1 00:00:00,157 --> 00:00:02,490 The following content is provided under a Creative 2 00:00:02,490 --> 00:00:04,030 Commons license. 3 00:00:04,030 --> 00:00:06,330 Your support will help MIT OpenCourseWare 4 00:00:06,330 --> 00:00:10,690 continue to offer high quality educational resources for free. 5 00:00:10,690 --> 00:00:13,320 To make a donation or view additional materials 6 00:00:13,320 --> 00:00:17,280 from hundreds of MIT courses, visit MIT OpenCourseWare 7 00:00:17,280 --> 00:00:20,040 at OCW.MIT.edu. 8 00:00:20,040 --> 00:00:27,180 PROFESSOR: Our guiding question through this investigation 9 00:00:27,180 --> 00:00:36,870 is going to be why is light produced? 10 00:00:50,730 --> 00:00:56,175 What can we tell about the source? 11 00:01:01,120 --> 00:01:01,620 Right? 12 00:01:01,620 --> 00:01:04,578 If we're looking at these bulbs over here. 13 00:01:04,578 --> 00:01:06,940 If we looked at a bulb, and we saw that it was just 14 00:01:06,940 --> 00:01:11,186 glowing red, we can tell that it was probably a low temperature 15 00:01:11,186 --> 00:01:13,860 without actually having to go up there and touch it. 16 00:01:13,860 --> 00:01:16,026 So we're going to think about how is light produced, 17 00:01:16,026 --> 00:01:20,910 and how does that help us figure out something about the source, 18 00:01:20,910 --> 00:01:23,534 because we want to know about what's going on out in space. 19 00:01:23,534 --> 00:01:24,576 You're going to be able to observe light made 20 00:01:24,576 --> 00:01:26,242 in an accretion disk around a black hole 21 00:01:26,242 --> 00:01:29,660 and be able to tell what temperature is it at, 22 00:01:29,660 --> 00:01:32,352 what kind of gas is it? 23 00:01:32,352 --> 00:01:34,200 Let's see if we learn a little bit more, 24 00:01:34,200 --> 00:01:37,184 because we only get light from objects in outer space. 25 00:01:37,184 --> 00:01:39,976 We can't go to the black hole and pick something up 26 00:01:39,976 --> 00:01:40,726 and bring it back. 27 00:01:40,726 --> 00:01:43,630 So we only get light. 28 00:01:43,630 --> 00:01:54,188 So we're going to do a review here of atomic structure, 29 00:01:54,188 --> 00:01:56,280 because the way that light is produced, 30 00:01:56,280 --> 00:01:59,543 we're going to find out, has to do with atoms. 31 00:01:59,543 --> 00:02:01,900 So I'm going to draw a couple of diagrams here, 32 00:02:01,900 --> 00:02:04,080 and, again, keep in mind some of those points 33 00:02:04,080 --> 00:02:06,182 that you guys wrote down about how 34 00:02:06,182 --> 00:02:10,290 your notes were different from here and [INAUDIBLE] yesterday. 35 00:02:10,290 --> 00:02:12,540 And try to keep those good note-taking habits in mind. 36 00:02:15,340 --> 00:02:15,840 OK. 37 00:02:15,840 --> 00:02:17,465 So I'm going to draw two pictures here. 38 00:02:20,464 --> 00:02:22,730 We're going to draw a helium atom, 39 00:02:22,730 --> 00:02:24,985 and we're going to draw another picture over here. 40 00:02:24,985 --> 00:02:27,110 So make sure to leave enough room for side-by-side. 41 00:02:31,110 --> 00:02:37,165 We all learned, hopefully, that atoms are made of a nucleus-- 42 00:02:40,630 --> 00:02:42,115 which is positively charged. 43 00:02:42,115 --> 00:02:47,065 And in a helium atom we have two protons. 44 00:02:47,065 --> 00:02:50,530 So the black circles here are protons. 45 00:02:50,530 --> 00:02:52,571 And we write the assigned symbol for proton as P 46 00:02:52,571 --> 00:02:54,985 with a little plus next to it. 47 00:02:54,985 --> 00:02:58,945 Why do I write a proton with a P with a little plus next to it? 48 00:02:58,945 --> 00:03:03,026 It's positively charged, right. 49 00:03:03,026 --> 00:03:04,901 And, remember, we're going to be a little bit 50 00:03:04,901 --> 00:03:09,010 confusing because we're going to talk about protons-- 51 00:03:09,010 --> 00:03:17,790 P-R-O-T-O-N-S-- versus photons. 