1 00:00:00,000 --> 00:00:05,473 we've discussed that General history of the solar system. 2 00:00:05,473 --> 00:00:10,661 And we're going to start now a process of trying to look at some of the outcomes in 3 00:00:10,661 --> 00:00:14,429 what we find in it. Today and a natural place to start is the 4 00:00:14,429 --> 00:00:17,764 planet we know best, the little blue marble we live on. 5 00:00:17,764 --> 00:00:22,520 Pedagogically, this makes sense because it's going to be interesting to see what 6 00:00:22,520 --> 00:00:26,920 the physics we understand and the, the natural history of the solar system 7 00:00:26,920 --> 00:00:29,910 that we've learned. Tell us about living on Earth. 8 00:00:29,910 --> 00:00:34,396 Scientifically, intellectually, it makes sense because we know more about Earth 9 00:00:34,396 --> 00:00:38,422 than about any other planet. Whatever theories we develop should first 10 00:00:38,422 --> 00:00:42,332 be tested where we can actually measure things, which is on Earth. 11 00:00:42,332 --> 00:00:44,690 So let's do a quick pass. Of course, 12 00:00:44,690 --> 00:00:49,176 Earth science is a huge field that I wouldn't be qualified to teach, even if I 13 00:00:49,176 --> 00:00:52,339 had the time. But let's do a quick pass and see what we 14 00:00:52,339 --> 00:00:56,751 can say about Earth as a planet. So first, first thing you see when you 15 00:00:56,751 --> 00:01:01,757 look at Earth, is that most of its surface is covered with liquid water, as 16 00:01:01,757 --> 00:01:04,802 we know that means that it has an atmosphere. 17 00:01:04,802 --> 00:01:10,282 Because liquid water can't exist without it, and indeed the earth is covered with 18 00:01:10,282 --> 00:01:15,424 an atmosphere. mostly nitrogen with about twenty percent molecular oxygen. 19 00:01:15,424 --> 00:01:20,972 and then below both of those is a rocky surface, a crust that is made mostly of 20 00:01:20,972 --> 00:01:23,819 silicates. Now, we don't expect this to be a good 21 00:01:23,819 --> 00:01:28,065 description of the composition of the entire Earth, because the Earth melted. 22 00:01:28,065 --> 00:01:30,580 we see that it is round because it melted. 23 00:01:30,580 --> 00:01:33,317 This means it underwent chemical differentiation. 24 00:01:33,317 --> 00:01:37,675 We expect the inside of the Earth to be different chemically and structurally 25 00:01:37,675 --> 00:01:42,127 than the outside. And indeed we find that the earth has an 26 00:01:42,127 --> 00:01:48,465 inner core which is rich in iron and nickel and heavier materials and then the 27 00:01:48,465 --> 00:01:53,920 silicates that make up the crust of the earth also make up an 28 00:01:53,920 --> 00:01:57,673 the, the layer below the crust, which is called the mantle. 29 00:01:57,673 --> 00:02:02,942 And so we have the very densest, heaviest elements having sunk differentially to 30 00:02:02,942 --> 00:02:06,629 the center and formed a core and outside that mantle. 31 00:02:06,629 --> 00:02:10,712 Now, this gives the Earth an average density of 5500 kilos per meter3. 32 00:02:10,712 --> 00:02:13,742 cube. remembering that rock is about 3,000 33 00:02:13,742 --> 00:02:18,286 kilos per meter3, cube, this is your first indication that there's something 34 00:02:18,286 --> 00:02:22,483 denser, underlying the Earth. The earth is an equilibrium system. 35 00:02:22,483 --> 00:02:25,911 Gravity, as we remember, is trying to crunch it down. 36 00:02:25,911 --> 00:02:31,290 The earth is at least, mostly a fluid. It is not a rigid object held up by its 37 00:02:31,290 --> 00:02:35,122 tensile strength. The reason, that earth does not collapse 38 00:02:35,122 --> 00:02:40,769 under the force of gravity it, just like our slinky, is that pressure increases as 39 00:02:40,769 --> 00:02:46,349 you go deeper down into the earth, each layer holding the layer above it against 40 00:02:46,349 --> 00:02:50,581 the, the force of gravity. And so, as you go down into the earth, 41 00:02:50,581 --> 00:02:54,857 pressure has become very, very high, density's become very high. 42 00:02:54,857 --> 00:03:00,994 we have density so, pressure so high at the center of the earth, that despite the 43 00:03:00,994 --> 00:03:06,799 fact that as we'll see temperatures are very high, The center of the earth is 44 00:03:06,799 --> 00:03:10,365 essentially solid metal. that's the inner core. 45 00:03:10,365 --> 00:03:13,810 This is surrounded by an outer core of liquid metal. 46 00:03:13,810 --> 00:03:18,446 And you can see on the graph at the bottom of the slide, that jumps in 47 00:03:18,446 --> 00:03:21,625 density as a function of distance from the core. 48 00:03:21,625 --> 00:03:26,858 distance from the center, increases to the right, and density is the vertical 49 00:03:26,858 --> 00:03:29,838 axis. And you see the, the different layers of 50 00:03:29,838 --> 00:03:34,607 the Earth, an inner solid core, a slightly less dense liquid outer core, a 51 00:03:34,607 --> 00:03:39,840 mantle that is essentially molten rock. And floating above all of it a crust. 52 00:03:39,840 --> 00:03:43,092 How do we know so much about the internal structure of the Earth? 53 00:03:43,092 --> 00:03:46,245 Nobody's ever been there. Well, it turns out that we can learn a 54 00:03:46,245 --> 00:03:50,549 lot about the mechanics and the structure of something by the propagation of sound 55 00:03:50,549 --> 00:03:52,751 waves. Earthquakes generate sound waves that 56 00:03:52,751 --> 00:03:54,952 propagate through the interior of the Earth. 57 00:03:54,952 --> 00:03:57,955 We can measure them way over on the other side of the Earth. 58 00:03:57,955 --> 00:04:01,508 Studying the time delays and the intensity patterns with which they 59 00:04:01,508 --> 00:04:05,781 propagate teaches us a lot and in. Particular indicates the size and mass 60 00:04:05,781 --> 00:04:10,494 fraction of the inner core which is solid and the outer core which is liquid 61 00:04:10,494 --> 00:04:14,780 because they have different properties as regard sound propagation. 62 00:04:14,780 --> 00:04:19,646 So within the understand the inside of the earth and one of the things that we 63 00:04:19,646 --> 00:04:24,389 know is that not only pressure and density increases you move deeper into 64 00:04:24,389 --> 00:04:29,132 the earth but temperature the inside of the earth is an extremely hard place, 65 00:04:29,132 --> 00:04:33,814 even the mantle is very hard molten rock and so what generates this heat. 66 00:04:33,814 --> 00:04:36,340 Well, there's Kelvin Helmholtz sheet that. 67 00:04:36,340 --> 00:04:41,347 Heated the earth as it was forming, remember, because gravitational potential 68 00:04:41,347 --> 00:04:46,618 energy was being converted into, heat. But, it turns out that the dominate heat 69 00:04:46,618 --> 00:04:50,440 producing process of the earth today is radioactive decay. 