a woman? Well, the secret must be in 705 00:45:36,103 --> 00:45:39,902 that equation. V is B squared divided by two 706 00:45:39,902 --> 00:45:43,171 mu zero. We have a coil which is about 707 00:45:43,171 --> 00:45:46,97 one square foot, so the area of that coil is 708 00:45:46,97 --> 00:45:51,425 about oh point one square meters. 709 00:45:51,425 --> 00:45:56,287 And so our goal was that we should be able to lift, 710 00:45:56,287 --> 00:46:01,344 let's say, a two hundred pounder, to give ourselves a 711 00:46:01,344 --> 00:46:06,11 little bit of leeway. And so we calculated that if 712 00:46:06,11 --> 00:46:10,584 the magnetic field, B, is about fifteen hundred 713 00:46:10,584 --> 00:46:14,96 gauss, which is about oh point one five tesla, 714 00:46:14,96 --> 00:46:22,059 that we would get very close. We know that mu zero is four pi 715 00:46:22,059 --> 00:46:25,882 times ten to the minus seven in SI units. 716 00:46:25,882 --> 00:46:31,521 And so you can calculate now what the force is on this area. 717 00:46:31,521 --> 00:46:36,299 The force is of course the pressure times the area, 718 00:46:36,299 --> 00:46:40,408 assuming that the magnetic field is uniform. 719 00:46:40,408 --> 00:46:45,951 And so you're going to get that this force is going to be B 720 00:46:45,951 --> 00:46:50,06 squared, which is oh point one five squared, 721 00:46:50,06 --> 00:46:54,456 divide by two, divide by four pi, 722 00:46:54,456 --> 00:46:59,541 times ten to the seventh, multiplied by the area, 723 00:46:59,541 --> 00:47:05,897 which is oh point one square meter, and you find that this is 724 00:47:05,897 --> 00:47:10,558 about nine hundred newtons. Ninety kilograms, 725 00:47:10,558 --> 00:47:13,947 as I said, a two hundred pounder. 726 00:47:13,947 --> 00:47:18,185 Then we tried to get this magnetic field, 727 00:47:18,185 --> 00:47:22,104 and believe me, Marcos and I and Bill, 728 00:47:22,104 --> 00:47:26,357 we really tried, but there was a problem, 729 00:47:26,357 --> 00:47:29,706 because we needed enormously high currents to get these 730 00:47:29,706 --> 00:47:32,063 magnetic fields. And at these very high 731 00:47:32,063 --> 00:47:35,164 currents, our circuit breakers would go every time. 732 00:47:35,164 --> 00:47:38,203 So we called up the physical plant, and they said, 733 00:47:38,203 --> 00:47:40,064 "Yeah, what do you expect, man? 734 00:47:40,064 --> 00:47:42,049 You need several hundred amperes. 735 00:47:42,049 --> 00:47:45,274 You think we can get seven hundred -- several hundred 736 00:47:45,274 --> 00:47:48,685 amperes out of this system? You have to redesign MIT for 737 00:47:48,685 --> 00:47:51,29 that." So it was very disappointing for us. 738 00:47:51,29 --> 00:47:56,438 So the best we could do was the strongest current that we could 739 00:47:56,438 --> 00:48:00,623 generate, we could only get a magnetic field of about three 740 00:48:00,623 --> 00:48:04,232 hundred and fifty gauss, which is four times lower. 741 00:48:04,232 --> 00:48:08,562 And the tragedy has it that the magnetic pressure goes with B 742 00:48:08,562 --> 00:48:11,81 squared, so that makes it sixteen times lower. 743 00:48:11,81 --> 00:48:15,779 So the two hundred pounder now becomes a twelve pounder. 744 00:48:15,779 --> 00:48:20,037 But that's the reason why when some women wrote me very nice 745 00:48:20,037 --> 00:48:24,223 email yesterday because they volunteered, why I had to say, 746 00:48:24,223 --> 00:48:33,43 "Thanks, but no thanks." But I want to keep my 747 00:48:33,43 --> 00:48:42,349 promise. I said I was going to levitate 748 00:48:42,349 --> 00:48:50,798 a woman, and I will. And here she is. 749 00:48:50,798 --> 00:48:58,543 [laughter]. And as far as we know, 750 00:48:58,543 --> 00:49:02,998 it is a woman. 751 00:49:02,998 --> 00:49:09,51 [laughter]. OK, give her some room. 752 00:49:09,51 --> 00:49:15,831 She deserves -- we ready for that? 753 00:49:15,831 --> 00:49:23,302 And there she goes. I levitated a woman. 754 00:49:23,302 --> 00:49:30,773 [laughter]. Have a great Spring 755 00:49:30,773 --> 49:36 Break. [applause].