52 00:03:17,790 --> 00:03:21,260 So proton and photon sound similar. 53 00:03:21,260 --> 00:03:25,295 A proton is a particle of matter that's positively charged. 54 00:03:25,295 --> 00:03:26,670 A photon is a particle of light-- 55 00:03:29,925 --> 00:03:32,250 it has no charge. 56 00:03:32,250 --> 00:03:35,167 So we also have in the nucleus we 57 00:03:35,167 --> 00:03:39,190 have a neutron, which we write as N with a little 0 58 00:03:39,190 --> 00:03:39,690 next to it. 59 00:03:39,690 --> 00:03:41,231 Why do I write a little 0 next to it? 60 00:03:44,410 --> 00:03:46,490 Because neutrons are neutral, and they 61 00:03:46,490 --> 00:03:51,610 have no electric charge, right? 62 00:03:51,610 --> 00:03:53,710 And so there are two-- 63 00:03:53,710 --> 00:03:55,760 there are two protons, there are two neutrons. 64 00:03:55,760 --> 00:04:00,280 Also, around the outside of the atom-- 65 00:04:00,280 --> 00:04:03,541 sometimes people-- there are several different models 66 00:04:03,541 --> 00:04:04,443 of atoms. 67 00:04:04,443 --> 00:04:06,250 You can talk about-- 68 00:04:06,250 --> 00:04:08,708 you know, if we had an electron-- 69 00:04:08,708 --> 00:04:10,773 which I'm going to draw it as a little tiny dot. 70 00:04:10,773 --> 00:04:12,705 Actually, let's draw it as-- 71 00:04:12,705 --> 00:04:15,603 just a little tiny dot. 72 00:04:15,603 --> 00:04:18,305 We write that as e with a minus next to it. 73 00:04:18,305 --> 00:04:21,215 Why do I write it with a minus sign? 74 00:04:21,215 --> 00:04:22,940 It's negatively charged. 75 00:04:22,940 --> 00:04:26,396 That is an electron. 76 00:04:26,396 --> 00:04:28,895 You guys have probably heard about electrons being in shells 77 00:04:28,895 --> 00:04:30,830 around the nucleus. 78 00:04:30,830 --> 00:04:32,440 There are a bunch of different models 79 00:04:32,440 --> 00:04:34,350 that people use to describe atoms. 80 00:04:34,350 --> 00:04:39,226 In reality, none of those models is actually truly correct. 81 00:04:39,226 --> 00:04:41,370 The closest thing that we've come to 82 00:04:41,370 --> 00:04:43,187 is a quantum mechanical interpretation 83 00:04:43,187 --> 00:04:45,800 of the atom, which has to do with this idea 84 00:04:45,800 --> 00:04:47,112 of electronic clouds. 85 00:04:47,112 --> 00:04:49,320 And it's not the atom that's orbiting like a planet-- 86 00:04:49,320 --> 00:04:53,049 we're going to find out why. 87 00:04:53,049 --> 00:04:58,056 But it's also that you can't, like, pick up an electron-- 88 00:04:58,056 --> 00:04:59,610 it's so small. 89 00:04:59,610 --> 00:05:00,610 They're spread out. 90 00:05:00,610 --> 00:05:02,110 What I'm just going to say is there 91 00:05:02,110 --> 00:05:05,184 are two electrons that are close, 92 00:05:05,184 --> 00:05:09,400 or bound to this nucleus. 93 00:05:09,400 --> 00:05:11,380 Something else you can think about them 94 00:05:11,380 --> 00:05:16,045 as being on an orbit around-- 95 00:05:16,045 --> 00:05:23,250 and you have them being on an orbit around the nucleus, OK? 96 00:05:23,250 --> 00:05:28,444 So this atom is-- 97 00:05:28,444 --> 00:05:36,025 it's neutral, which means it has no net charge. 98 00:05:36,025 --> 00:05:49,150 The neutral equals net charge of 0, because I have two protons, 99 00:05:49,150 --> 00:05:51,986 I have two neutrons that have no charge, 100 00:05:51,986 --> 00:05:58,090 and then I have two electrons, so the net charge is 0. 101 00:06:03,359 --> 00:06:07,553 We are going to find out that it is also important in astronomy 102 00:06:07,553 --> 00:06:09,808 to think about ions. 103 00:06:09,808 --> 00:06:14,446 Over here I'm going to draw a helium ion. 104 00:06:17,320 --> 00:06:20,790 An ion is like this-- 105 00:06:20,790 --> 00:06:26,280 again, you still have two protons, you have two neutrons. 106 00:06:26,280 --> 00:06:30,996 But instead we only have, let's say, one electron. 107 00:06:30,996 --> 00:06:36,900 This is proton, proton, neutron. 