70 00:04:50,440 --> 00:04:55,447 The heavy elements, which are the ones prone to radioactive decay, would have 71 00:04:55,447 --> 00:04:58,863 sunk under chemical differentiation and you're 72 00:04:58,863 --> 00:05:03,210 concentrated in the core. So the core is generating heat and this 73 00:05:03,210 --> 00:05:08,261 heat is carried up through the manto by convective process is we have here the 74 00:05:08,261 --> 00:05:11,329 process that we said we might discover some day. 75 00:05:11,329 --> 00:05:16,316 Here we have a fluid the manto heated from below by the harder core by the 76 00:05:16,316 --> 00:05:21,622 radio active and to some extend still Kat Von Helmholtz process is going on in the 77 00:05:21,622 --> 00:05:24,981 core and so. This causes warm fluid to rise in some 78 00:05:24,981 --> 00:05:29,832 places, cooler fluid to sink in other places, and sets up these convection 79 00:05:29,832 --> 00:05:34,084 cells in which the fluid circulates. This has several effects. 80 00:05:34,084 --> 00:05:38,403 One is it carries the heat out from the core, heating the surface. 81 00:05:38,403 --> 00:05:43,852 The surface looses the heat to radiation. Every square meter of earth looses about 82 00:05:43,852 --> 00:05:47,669 87 watts of. about the amount of energy is takes to 83 00:05:47,669 --> 00:05:52,269 run an average light bulb to radiation, because of inner heating. 84 00:05:52,269 --> 00:05:57,157 80% of that is radioactive. the other effect, of course, is that the 85 00:05:57,157 --> 00:06:02,836 horizontal component of the flow at the top of the [INAUDIBLE] drags the crust 86 00:06:02,836 --> 00:06:06,430 around. Whereas, crust is broken into plates, and 87 00:06:06,430 --> 00:06:10,600 the motion of these plates tell us a lot about the nature. 88 00:06:10,600 --> 00:06:15,460 Of geological processes on earth, the fact that earth has this 89 00:06:15,460 --> 00:06:20,519 Plated crusts and that these crusts are moving leads to recognizable features. 90 00:06:20,519 --> 00:06:24,605 For example the mountain ridges. The linear ridges of mountains. 91 00:06:24,605 --> 00:06:29,599 The Rockies and the Andes that line plate boundaries where plates are coming 92 00:06:29,599 --> 00:06:34,334 together and one of them gets, pushed up. Or the Alps and the Himalayas. 93 00:06:34,334 --> 00:06:39,393 Uh,, these are linear mountain structures that reflect the fact that plates are 94 00:06:39,393 --> 00:06:43,868 being crunched together. on the other side when two plates come 95 00:06:43,868 --> 00:06:46,722 together one of them will typically fold up. 96 00:06:46,722 --> 00:06:50,541 The other will Fold under, and crust will be subducted. 97 00:06:50,541 --> 00:06:55,911 And the result of this plate techtonic process is that the Earth's crust is by 98 00:06:55,911 --> 00:07:00,058 and large very young. This is why we never found old rocks on 99 00:07:00,058 --> 00:07:03,309 Earth Not only our features on Earth, mountains 100 00:07:03,309 --> 00:07:09,039 and valleys subject to erosion by air and water and natural processes, but the very 101 00:07:09,039 --> 00:07:14,125 atoms that are formed across are in a geologic sense very young and constantly 102 00:07:14,125 --> 00:07:17,538 being interchanged. Crust is constantly subducted and 103 00:07:17,538 --> 00:07:21,851 reabsorbed into the mantle. And then new crust is constantly formed 104 00:07:21,851 --> 00:07:25,907 by volcanic processes. So that most of the atoms we find on the 105 00:07:25,907 --> 00:07:30,350 Earth's crust today are only a few hundred millions years old, and 106 00:07:30,350 --> 00:07:32,859 By, geologic standards that's quite young. 107 00:07:32,859 --> 00:07:37,572 So, we have heat rising from the bottom and then heat being radiated out into 108 00:07:37,572 --> 00:07:41,183 space and, this is the beginnings of earth's energy balance. 109 00:07:41,183 --> 00:07:46,080 Of course if earth relied on internal heat it would have been a very cold place 110 00:07:46,080 --> 00:07:49,752 by now, it's been around losing energy for billions of years. 111 00:07:49,752 --> 00:07:53,032 What keeps earth from freezing. Is the sunlight. 112 00:07:53,032 --> 00:08:00,029 So we can set up understanding, and her's a nice application of the physics we 113 00:08:00,029 --> 00:08:03,292 learned. We can set up an understanding of what 114 00:08:03,292 --> 00:08:07,798 maintains the constant temperature of Earth by setting up a sort of balance 115 00:08:07,798 --> 00:08:10,526 problem. Earth absorbs energy from sunlight as 116 00:08:10,526 --> 00:08:14,617 radiation with this additional small contribution from internal heat. 117 00:08:14,617 --> 00:08:19,242 But if you remember that every square meter of Earth absorbs about 1300 Watts 118 00:08:19,242 --> 00:08:22,444 in sunlight. The 100 Watts in internal energy is not a 119 00:08:22,444 --> 00:08:25,231 big deal. At the same time, Earth is losing heat 120 00:08:25,231 --> 00:08:29,856 because it's a dense object sitting in space, it has a temperature, it radiates 121 00:08:29,856 --> 00:08:34,535 a black body spectrum out into space. If Earth is too hot, if Earth gets warm, 122 00:08:34,535 --> 00:08:37,912 it radiates more because of the Stefan Boltzmann law. 123 00:08:37,912 --> 00:08:41,030 And that cools it down until. The rates are equal. 124 00:08:41,030 --> 00:08:44,017 If Earth gets to cool, if [INAUDIBLE] decreases. 125 00:08:44,017 --> 00:08:47,640 And then, that causes the sunlight to continue to heat it. 126 00:08:47,640 --> 00:08:50,309 In equilibrium, this is stable equilibrium. 127 00:08:50,309 --> 00:08:55,583 And one expects the rate at which earth absorbs energy, and the rate at which it 128 00:08:55,583 --> 00:08:59,650 loses energy to be equal. And you can use that to figure out how 129 00:08:59,650 --> 00:09:02,700 warm earth should be. So earth is a dense object. 130 00:09:02,700 --> 00:09:06,831 As a first approximation, let's imagine the earth as a black body. 131 00:09:06,831 --> 00:09:09,310 How does it work? Well, here's the Earth. 132 00:09:09,310 --> 00:09:13,830 It's a sphere of radius r and it's orbiting here in space. 133 00:09:13,830 --> 00:09:20,203 And if I imagine that the sun is over to the left then there is a flux of sunlight 134 00:09:20,203 --> 00:09:25,613 impinging on the earth from the left. And this flux is given by the solar 135 00:09:25,613 --> 00:09:31,320 constant which is related to the solar luminosity by our energy conservation 136 00:09:31,320 --> 00:09:36,989 relation that says the solar flux. Is distributed uniformly over a sphere 137 00:09:36,989 --> 00:09:41,027 whose radius is the distance from Earth to sun. 138 00:09:41,027 --> 00:09:44,500 So D here stands for one astronomical unit. 139 00:09:44,500 --> 00:09:47,195 Now how much of this light does the Earth absorb? 140 00:09:47,195 --> 00:09:51,541 Well, it's clear that only half of the Earth at any given time is absorbing any 141 00:09:51,541 --> 00:09:55,668 light at all, because the right half of the Earth is not absorbing anything. 142 00:09:55,668 --> 00:10:00,124 But, it's also true that radiation is not uniformly absorbed, depends on where you 143 00:10:00,124 --> 00:10:02,710 are on Earth in this equinoctial configuration. 144 00:10:02,710 --> 00:10:07,552 Light is absorbed most efficiently at the equator, and this is easiest to 145 00:10:07,552 --> 00:10:12,594 understand by realizing that if I replace the earth by this disc, which sits 146 00:10:12,594 --> 00:10:17,835 perpendicular to the direction to the sun, and has a radius equal to the radius 147 00:10:17,835 --> 00:10:22,954 of the earth, then this disc would, May produce the exact same shadow over to 148 00:10:22,954 --> 00:10:25,808 the right, as does the ball that is the Earth. 149 00:10:25,808 --> 00:10:30,819 Basically, if I chop off everything but the central disc, then I am not changing 150 00:10:30,819 --> 00:10:33,927 the shadow. And so effectively as an absorber, the 151 00:10:33,927 --> 00:10:38,811 absorbent area of the Earth is the same as the absorbent area of this, as the 152 00:10:38,811 --> 00:10:42,364 actual geometric area of this disc, which is just u03c0r^2. 153 00:10:42,364 --> 00:10:47,502 And remember, that the, the full surface of the Earth is 4u03c0r^2 but it absorbs 154 00:10:47,502 --> 00:10:51,878 as though its area were u03c0r^2. And so, now I have the area, I have the 155 00:10:51,878 --> 00:10:55,474 incoming Power per unit area I can f- figure out 156 00:10:55,474 --> 00:11:01,073 the total incoming flux by multiply them and the total rate at which the Earth 157 00:11:01,073 --> 00:11:04,900 absorbs energy from the sun is therefore pi R squared. 158 00:11:04,900 --> 00:11:09,603 Times. The solar luminosity divided by four Pi 159 00:11:09,603 --> 00:11:12,874 times the square of the distance to the sun. 160 00:11:12,874 --> 00:11:18,450 I can improve upon that calculation because the Stefan Boltzmann law tells 161 00:11:18,450 --> 00:11:24,398 me, that the solar luminosity is equal to four Pi times the radius of the sun 162 00:11:24,398 --> 00:11:30,048 squared, that's the surface area with which the sun radiates times sigma, the 163 00:11:30,048 --> 00:11:35,550 Stefan Boltzmann constant, times the fourth power of the solar temperature. 164 00:11:35,550 --> 00:11:41,695 So I can plug this into that. And I obtain when I put this expression 165 00:11:41,695 --> 00:11:43,210 into there. I get. 166 00:11:43,210 --> 00:11:49,879 U03c0*the radius of Earth^2*4u03c0s cancel all over the place* u03c3 fourth 167 00:11:49,879 --> 00:11:58,516 power of the solar temperature, and then I get the radius of the Sun^2, divided by 168 00:11:58,516 --> 00:12:03,163 the distance to the Sun^2. So this is my expression. 169 00:12:03,163 --> 00:12:07,210 I will rewrite it more neatly, but this gives me a. 170 00:12:07,210 --> 00:12:12,478 [INAUDIBLE] expression for the total rate at which the earth receives sunlight. 171 00:12:12,478 --> 00:12:17,064 Some pi's have been cancelled, and I've rearranged the factor slightly, but 172 00:12:17,064 --> 00:12:22,332 notice that this is, depends on both the sun size and temperature and our distance 173 00:12:22,332 --> 00:12:25,307 from the sun as well as the radius of the earth. 174 00:12:25,307 --> 00:12:30,307 Okay, this is the rate at which earth would absorb energy, how about the rate 175 00:12:30,307 --> 00:12:35,460 at which it looses energy, well that too is not very difficult to realize. 176 00:12:35,460 --> 00:12:39,907 the earth is a black body. It has a temperature T(f) and if the 177 00:12:39,907 --> 00:12:45,555 earth is at some temperature then each square meter of earth radiates according 178 00:12:45,555 --> 00:12:51,132 to the Stefanu2013Boltzmann law at the rate sigma T to the fourth in the space 179 00:12:51,132 --> 00:12:56,214 multiply that by the radiating worth which is the full surface of earth. 180 00:12:56,214 --> 00:13:01,580 Since earth radiates in all directions and that will give you an expression. 181 00:13:01,580 --> 00:13:06,194 For the rate at which Earth loses energy by radiating it out to space. 182 00:13:06,194 --> 00:13:10,874 And so again, re-writing that more cleanly we have now an expression. 183 00:13:10,874 --> 00:13:15,950 And thermodynamic equilibrium will determine the Earth's temperature in such 184 00:13:15,950 --> 00:13:18,719 a way that these two quantities are equal. 185 00:13:18,719 --> 00:13:23,597 So my job is to equate this to that. That will give me an equation for the 186 00:13:23,597 --> 00:13:28,278 surface temperature of Earth in terms of properties of everything else. 187 00:13:28,278 --> 00:13:31,980 And let's see. Well, we have some cancellations here. 188 00:13:31,980 --> 00:13:36,173 Fortunately the sigmas cancel. That's good, they don't need to remember 189 00:13:36,173 --> 00:13:38,210 the value of sigma. The pis cancel. 190 00:13:38,210 --> 00:13:41,313 Most importantly the radius of earth cancels. 191 00:13:41,313 --> 00:13:44,278 The size of earth is completely irrelevant. 192 00:13:44,278 --> 00:13:49,518 Any object asteroid, Jupiter, Earth. Whatever you put at Earth's distance from 193 00:13:49,518 --> 00:13:55,311 the sun will have the same equilibrium temperature and rearranging things I find 194 00:13:55,311 --> 00:14:00,827 an expression that says the temperature of the earth to the fourth power is the 195 00:14:00,827 --> 00:14:04,000 solar temperature to the fourth power times... 