108 00:06:36,900 --> 00:06:42,312 And you can think about this as having an electron go in orbit. 109 00:06:42,312 --> 00:06:48,540 In this case, this is charged, which means 110 00:06:48,540 --> 00:06:55,106 it has a net charge of what? 111 00:06:55,106 --> 00:06:58,050 What is the net charge here? 112 00:06:58,050 --> 00:06:58,550 Right. 113 00:06:58,550 --> 00:07:01,010 We have two positive charges, one negative charge. 114 00:07:01,010 --> 00:07:04,770 We have a net charge of plus 1. 115 00:07:04,770 --> 00:07:06,270 All right. 116 00:07:06,270 --> 00:07:08,969 So this should be review. 117 00:07:08,969 --> 00:07:13,604 We have an uneven number of protons and neutrons-- 118 00:07:13,604 --> 00:07:16,811 or, I'm sorry-- protons and electrons, 119 00:07:16,811 --> 00:07:18,060 because here we only have one. 120 00:07:24,060 --> 00:07:27,526 How do we go from an atom to an ion? 121 00:07:27,526 --> 00:07:30,545 We go from here to here-- 122 00:07:30,545 --> 00:07:33,440 the process that Frank talked to us a little bit yesterday-- 123 00:07:33,440 --> 00:07:36,828 ionization. 124 00:07:36,828 --> 00:07:40,216 Ionization means the creation of ions. 125 00:07:40,216 --> 00:07:49,342 We're going to define ionization this way. 126 00:07:49,342 --> 00:07:51,869 How would we-- you know, apparently [INAUDIBLE] we 127 00:07:51,869 --> 00:07:53,872 lost an electron. 128 00:07:53,872 --> 00:07:57,050 We lost an electron somehow. 129 00:07:57,050 --> 00:08:10,872 Ionization is when an atom loses one or more electrons. 130 00:08:14,344 --> 00:08:17,320 And it loses it if-- 131 00:08:21,288 --> 00:08:25,256 if [INAUDIBLE] and we're going to put [INAUDIBLE] 132 00:08:25,256 --> 00:08:27,736 in parenthesis or a type of quotation marks-- 133 00:08:27,736 --> 00:08:45,979 if bumped hard enough by another atom or a photon. 134 00:08:48,937 --> 00:08:53,090 So if one atom comes in and bumps into another one, 135 00:08:53,090 --> 00:08:56,684 you can kind of bump away one of those electrons. 136 00:08:56,684 --> 00:08:57,670 Right? 137 00:08:57,670 --> 00:09:00,172 And so if you've ever seen somebody playing football-- 138 00:09:00,172 --> 00:09:01,630 two football players come together, 139 00:09:01,630 --> 00:09:03,582 sometimes one loses their helmet, 140 00:09:03,582 --> 00:09:08,710 or loses like a pad out of their shoulder pads or side pads? 141 00:09:08,710 --> 00:09:10,210 Kind of the same thing-- 142 00:09:10,210 --> 00:09:11,710 when you bump two things together, 143 00:09:11,710 --> 00:09:14,044 sometimes this can fly out. 144 00:09:14,044 --> 00:09:15,710 Same thing happens if you take a photon, 145 00:09:15,710 --> 00:09:18,202 and you have the photon bump against the atom, 146 00:09:18,202 --> 00:09:21,154 it can cause ionization. 147 00:09:21,154 --> 00:09:22,630 OK. 148 00:09:22,630 --> 00:09:26,880 So, you're going to say that-- 149 00:09:26,880 --> 00:09:31,000 in most cases, if you have a bunch of helium atoms together, 150 00:09:31,000 --> 00:09:34,140 we call that a gas. 151 00:09:34,140 --> 00:09:39,475 So helium atoms altogether form a gas. 152 00:09:39,475 --> 00:09:43,175 If you have a bunch of ions all together, 153 00:09:43,175 --> 00:09:47,870 you can make a plasma. 154 00:09:47,870 --> 00:09:50,339 And a plasma is something we're just 155 00:09:50,339 --> 00:10:00,005 going to define as a gas made of ions. 156 00:10:00,005 --> 00:10:02,440 AUDIENCE: Is that kind of like a plasma TV? 157 00:10:02,440 --> 00:10:03,901 PROFESSOR: Not quite. 158 00:10:03,901 --> 00:10:07,295 A plasma TV, I honestly don't know exactly how that works. 159 00:10:07,295 --> 00:10:08,961 It might have something to do with that. 160 00:10:08,961 --> 00:10:10,949 That's a good question. 161 00:10:10,949 --> 00:10:14,428 So if you want to research that and find that out for us, 162 00:10:14,428 --> 00:10:15,920 that would be great! 163 00:10:15,920 --> 00:10:18,935 So the plasma is a gas made of ions. 164 00:10:18,935 --> 00:10:22,380 We're going to find out that a lot of astrophysical forces 165 00:10:22,380 --> 00:10:25,110 are plasma. 166 00:10:25,110 --> 00:10:28,730 Now we are going to take a look now-- 167 00:10:28,730 --> 00:10:32,140 we're going to make a couple of observations about 168 00:10:32,140 --> 00:10:37,130 a simulation, or a model, of what people think-- 169 00:10:37,130 --> 00:10:39,130 or what people know pretty well-- 170 00:10:39,130 --> 00:10:40,630 it's a pretty standard model-- 171 00:10:40,630 --> 00:10:42,088 of what happens on an atomic scale. 172 00:10:44,816 --> 00:10:45,940 So what we're going to do-- 173 00:10:45,940 --> 00:10:48,123 we're going to look at a very, very small box that 174 00:10:48,123 --> 00:10:48,998 is filled with atoms. 175 00:10:51,870 --> 00:10:53,024 These atoms represent air. 176 00:10:53,024 --> 00:10:57,172 So, Peter, can we put up the atoms in motion simulation? 177 00:11:02,142 --> 00:11:07,687 So-- we already took pictures of these, so I'm going to-- 178 00:11:07,687 --> 00:11:08,603 let's take these down. 179 00:11:27,986 --> 00:11:28,530 There. 180 00:11:28,530 --> 00:11:29,071 Are we there? 181 00:11:29,071 --> 00:11:30,193 OK. 182 00:11:30,193 --> 00:11:33,905 So this is a simulation, or a model of a box of air. 183 00:11:33,905 --> 00:11:35,696 It's a really, really small box, because we 184 00:11:35,696 --> 00:11:39,230 can see the individual atoms. 185 00:11:39,230 --> 00:11:45,876 What I want you to do is, I want you to watch one atom-- 186 00:11:45,876 --> 00:11:48,425 like pick out one-- they're moving slowly enough so 187 00:11:48,425 --> 00:11:49,520 that you can see it. 188 00:11:49,520 --> 00:11:50,890 I want you to watch it-- 189 00:11:50,890 --> 00:11:52,681 and I want you to watch what happens to it. 190 00:11:56,707 --> 00:11:57,693 It might be hard. 191 00:11:57,693 --> 00:11:59,068 If you lose it, pick another one. 192 00:12:08,046 --> 00:12:09,892 Oh, I lost it. 193 00:12:09,892 --> 00:12:11,600 Sometime it's easier to pick the red ones 194 00:12:11,600 --> 00:12:12,850 because there's fewer of them. 195 00:12:17,590 --> 00:12:24,905 How would you describe the motion of your one atom? 196 00:12:24,905 --> 00:12:26,395 Let's make some observations. 197 00:12:26,395 --> 00:12:27,811 So Mandy, what do you want to say? 198 00:12:27,811 --> 00:12:31,510 AUDIENCE: [INAUDIBLE] 199 00:12:31,510 --> 00:12:36,560 PROFESSOR: Atom goes-- one atom can go a little fast and slow. 200 00:12:36,560 --> 00:12:41,150 Goes fast and slow. 201 00:12:41,150 --> 00:12:43,966 But it changes speed after it-- 202 00:12:43,966 --> 00:12:46,778 after it bumps into another atom. 203 00:12:46,778 --> 00:12:52,896 Goes fast and slow, let's say, after bumps, 204 00:12:52,896 --> 00:12:55,720 which we call collisions. 205 00:12:55,720 --> 00:12:57,909 OK, so bumps-- collisions-- 206 00:13:00,630 --> 00:13:02,495 with other atoms. 207 00:13:05,020 --> 00:13:05,520 OK. 208 00:13:05,520 --> 00:13:08,660 How would someone else describe them? 209 00:13:08,660 --> 00:13:10,270 I see, and then I heard [? Jalen ?] 210 00:13:10,270 --> 00:13:11,228 start to say something. 211 00:13:11,228 --> 00:13:12,120 See? 212 00:13:12,120 --> 00:13:13,370 AUDIENCE: They're just random. 213 00:13:15,410 --> 00:13:17,290 Just go anywhere. 214 00:13:17,290 --> 00:13:21,089 PROFESSOR: OK, so the direction seems to be kind of random. 215 00:13:21,089 --> 00:13:29,900 So the direction of motion is random. 216 00:13:29,900 --> 00:13:31,320 OK, what else? 217 00:13:31,320 --> 00:13:33,247 [? Jalen, ?] are you going to say something? 218 00:13:33,247 --> 00:13:33,788 Are you sure? 219 00:13:38,252 --> 00:13:41,930 OK, so the direction gets changed when it's bumped. 