196 00:14:04,000 --> 00:14:09,362 This ratio, the radius of the sun divided by the distance to the sun and then I 197 00:14:09,362 --> 00:14:14,202 want to square this so that four will turn into two when I break inside the 198 00:14:14,202 --> 00:14:19,238 square and this tells you no I computed for earth and sun than any object in 199 00:14:19,238 --> 00:14:24,405 equilibrium with the radiation of any star if you know the temperature and the 200 00:14:24,405 --> 00:14:29,637 size of the star and your distance from the star you can compute in equilibrium 201 00:14:29,637 --> 00:14:32,972 temperature. So now I can apply this to earth and I 202 00:14:32,972 --> 00:14:35,760 find. 278 degrees Calvin. 203 00:14:35,760 --> 00:14:40,623 This is eight degrees Celsius. It's pretty chilly so maybe some of our 204 00:14:40,623 --> 00:14:45,102 approximations need to be looked at. But it's not outside the ballpark. 205 00:14:45,102 --> 00:14:48,238 So roughly earth is in equilibrium with sunlight. 206 00:14:48,238 --> 00:14:51,206 This is good. Let's refine our calculation a little 207 00:14:51,206 --> 00:14:55,592 bit, and the first thing you notice is that Earth is not black of course, Earth 208 00:14:55,592 --> 00:14:58,235 is blue. It's blue because it reflects sunlight. 209 00:14:58,235 --> 00:15:02,564 If Earth absorbed all of the sunlight, the only radiation it emitted would be 210 00:15:02,564 --> 00:15:07,006 infrared, and to our eyes it would appear black, not the beautiful blue color it 211 00:15:07,006 --> 00:15:09,705 appears. In fact, Earth reflects about a third of 212 00:15:09,705 --> 00:15:13,753 the light that hits it, this is characteristic of any astronomical 213 00:15:13,753 --> 00:15:16,340 object. We call it the albedo, and we donate it 214 00:15:16,340 --> 00:15:19,375 by the letter A. What this means though is that about a 215 00:15:19,375 --> 00:15:22,112 third of the sunlight never goes to warming Earth. 216 00:15:22,112 --> 00:15:25,944 It just bounces straight back out into space making us look beautiful. 217 00:15:25,944 --> 00:15:30,433 And what that means is that only about two thirds, a fraction of one minus A, of 218 00:15:30,433 --> 00:15:33,390 the incoming radiation is available to warm the Earth. 219 00:15:33,390 --> 00:15:36,796 This means the Earth needs to be cooler, to be in equilibrium. 220 00:15:36,796 --> 00:15:41,430 plugging that back into our equation, we find that we get the same expression as 221 00:15:41,430 --> 00:15:45,171 before, for the Earth's equilibrium temperature, except when we take 222 00:15:45,171 --> 00:15:49,415 everything to the one-fourth power. we find that this factor of 1-A, about 223 00:15:49,415 --> 00:15:52,040 two-thirds to the power of one-fourth comes up. 224 00:15:52,040 --> 00:15:56,486 This further lowers the expected temperature and given the reflected light 225 00:15:56,486 --> 00:16:01,229 you'd expect the earth surface temperate to average out to a chilly 248 degrees 226 00:16:01,229 --> 00:16:04,016 kelvin. This is way below the freezing point of 227 00:16:04,016 --> 00:16:06,981 water. if this were the temperature of earth, 228 00:16:06,981 --> 00:16:11,746 life on earth would be very different at least than it is now, of course There's 229 00:16:11,746 --> 00:16:16,832 another thing we've neglected in our calculation and this is the effect of the 230 00:16:16,832 --> 00:16:19,623 atmosphere. Or the infamous greenhouse effect. 231 00:16:19,623 --> 00:16:23,520 So what does this mean? Again we can do the physics so lets try 232 00:16:23,520 --> 00:16:27,659 to look at what it means. the nature of the greenhouse effect is 233 00:16:27,659 --> 00:16:32,416 that the incoming light, the sun at 3,000 degrees kelvin produces most of its 234 00:16:32,416 --> 00:16:36,546 energy in visible light and this Penetrates the atmosphere, we talked 235 00:16:36,546 --> 00:16:40,254 about visible light penetrating the atmosphere, and is absorbed by the 236 00:16:40,254 --> 00:16:44,124 ground, heating up the ground. The ground in turn radiates mostly 237 00:16:44,124 --> 00:16:48,260 infra-red light because the ground is at 300 kelvin, not 3,000. 238 00:16:48,260 --> 00:16:53,536 And this infrared light we with saw when we talked about the optical properties of 239 00:16:53,536 --> 00:16:57,093 the atmosphere is a large extend absorbed by the atmosphere. 240 00:16:57,093 --> 00:17:01,598 Well this heats the atmosphere and the atmosphere then radiates because the 241 00:17:01,598 --> 00:17:06,282 atmosphere is one by some of the heat that the atmosphere radiates goes out in 242 00:17:06,282 --> 00:17:09,068 the space. Some of the heat is re-radiated back 243 00:17:09,068 --> 00:17:13,810 towards the surface moving the surface the net result is that the surface is 244 00:17:13,810 --> 00:17:18,494 warmer then would be a black body that lost as much energy the spaces does the 245 00:17:18,494 --> 00:17:20,805 earth. This might sound, or should sound 246 00:17:20,805 --> 00:17:24,183 confusing to you. I know it confuses the daylights out of 247 00:17:24,183 --> 00:17:26,553 me. But what usually helps me is to do a 248 00:17:26,553 --> 00:17:29,693 calculation. So let's try to do a calculation and see 249 00:17:29,693 --> 00:17:33,190 if physics explains to us how this greenhouse effect works. 250 00:17:33,190 --> 00:17:37,658 So lets do a simple model, a simple model will have two components. 251 00:17:37,658 --> 00:17:42,397 It'll have an atmosphere that is transparent to visible radiation but 252 00:17:42,397 --> 00:17:47,204 absorbs a fraction G, I'll call it the green house fraction of infrared 253 00:17:47,204 --> 00:17:52,621 radiation that hits it and then will have a surface and so here's how my simple 254 00:17:52,621 --> 00:17:55,938 model looks. I have here the surface of earth and 255 00:17:55,938 --> 00:17:59,730 above it decoupled from it other than through radiation. 256 00:17:59,730 --> 00:18:04,533 I'm going to imagine, is something called the atmosphere, and between some 257 00:18:04,533 --> 00:18:08,284 transparent medium. So, of course, this is an approximation, 258 00:18:08,284 --> 00:18:11,641 but it will give us a ballpark of what is going on. 259 00:18:11,641 --> 00:18:16,576 And then, sunlight, visible sunlight, which I'll draw in blue, is impinging at 260 00:18:16,576 --> 00:18:21,248 some rate for every square meter something like a quarter of the solar 261 00:18:21,248 --> 00:18:24,802 constant effectively, is impinging upon the atmosphere. 