220 00:13:41,930 --> 00:13:44,250 And it has to do from where it's bumped, and bumped 221 00:13:44,250 --> 00:13:47,470 on one side, the direction kind of changes to that direction. 222 00:13:47,470 --> 00:13:55,337 OK, so the direction of motion is random, but changed by bumps 223 00:13:55,337 --> 00:13:58,930 or by [INAUDIBLE] What else? 224 00:13:58,930 --> 00:13:59,550 Anything else? 225 00:13:59,550 --> 00:14:00,966 AUDIENCE: Would you say it travels 226 00:14:00,966 --> 00:14:02,236 in a straight direction? 227 00:14:02,236 --> 00:14:02,819 PROFESSOR: Ah! 228 00:14:02,819 --> 00:14:05,967 In between bumps, it's always traveling in a straight line. 229 00:14:09,376 --> 00:14:16,194 So between bumps, travels in straight line. 230 00:14:21,051 --> 00:14:21,551 OK. 231 00:14:21,551 --> 00:14:24,510 What else? 232 00:14:24,510 --> 00:14:28,067 Are all of these moving at the same speed? 233 00:14:28,067 --> 00:14:29,900 All right, we said that in between the bumps 234 00:14:29,900 --> 00:14:31,850 it could be going a different speed. 235 00:14:31,850 --> 00:14:33,310 So there are some that are always 236 00:14:33,310 --> 00:14:35,310 moving sl-- there are some that are moving slow, 237 00:14:35,310 --> 00:14:36,907 some that are moving fast. 238 00:14:36,907 --> 00:14:39,240 Are the ones that are moving fast always the same atoms? 239 00:14:41,666 --> 00:14:42,166 No. 240 00:14:42,166 --> 00:14:43,755 So, like, right now I see a red one 241 00:14:43,755 --> 00:14:45,380 in the middle that's going really slow, 242 00:14:45,380 --> 00:14:46,766 and down there it just took off. 243 00:14:46,766 --> 00:14:50,832 That's one slow again, the other one just took off. 244 00:14:50,832 --> 00:14:55,240 So what I'm going to have Peter do now is he is going to-- 245 00:14:55,240 --> 00:14:56,910 we're going to turn the temperature 246 00:14:56,910 --> 00:14:59,670 of the floor of this box up. 247 00:14:59,670 --> 00:15:02,435 Right now we're at 30 Kelvin, which 248 00:15:02,435 --> 00:15:04,060 is a very, very cold temperature, which 249 00:15:04,060 --> 00:15:07,090 explains some of the things that we're about to see. 250 00:15:07,090 --> 00:15:11,550 So let's turn up to 100 Kelvin, which is still very, very cold. 251 00:15:11,550 --> 00:15:13,300 That's the temperature of clouds in space. 252 00:15:13,300 --> 00:15:15,560 But I want you to watch what happ-- 253 00:15:15,560 --> 00:15:16,110 pick an atom. 254 00:15:16,110 --> 00:15:18,790 And I want you to watch what happens to that atom. 255 00:15:26,235 --> 00:15:27,610 It's easier to watch the red one. 256 00:15:36,045 --> 00:15:36,920 Some of them are red. 257 00:15:36,920 --> 00:15:40,132 It's the simulation of what air is like. 258 00:15:40,132 --> 00:15:45,354 Red represents oxygen. And blue represents [INAUDIBLE] OK, 259 00:15:45,354 --> 00:15:47,020 so keep your eye on one of the red ones. 260 00:15:52,450 --> 00:15:59,496 OK, now I want you to describe at a higher temperature-- 261 00:15:59,496 --> 00:16:04,905 a higher temperature-- how do you describe the motion now? 262 00:16:04,905 --> 00:16:07,196 How was the motion of one that's different from the way 263 00:16:07,196 --> 00:16:09,080 it was before. 264 00:16:09,080 --> 00:16:15,700 [INAUDIBLE] 265 00:16:15,700 --> 00:16:19,700 OK, now before we said it goes fast and slow. 266 00:16:19,700 --> 00:16:20,606 So what? 267 00:16:20,606 --> 00:16:22,231 How can we be more specific about that? 268 00:16:30,686 --> 00:16:34,170 It became-- it's-- when you say it became more faster, 269 00:16:34,170 --> 00:16:39,635 you mean to say it became faster or it was more fast. 270 00:16:39,635 --> 00:16:40,135 OK. 271 00:16:40,135 --> 00:16:43,850 Let's just say it-- it's faster. 272 00:16:43,850 --> 00:16:45,182 So, Ricky, you were saying? 273 00:16:45,182 --> 00:16:46,390 You wanted to add on to that? 274 00:16:49,510 --> 00:16:50,700 OK. 275 00:16:50,700 --> 00:16:51,665 It picks up speed. 276 00:16:51,665 --> 00:16:52,165 See? 277 00:16:52,165 --> 00:16:58,610 AUDIENCE: [INAUDIBLE] 278 00:16:58,610 --> 00:16:59,694 PROFESSOR: OK. 279 00:16:59,694 --> 00:17:01,367 If the temperature is high-- 280 00:17:01,367 --> 00:17:02,530 so let's write this down. 281 00:17:02,530 --> 00:17:11,060 Higher temperature means, let's say, faster motion. 282 00:17:11,060 --> 00:17:12,030 OK? 283 00:17:12,030 --> 00:17:13,089 Let's check it out. 284 00:17:15,720 --> 00:17:17,127 So here the temperature is high. 285 00:17:17,127 --> 00:17:19,003 But watch that guy-- oh, well. 286 00:17:19,003 --> 00:17:23,260 Watch that guy right there, right? 287 00:17:23,260 --> 00:17:26,560 Are they all moving faster? 288 00:17:26,560 --> 00:17:28,240 Or are there still some them slow? 289 00:17:28,240 --> 00:17:37,360 AUDIENCE: [INAUDIBLE] 290 00:17:37,360 --> 00:17:38,540 PROFESSOR: OK, so where-- 291 00:17:38,540 --> 00:17:40,180 at what temperature, look! 292 00:17:40,180 --> 00:17:41,596 High temperature, low temperature. 293 00:17:41,596 --> 00:17:47,680 Do they move slower or longer? 294 00:17:47,680 --> 00:17:49,020 [INAUDIBLE] Right? 295 00:17:49,020 --> 00:17:53,154 So right here, this guy is kind of moving slow a little bit. 296 00:17:53,154 --> 00:17:54,128 Oh, now he took off. 297 00:17:54,128 --> 00:17:55,089 Right? 298 00:17:55,089 --> 00:17:55,589 Jason. 299 00:17:55,589 --> 00:18:07,764 AUDIENCE: Last time [INAUDIBLE] 300 00:18:07,764 --> 00:18:08,770 PROFESSOR: OK. 301 00:18:08,770 --> 00:18:09,910 Can we still see-- 302 00:18:09,910 --> 00:18:12,201 can you watch for some red particles still moving slow, 303 00:18:12,201 --> 00:18:13,582 though? 304 00:18:13,582 --> 00:18:14,905 AUDIENCE: A little bit. 305 00:18:14,905 --> 00:18:15,790 PROFESSOR: OK. 306 00:18:15,790 --> 00:18:17,600 There's a slow one there. 307 00:18:17,600 --> 00:18:20,860 So what we're going to say is, on average, 308 00:18:20,860 --> 00:18:23,062 the speed of these particles has increased. 309 00:18:23,062 --> 00:18:24,770 There are still some that are going slow. 310 00:18:24,770 --> 00:18:27,290 There are stil some that are going fast. 311 00:18:27,290 --> 00:18:31,440 But overall, they're all living just a little bit faster, 312 00:18:31,440 --> 00:18:33,120 because the slower ones, you know. 313 00:18:33,120 --> 00:18:36,267 Peter, let's turn the temperature back down. 314 00:18:36,267 --> 00:18:37,600 We got down to, like, 10, maybe. 315 00:18:45,520 --> 00:18:48,275 So we're still bouncing. 316 00:18:48,275 --> 00:18:51,135 What's happening to the number of collisions? 317 00:18:51,135 --> 00:18:53,975 AUDIENCE: [INAUDIBLE] 318 00:18:53,975 --> 00:18:56,972 PROFESSOR: We can go longer without having a collision. 319 00:18:56,972 --> 00:19:01,070 So at high temperature there was a faster average motion. 320 00:19:05,159 --> 00:19:06,950 There were still some that were going slow, 321 00:19:06,950 --> 00:19:08,533 there still some that were going fast. 322 00:19:08,533 --> 00:19:11,680 But in general, most of them had speeded up just a little bit. 323 00:19:11,680 --> 00:19:13,430 There was faster motion, there was also 324 00:19:13,430 --> 00:19:22,660 more collisions, more bounces, or I guess you said bumps. 325 00:19:22,660 --> 00:19:24,600 More collisions. 326 00:19:24,600 --> 00:19:32,332 And at a lower temperature, we have on average-- 327 00:19:32,332 --> 00:19:34,290 there's still some of them that are moving slow 328 00:19:34,290 --> 00:19:37,900 and some that are moving faster, but the average speed 329 00:19:37,900 --> 00:19:39,714 is a little bit slower. 330 00:19:39,714 --> 00:19:45,760 And I can watch one atom for much longer in between bumps. 331 00:19:45,760 --> 00:19:54,240 So lower temperature has slower average motion 332 00:19:54,240 --> 00:20:00,796 and it has less collisions, like maybe less collisions 333 00:20:00,796 --> 00:20:03,236 per second. 