262 00:18:24,802 --> 00:18:29,080 The quarter common of the geometric factors that we talked about. 263 00:18:29,080 --> 00:18:31,940 Now, the atmosphere is transparent to this. 264 00:18:31,940 --> 00:18:35,208 So this light goes right through the atmosphere. 265 00:18:35,208 --> 00:18:38,204 And, indeed, some fraction of it is reflected. 266 00:18:38,204 --> 00:18:43,720 Some of it might bounce off the clouds. So there's some contribution by clouds to 267 00:18:43,720 --> 00:18:47,193 the albedo. Some contribution by the ground and the 268 00:18:47,193 --> 00:18:51,347 oceans and ice caps. And the visible reflected radiation goes 269 00:18:51,347 --> 00:18:53,390 off into space. The net result. 270 00:18:53,390 --> 00:18:58,422 Is that there is a flux which I will call fn which is something we computed that 271 00:18:58,422 --> 00:19:03,454 amount per the, the, the power per square meter of visible light that comes in and 272 00:19:03,454 --> 00:19:06,250 is absorbed by the ground heating the ground. 273 00:19:06,250 --> 00:19:10,810 So far the atmosphere has played no essential role but here things become 274 00:19:10,810 --> 00:19:13,892 interesting. Now the ground of course being a warm 275 00:19:13,892 --> 00:19:17,960 object radiates that light up as infrared radiation so there's 276 00:19:17,960 --> 00:19:25,846 Fact power sigma, T ground to the fourth will give the ground to temperature of T 277 00:19:25,846 --> 00:19:29,680 ground. Lets give it a temperature of t ground 278 00:19:29,680 --> 00:19:35,406 and we will give the atmospherial temperature t atmosphere which need not 279 00:19:35,406 --> 00:19:41,430 be the same and the ground will radiate at the rate that different Boltsman tell 280 00:19:41,430 --> 00:19:47,157 it to radiate and now the news is that this doesn't go into space it pinches 281 00:19:47,157 --> 00:19:53,255 upon the atmosphere where a fraction G of this radiation is actually absorbed and 282 00:19:53,255 --> 00:19:58,160 goes to heating the atmosphere whereas the fraction 1-G. 283 00:19:58,160 --> 00:20:03,790 Goes out into space. So far so good, but now we have a warm 284 00:20:03,790 --> 00:20:08,925 atmosphere, at a temperature ta, and this too radiates we'll imagine that it too 285 00:20:08,925 --> 00:20:13,930 radiates like a black body, so there is a flux of sigma times the atmospheric 286 00:20:13,930 --> 00:20:17,685 temperature to the fourth. In every direction, emitted by the 287 00:20:17,685 --> 00:20:22,240 atmosphere, this layer, this imaginary layer that I've called the atmosphere. 288 00:20:22,240 --> 00:20:26,977 And of course, this gets lost to space. The infrared light that's directed down, 289 00:20:26,977 --> 00:20:31,411 is directed in all directions, but eventually what's directed up, goes out 290 00:20:31,411 --> 00:20:34,630 to space. What's directed down will eventually hit 291 00:20:34,630 --> 00:20:39,282 the ground and be absorbed. Because the ground is absorbent of both 292 00:20:39,282 --> 00:20:42,370 infrared. And the visible radiation, and now, what 293 00:20:42,370 --> 00:20:45,893 do we need to do? This is our picture, and what we need to 294 00:20:45,893 --> 00:20:50,281 do, is use the fact that both the temperature of the surface and of the 295 00:20:50,281 --> 00:20:55,040 atmosphere are going to be constant. To find equilibrium conditions, we can do 296 00:20:55,040 --> 00:20:59,119 some elaborate Physics here and I, I consider this a good thing. 297 00:20:59,119 --> 00:21:02,962 And so what do we have? Well We have equilibrium conditions for 298 00:21:02,962 --> 00:21:05,738 the surface and for the atmosphere separately. 299 00:21:05,738 --> 00:21:09,722 For each of them, the incoming, power must equal the outgoing power. 300 00:21:09,722 --> 00:21:13,765 So for example for the ground the incoming power has two pieces. 301 00:21:13,765 --> 00:21:18,473 It has incoming, visible radiation from the sun at a rate Fn which we computed 302 00:21:18,473 --> 00:21:22,758 before, plus the infrared light it absorbs from the atmosphere, and that 303 00:21:22,758 --> 00:21:27,285 must equal the rate at which the ground radiates, and for the atmosphere, it 304 00:21:27,285 --> 00:21:31,842 loses at a rate energy both to the. To space and back down to the ground. 305 00:21:31,842 --> 00:21:35,163 And thus it must equal the rate at which it absorbs energy. 306 00:21:35,163 --> 00:21:39,271 So we have two sets of equations. I'll erase the model, so that we can see 307 00:21:39,271 --> 00:21:43,214 them more clearly in a minute. But this is the equilibrium condition 308 00:21:43,214 --> 00:21:48,019 that says that what the ground emits is equal to the sum of the two sources from 309 00:21:48,019 --> 00:21:51,459 which it absorbs. And this is the equation which says that 310 00:21:51,459 --> 00:21:55,790 what the atmosphere emits in both directions is equal to what it absorbs. 311 00:21:55,790 --> 00:21:59,824 Let's clear out this nonsense. And now we have just two equations to 312 00:21:59,824 --> 00:22:02,196 solve. And these will determine both the 313 00:22:02,196 --> 00:22:05,696 temperature of the atmosphere and the temperature of earth. 314 00:22:05,696 --> 00:22:08,759 And so The first thing I will do is I will move 315 00:22:08,759 --> 00:22:12,374 this two over here. And now I recognize that I can take this 316 00:22:12,374 --> 00:22:16,213 sigma t a to the fourth. Plug it in for that sigma T to the 317 00:22:16,213 --> 00:22:20,795 fourth, get an equation that has nothing about the atmosphere in it. 318 00:22:20,795 --> 00:22:24,898 And it's all about the temperature of the, of the surface. 319 00:22:24,898 --> 00:22:30,095 And it will be sigma T surface to the fourth is equal to G over two, sigma T 320 00:22:30,095 --> 00:22:33,430 surface to the fourth. Plus F N. 321 00:22:33,430 --> 00:22:37,479 And then moving that over to the left I'll get an equation for the temperature 322 00:22:37,479 --> 00:22:41,374 of the surface to the fourth and then the other thing I realize is that the 323 00:22:41,374 --> 00:22:45,372 temperature of the atmosphere to the fourth is just g over two temperature of 324 00:22:45,372 --> 00:22:49,043 surface to the fourth. Since G is a number smaller than one it's 325 00:22:49,043 --> 00:22:52,254 a fraction. this tells me the atmosphere is cooler 326 00:22:52,254 --> 00:22:54,616 than Earth. This is how the model works. 327 00:22:54,616 --> 00:22:59,461 Basically some of the radiating part that space sees is the atmosphere which is 328 00:22:59,461 --> 00:23:03,398 cooler than the surface. So it doesn't lose as much as if it were 329 00:23:03,398 --> 00:23:06,972 a black body at the atmospheric, at the surface temperature. 330 00:23:06,972 --> 00:23:11,999 And indeed, solving the equation for the temperature of the surface we find this 331 00:23:11,999 --> 00:23:15,294 equation which is. Essentially the equation we found before 332 00:23:15,294 --> 00:23:19,756 that sigma to the fourth is equal to the flux with this little factor of one minus 333 00:23:19,756 --> 00:23:23,412 G over two in front of it. And this means the earth has to be warmer 334 00:23:23,412 --> 00:23:27,929 because this is a number smaller than one then it would otherwise by precisely this 335 00:23:27,929 --> 00:23:30,509 factor. The temperature you find with greenhouse 336 00:23:30,509 --> 00:23:34,757 is bigger than the temperature you would find without it by this factor of one 337 00:23:34,757 --> 00:23:38,843 minus over two, number smaller than one raised to the minus one fourth power. 338 00:23:38,843 --> 00:23:43,280 So that raises the temperature. This is how the greenhouse effect 339 00:23:43,280 --> 00:23:47,317 Causes the surface to be hotter. I don't know if that helped you but it 340 00:23:47,317 --> 00:23:51,467 certainly clarifies things for me. That's a simple model in truth there's 341 00:23:51,467 --> 00:23:55,334 all kind of complications and different mechanisms of heat transfer. 342 00:23:55,334 --> 00:23:57,295 This is More complete version. 343 00:23:57,295 --> 00:24:00,089 the salient part, though, the big fat arrows. 344 00:24:00,089 --> 00:24:03,671 the ones that count are precisely the ones we mentioned. 345 00:24:03,671 --> 00:24:07,801 We see here, the energy emitted as infrared radiation by the surface. 346 00:24:07,801 --> 00:24:10,594 Some fraction of which travels out into space. 347 00:24:10,594 --> 00:24:14,237 And the rest is radiated back and reabsorbed by the surface. 348 00:24:14,237 --> 00:24:17,760 This is that greenhouse effect that we were talking about. 349 00:24:17,760 --> 00:24:22,289 And so putting that into our equations this is what we found, I take the old 350 00:24:22,289 --> 00:24:26,760 equation and modify it by this green house factor and I find that with all 351 00:24:26,760 --> 00:24:31,230 these factors in place I can determine the surface temperature of earth of 352 00:24:31,230 --> 00:24:36,058 course I don't measure the green house factor what I've done here is fudged I've 353 00:24:36,058 --> 00:24:40,886 adopted a green house factors such that the resulting so temperature of earth is 354 00:24:40,886 --> 00:24:45,868 the agrees with your measured 292 kelvin. Comfortably above the freezing point of 355 00:24:45,868 --> 00:24:48,851 water. And so the atmospheric greenhouse effect 356 00:24:48,851 --> 00:24:53,230 without an atmosphere earth would be frozen and uninhabitable and 357 00:24:53,230 --> 00:24:58,225 The comfortable temperatures we enjoy depend on the precise values of A and G 358 00:24:58,225 --> 00:25:02,551 that we currently have. A can change due to different amounts of 359 00:25:02,551 --> 00:25:06,251 cloud cover, ice cover. G can change due to changes in the 360 00:25:06,251 --> 00:25:10,405 chemical composition of the atmosphere. We hear a lot about this. 361 00:25:10,405 --> 00:25:15,534 This can change the temperature, notice that even though A and G appear only in 362 00:25:15,534 --> 00:25:20,272 this one fourth power which makes the temperature relatively insensitive. 363 00:25:20,272 --> 00:25:25,855 Because we're dealing with a temperate of 300 degrees, one part in a hundred change 364 00:25:25,855 --> 00:25:30,529 in temperature is a three degree centigrade change which completely and 365 00:25:30,529 --> 00:25:36,061 drastically alters climate conditions. Moreover the real thing that people are 366 00:25:36,061 --> 00:25:41,315 worried about when they talk about changes in the earth's greenhouse 367 00:25:41,315 --> 00:25:47,080 scenario is that a small change in G. For example increasing G makes this 368 00:25:47,080 --> 00:25:51,678 denominator a little bit small. What this causes is the earth's 369 00:25:51,678 --> 00:25:56,130 temperature on the surface to rise a bit. This might cause 370 00:25:56,130 --> 00:26:01,069 polar ice caps to melt, reducing A, which further raises the temperature and can 371 00:26:01,069 --> 00:26:04,633 lead to non-linear effects that enhance the change. 372 00:26:04,633 --> 00:26:09,760 This is but, but, that's why people think we're very sensitive to the precise value 373 00:26:09,760 --> 00:26:14,200 of A and G, with which we live, in order to keep our temperatures stable. 374 00:26:14,200 --> 00:26:18,851 So, we've managed to understand one property of the physics of earth, based 375 00:26:18,851 --> 00:26:21,743 upon the physics that we learned last week. 376 00:26:21,743 --> 00:26:26,772 So it's good that we have an atmosphere, but given what we've tal, said about the 377 00:26:26,772 --> 00:26:30,230 creation of Earth, one could ask where did it come from. 378 00:26:30,230 --> 00:26:35,059 When the Earth formed, it was forming as silicates and aluminum and calcium., And 379 00:26:35,059 --> 00:26:39,467 I didn't say anything about water. Some amount of water can be trapped in 380 00:26:39,467 --> 00:26:42,787 minerals, we know. And I didn't say anything about gases. 381 00:26:42,787 --> 00:26:47,436 Earth was too small a planet to trap hydrogen and helium, which were the only 382 00:26:47,436 --> 00:26:50,696 gases plentiful. And indeed, when Earth formed, it would 383 00:26:50,696 --> 00:26:55,383 have had a very tenuous atmosphere. But when you cook rocks, earth formed it 384 00:26:55,383 --> 00:27:00,052 was very hot the rocks on the surface were cooked and they released carbon 385 00:27:00,052 --> 00:27:05,020 dioxide and nitrogen that can be trapped in minerals and is released when you heat 386 00:27:05,020 --> 00:27:08,123 them. this gave us the beginnings of an 387 00:27:08,123 --> 00:27:13,881 atmosphere of nitrogen and mostly carbon dioxide, much was released when the 388 00:27:13,881 --> 00:27:19,867 cooking rocks on the interior of earth in the manto were a, a, a available by vo, 389 00:27:19,867 --> 00:27:26,080 volcanic action and so this releases gases from the interior of the planet and 390 00:27:26,080 --> 00:27:29,792 we get bergining atmosphere of carbon dioxide. 