334 00:20:03,236 --> 00:20:06,652 But in this case, most of them aren't 335 00:20:06,652 --> 00:20:08,116 colliding with each other. 336 00:20:13,484 --> 00:20:16,412 Can we turn the lights back up? 337 00:20:20,910 --> 00:20:26,960 We're going to-- put it back over here, put it on this side. 338 00:20:35,310 --> 00:20:38,010 Just like we did with the particle model of light, 339 00:20:38,010 --> 00:20:41,512 and we made a nice simple model we 340 00:20:41,512 --> 00:20:42,970 could use to make some predictions, 341 00:20:42,970 --> 00:20:46,010 we're going to do the same things with-- 342 00:20:46,010 --> 00:20:47,960 I guess I didn't label this. 343 00:20:47,960 --> 00:20:54,197 These are observations of atoms in a gas. 344 00:21:01,829 --> 00:21:07,940 I'm now going to introduce a model of light production. 345 00:21:12,350 --> 00:21:14,400 It has to do with all of these observations 346 00:21:14,400 --> 00:21:15,901 that we've been taking. 347 00:21:15,901 --> 00:21:17,650 What have we seen with these [INAUDIBLE]?? 348 00:21:17,650 --> 00:21:23,165 What did we see with that simulation? 349 00:21:23,165 --> 00:21:46,580 And I'm just going to say, when a charged particle, 350 00:21:46,580 --> 00:21:50,480 and that could mean an ion, it could mean an electron just 351 00:21:50,480 --> 00:21:53,804 by itself, but there has to be a charged particle-- 352 00:21:53,804 --> 00:21:56,030 and particle is just our word for a very small amount 353 00:21:56,030 --> 00:21:57,600 of something-- 354 00:21:57,600 --> 00:22:12,466 when a charged particle bounces, or, if some of you 355 00:22:12,466 --> 00:22:20,935 are familiar with this word, accelerates, which we mean-- 356 00:22:20,935 --> 00:22:24,510 so "ie" means I've gotten to know one of you, for example-- 357 00:22:24,510 --> 00:22:35,012 or what we really mean is, if it changes velocity or direction. 358 00:22:38,330 --> 00:22:40,930 So if something is accelerating, it's 359 00:22:40,930 --> 00:22:43,144 either changing its velocity or changing 360 00:22:43,144 --> 00:22:44,810 the direction of its velocity-- velocity 361 00:22:44,810 --> 00:22:50,662 is actually a factor in both the speed as well as the direction. 362 00:22:50,662 --> 00:22:53,120 But an easier way to think about it is sometimes is bounce. 363 00:22:53,120 --> 00:22:56,710 If something bounces, like [INAUDIBLE] said before, 364 00:22:56,710 --> 00:22:59,730 you change the direction of the atom's motion. 365 00:22:59,730 --> 00:23:02,756 And in between the bounces, Juan said, all of the atoms 366 00:23:02,756 --> 00:23:03,630 go in straight lines. 367 00:23:03,630 --> 00:23:06,380 So if they're going in a straight line, no bounce. 368 00:23:06,380 --> 00:23:09,965 If they bounce off of another atom then 369 00:23:09,965 --> 00:23:11,462 that's the acceleration. 370 00:23:11,462 --> 00:23:16,707 So when a charged particle bounces or accelerates 371 00:23:16,707 --> 00:23:22,108 it emits a photon of light. 372 00:23:28,000 --> 00:23:29,950 So you remember when we said yesterday 373 00:23:29,950 --> 00:23:34,444 that light, a lot of people talk about the electromagnetic 374 00:23:34,444 --> 00:23:35,806 spectrum? 375 00:23:35,806 --> 00:23:38,563 Well, electromagnetism is the name 376 00:23:38,563 --> 00:23:43,664 of the force that holds nucleus and electrons together. 377 00:23:43,664 --> 00:23:46,686 It's the electrostatic force. 378 00:23:46,686 --> 00:23:48,880 The electromagnetic force that comes into 379 00:23:48,880 --> 00:23:51,440 play when the charges are moving. 380 00:23:51,440 --> 00:23:54,119 So if we're bouncing around, we're 381 00:23:54,119 --> 00:23:55,945 moving some of these charged particles 382 00:23:55,945 --> 00:24:00,290 that have an electromagnetic force that's acting. 383 00:24:00,290 --> 00:24:04,246 We can make the connection that a photon of light 384 00:24:04,246 --> 00:24:07,084 could come out of that interaction. 