391 00:27:29,792 --> 00:27:33,977 where water came from. Well there's some water as I said trapped 392 00:27:33,977 --> 00:27:36,946 in the rocks. But calculations show that, that probably 393 00:27:36,946 --> 00:27:41,173 would not have been enough to Create the oceans that we see on Earth 394 00:27:41,173 --> 00:27:44,144 today. And what there was of water would have 395 00:27:44,144 --> 00:27:47,733 been baked out and lost to space early in Earth's history. 396 00:27:47,733 --> 00:27:52,313 So it is, imagine that most of the water that forms our oceans, was in fact 397 00:27:52,313 --> 00:27:55,160 imported from farther out in the solar system. 398 00:27:55,160 --> 00:28:00,226 In the form of ice, where it formed in the form of ice and then fell in and 399 00:28:00,226 --> 00:28:05,032 impacted Earth during that heavy bombardment period 3.8, 3.9 billion years 400 00:28:05,032 --> 00:28:07,955 ago. And so the fact that we have water, is 401 00:28:07,955 --> 00:28:12,112 the result of import. And water creates a dynamic water falls 402 00:28:12,112 --> 00:28:14,970 as ice on the surface of Earth, it boils off. 403 00:28:14,970 --> 00:28:19,822 Up it accumulates as water vapor in the 404 00:28:19,822 --> 00:28:24,419 atmosphere. It condenses and creates the oceans that 405 00:28:24,419 --> 00:28:26,760 we see. And the rain that 406 00:28:26,760 --> 00:28:29,924 Falls from the atmosphere where the water condenses. 407 00:28:29,924 --> 00:28:33,941 Dissolves carbon dioxide. Which is then dissolved in the ocean and 408 00:28:33,941 --> 00:28:37,411 is fixed in sediments. This process is accelerated by the 409 00:28:37,411 --> 00:28:41,367 emergence of continents about three and a half billion years ago. 410 00:28:41,367 --> 00:28:45,810 And that, amount of carbon dioxide in the atmosphere is gradually removed. 411 00:28:45,810 --> 00:28:48,366 Leaving us with an atmosphere of nitrogen. 412 00:28:48,366 --> 00:28:51,592 That'd three-fourths of the atmosphere already there. 413 00:28:51,592 --> 00:28:54,148 With traces of water vapor, we need oxygen. 414 00:28:54,148 --> 00:28:58,969 The other quarter, oxygen. Free oxygen on earth is produced by the 415 00:28:58,969 --> 00:29:05,183 beginnings of plant life actually. initially this goes to oxidize sediments 416 00:29:05,183 --> 00:29:09,480 of iron and sulfur. But eventually as these are fully 417 00:29:09,480 --> 00:29:15,694 oxidized, free oxygen takes begins to exist in the earth's atmosphere starting 418 00:29:15,694 --> 00:29:18,380 about 700,000,000 years ago. And 419 00:29:18,380 --> 00:29:23,308 The, the level of oxygen approaches twenty% that we comfortable breathe 420 00:29:23,308 --> 00:29:25,350 today. There is therefore the. 421 00:29:25,350 --> 00:29:31,047 The atmosphere both enables the existence of life and is shaped by the existence of 422 00:29:31,047 --> 00:29:35,728 life all the oxygen we breath was produced locally in photosynthetic 423 00:29:35,728 --> 00:29:36,610 processes. So. 424 00:29:36,610 --> 00:29:39,514 This is how we think about, about the chemistry of the atmosphere. 425 00:29:39,514 --> 00:29:42,278 What does the. Physics tell us about the atmosphere. 426 00:29:42,278 --> 00:29:45,688 Well the atmosphere like the mantle is a convective system. 427 00:29:45,688 --> 00:29:49,849 It's heated from below because the ground is warmer than the atmosphere. 428 00:29:49,849 --> 00:29:54,589 So air down close to the ground is heated by conduction through the atmos through 429 00:29:54,589 --> 00:29:58,056 the from the ground. And then we have a fluid which is being 430 00:29:58,056 --> 00:30:01,120 heated from below. This drives convection patterns 431 00:30:01,120 --> 00:30:06,627 The differential heating shapes where the convection cells are going to fall, so 432 00:30:06,627 --> 00:30:11,652 globally, Earth has in each hemisphere these three large convection cells. 433 00:30:11,652 --> 00:30:16,953 The air, hot air rises near the Equator, cooler air sinks at mid latitudes, and 434 00:30:16,953 --> 00:30:21,634 this there's one more instance of hot air rising at upper latitudes. 435 00:30:21,634 --> 00:30:24,663 And, of course, at the poles there, is sinking. 436 00:30:24,663 --> 00:30:30,170 And so, we build these three convection cells, this drives northerly or southerly 437 00:30:30,170 --> 00:30:34,552 surface winds, and Global pattern of circulation cells will repeat and be 438 00:30:34,552 --> 00:30:37,580 generic and so. We should enjoy what we can of the 439 00:30:37,580 --> 00:30:40,179 physics of it. So, we've talked about the Earth's 440 00:30:40,179 --> 00:30:42,720 surface. We've talked about the Earth's interior. 441 00:30:42,720 --> 00:30:46,902 The Earth doesn't stop at the atmosphere which tapers off into planetary space. 442 00:30:46,902 --> 00:30:51,242 There's one important property of Earth that extends way beyond the extent of the 443 00:30:51,242 --> 00:30:53,889 atmosphere, and this is the Earth's magnetic field. 444 00:30:53,889 --> 00:30:57,330 As anybody who has used the compass knows that there is a magnet. 445 00:30:57,330 --> 00:31:01,745 Earth is a magnet whose poles roughly align with the axis of rotation. 446 00:31:01,745 --> 00:31:07,007 in fact, the earth's self magnetic pole is roughly aligned with the earth's north 447 00:31:07,007 --> 00:31:10,455 geographic pole. These are just a matter of convention we 448 00:31:10,455 --> 00:31:15,173 call north pole of a magnet, the part that faces north since opposite poles of 449 00:31:15,173 --> 00:31:19,709 a magnet attract, this means that it's being attracted to the earth's south 450 00:31:19,709 --> 00:31:22,673 magnetic pole. So here's a picture of the earth's 451 00:31:22,673 --> 00:31:25,939 magnetic field. The fields, remember, start at the north 452 00:31:25,939 --> 00:31:29,450 magnetic pole and enter into the South magnetic pole. 453 00:31:29,450 --> 00:31:34,507 And what this picture suggests us, other than the South pole is in the North, is 454 00:31:34,507 --> 00:31:37,899 that the source of the magnet is in the earth's core. 455 00:31:37,899 --> 00:31:41,804 This is indeed true. the process by which the earth's core 456 00:31:41,804 --> 00:31:44,812 generates a magnetic field are non-trivial one. 457 00:31:44,812 --> 00:31:49,933 It's called the geo dynamo, and it has to do with confluence of three factors. 458 00:31:49,933 --> 00:31:53,070 One is that the core is being heated from within. 