385 00:24:10,880 --> 00:24:14,370 So we're going to find out that there's 386 00:24:14,370 --> 00:24:21,790 two general ways that you can produce this light. 387 00:24:21,790 --> 00:24:26,260 You can either produce it, we're going to say, 388 00:24:26,260 --> 00:24:29,072 thermal light production. 389 00:24:34,964 --> 00:24:37,046 And if we're talking about thermal, 390 00:24:37,046 --> 00:24:39,890 that means it has something to do with temperature. 391 00:24:39,890 --> 00:24:42,971 What do you see over there when we saw the higher temperature? 392 00:24:42,971 --> 00:24:45,857 What happens to the particles? 393 00:24:45,857 --> 00:24:47,781 AUDIENCE: They sped up. 394 00:24:47,781 --> 00:24:48,743 PROFESSOR: Sped up. 395 00:24:48,743 --> 00:24:49,690 What else? 396 00:24:49,690 --> 00:24:51,148 AUDIENCE: They had more collisions. 397 00:24:51,148 --> 00:24:52,647 PROFESSOR: They had more collisions, 398 00:24:52,647 --> 00:24:54,980 they had more bounces. 399 00:24:54,980 --> 00:24:57,150 So thermal light production happens 400 00:24:57,150 --> 00:25:03,290 when particles bounce because of their thermal motion. 401 00:25:03,290 --> 00:25:14,529 So thermal light production happens when particles bounce 402 00:25:14,529 --> 00:25:22,000 because of thermal motion. 403 00:25:22,000 --> 00:25:24,790 Now thermal motion, what does that mean? 404 00:25:24,790 --> 00:25:28,050 That's just the motion of every particle, 405 00:25:28,050 --> 00:25:31,360 like we saw in that simulation. 406 00:25:31,360 --> 00:25:34,800 And the bouncing, if you have a higher temperature, 407 00:25:34,800 --> 00:25:38,350 you have things moving at a faster speed, 408 00:25:38,350 --> 00:25:41,234 and you can have more thermal motion. 409 00:25:41,234 --> 00:25:44,630 So these are the particle bounce because of thermal motion. 410 00:25:44,630 --> 00:25:46,025 Its bounce [INAUDIBLE] 411 00:25:46,025 --> 00:25:48,250 And again, just to make it clear, 412 00:25:48,250 --> 00:25:49,748 let's define temperature. 413 00:25:54,194 --> 00:26:02,592 Temperature is going to be a measurement 414 00:26:02,592 --> 00:26:14,448 of the average kinetic energy of moving particles. 415 00:26:18,251 --> 00:26:20,376 And some of you may have heard about kinetic energy 416 00:26:20,376 --> 00:26:22,380 and some of you may not have. 417 00:26:22,380 --> 00:26:24,950 But kinetic energy is related to how fast 418 00:26:24,950 --> 00:26:26,460 the particles are going. 419 00:26:26,460 --> 00:26:29,410 So we said before, if we had a low temperature 420 00:26:29,410 --> 00:26:32,045 you had a low average speed. 421 00:26:32,045 --> 00:26:33,670 There was some that were moving faster, 422 00:26:33,670 --> 00:26:35,336 there were some that were moving slower. 423 00:26:35,336 --> 00:26:37,460 But on average, they were moving slower. 424 00:26:37,460 --> 00:26:40,155 At a higher temperature, we saw that the particles 425 00:26:40,155 --> 00:26:41,286 moved faster. 426 00:26:41,286 --> 00:26:43,880 They have a higher average speed, or higher 427 00:26:43,880 --> 00:26:45,944 average kinetic energy. 428 00:26:45,944 --> 00:26:49,390 So temperature is a measurement of how fast they're moving, 429 00:26:49,390 --> 00:26:52,360 or how much energy they're moving. 430 00:26:52,360 --> 00:26:56,670 We can also have something called 431 00:26:56,670 --> 00:27:02,550 nonthermal light production. 432 00:27:08,812 --> 00:27:11,020 I'm oversimplifying this for us because it's actually 433 00:27:11,020 --> 00:27:12,370 a little bit more complex. 434 00:27:12,370 --> 00:27:14,250 But as I say, nonthermal light production 435 00:27:14,250 --> 00:27:17,288 is anything else, anything else that 436 00:27:17,288 --> 00:27:19,350 doesn't have to do with the bouncing because 437 00:27:19,350 --> 00:27:21,150 of the thermal motion. 438 00:27:21,150 --> 00:27:23,900 And I'll give you an example.