459 00:31:53,070 --> 00:31:59,372 By the hotter inner core. So we have a liquid, metal-rich outer 460 00:31:59,372 --> 00:32:05,124 core full of conducting Elements like iron and nickel, its being 461 00:32:05,124 --> 00:32:11,157 heating from below which drives convection along with the rotation of the 462 00:32:11,157 --> 00:32:14,997 earth... This drives this conducted flow leads to 463 00:32:14,997 --> 00:32:19,620 instability that generates a spontaneous magnetic field 464 00:32:19,620 --> 00:32:23,614 It's not a trivial problem. It's not one that I'm going to write the 465 00:32:23,614 --> 00:32:26,810 equations for. But it's true that it's, it's 466 00:32:26,810 --> 00:32:31,700 Understood phenomenon, that rotation along with convection leads to 467 00:32:31,700 --> 00:32:36,322 spontaneous magnetic field. and that this field will roughly align 468 00:32:36,322 --> 00:32:41,212 itself with the axis of rotation. What is not true is, that the South Pole 469 00:32:41,212 --> 00:32:44,696 needs to be at the Earth's geographic North Pole. 470 00:32:44,696 --> 00:32:50,122 And indeed the geologic records shows that it always hasn't been that way, the 471 00:32:50,122 --> 00:32:54,991 field reverses polarity in a rather unpredictable way On average every 472 00:32:54,991 --> 00:33:00,363 500,000 years and here's a geologic record of the earth's polarity. 473 00:33:00,363 --> 00:33:03,970 Regions that are in black indicate a polarity 474 00:33:03,970 --> 00:33:06,587 Sorry. Regions in white indicate a polarity the 475 00:33:06,587 --> 00:33:10,375 same as the current polarity. Regions in black indicate the opposite 476 00:33:10,375 --> 00:33:12,825 polarity. And you see that the intervals are 477 00:33:12,825 --> 00:33:15,722 somewhat random. There are long periods of stability, 478 00:33:15,722 --> 00:33:19,286 periods of rapid change. We're now in a relatively stable period. 479 00:33:19,286 --> 00:33:23,241 But there are indications that the Earth's magnetic field might change. 480 00:33:23,241 --> 00:33:27,028 here is a, sort of, simulation of the Earth's magnetic field lines. 481 00:33:27,028 --> 00:33:29,980 Including the complicated structure near, the core. 482 00:33:29,980 --> 00:33:34,102 And here's what the field looks like. During polarity change, as I said, the 483 00:33:34,102 --> 00:33:35,940 process in complicated. What else? 484 00:33:35,940 --> 00:33:39,327 Does the field do for us? Why am I making such a big deal? 485 00:33:39,327 --> 00:33:42,120 the next demo might help us understand this. 486 00:33:42,120 --> 00:33:46,645 The interaction of charged particles and magnetic fields will come up again and 487 00:33:46,645 --> 00:33:50,095 again in this class. So this apparatus will allow us to study 488 00:33:50,095 --> 00:33:52,697 it. inside that glass bulb is that shiny 489 00:33:52,697 --> 00:33:55,525 horizontal metal structure. That's an electron gun. 490 00:33:55,525 --> 00:33:59,885 When I turn on a current. There is a piece of metal there that will 491 00:33:59,885 --> 00:34:02,973 be heated, will emit electrons. Those will be accelerated. 492 00:34:02,973 --> 00:34:06,887 And the electron gun will emit a horizontal beam of electrons into the 493 00:34:06,887 --> 00:34:08,982 bulb. Now, the bulb is nearly evacuated. 494 00:34:08,982 --> 00:34:12,401 So those electrons will be able to, propagate rather freely. 495 00:34:12,401 --> 00:34:15,984 But there's a very tenuous atmosphere of some inert gas in there. 496 00:34:15,984 --> 00:34:20,174 And this means that, when the electrons hit the gas, they will ionize the gas. 497 00:34:20,174 --> 00:34:24,310 The gas will glow, and we'll be able to trace where the electrons are going. 498 00:34:24,310 --> 00:34:29,132 Surrounding the bulb are these big coils. When I turn a current on through the 499 00:34:29,132 --> 00:34:33,892 coils, then as we saw eh, last weeks discussion of magnetism, we'll create a 500 00:34:33,892 --> 00:34:38,220 magnetic field penetrating the coils. So in the view we're seeing, that 501 00:34:38,220 --> 00:34:42,733 magnetic field will be directed from where we are sitting into the black 502 00:34:42,733 --> 00:34:45,540 backdrop. And we'll see what that does to the 503 00:34:45,540 --> 00:34:49,291 motion of the charged particles. Remember that, moving charges are 504 00:34:49,291 --> 00:34:53,042 affected by magnetic fields. So here, we've dimmed the light, we've 505 00:34:53,042 --> 00:34:57,024 turned down the electron beam. We see a, a nice straight electron beam, 506 00:34:57,024 --> 00:34:59,967 and we see that it ionizes the atoms of the gas. 507 00:34:59,967 --> 00:35:03,372 And the gas glows so we can trace the position of the beam. 508 00:35:03,372 --> 00:35:07,989 And now I'm going to turn on the magnetic field, which in this image, points from 509 00:35:07,989 --> 00:35:10,240 right to left. And we see that, when the. 510 00:35:10,240 --> 00:35:15,317 Beam impinges upon the electrons. The straight electron beam is curled into 511 00:35:15,317 --> 00:35:20,265 this helical shape, and as I turn up the current in the coils, increasing the 512 00:35:20,265 --> 00:35:25,278 magnetic field, the helix tightens more and more, we see what happens when the 513 00:35:25,278 --> 00:35:30,160 electrons hit the bob, the beam is absorbed. But by increasing the intensity 514 00:35:30,160 --> 00:35:35,433 of the beam, I can tighten the helix, make the radius about with which the 515 00:35:35,433 --> 00:35:38,490 electrons move smaller in other words increasing 516 00:35:38,490 --> 00:35:41,948 their acceleration since their speed is pretty much constant. 517 00:35:41,948 --> 00:35:44,328 I weaken the field and the helix opens up. 518 00:35:44,328 --> 00:35:48,573 This is clear perhaps in this view. We're viewing it from the side The beam 519 00:35:48,573 --> 00:35:52,933 is moving to the right, and when I turn on the magnetic field, which now points 520 00:35:52,933 --> 00:35:56,734 away from us I see a circle which is the projection of the helix. 521 00:35:56,734 --> 00:35:59,752 When I tilt it, you see, there's still a helix going on. 522 00:35:59,752 --> 00:36:04,391 But viewed from the side, it looks like a circle, and the radius of the circle is 523 00:36:04,391 --> 00:36:06,963 smaller when I make the magnetic field strong. 524 00:36:06,963 --> 00:36:10,820 And as I turn the magnetic field down, the radius of the circle grows. 525 00:36:10,820 --> 00:36:16,208 So now, we understand what the magnetic field can do for us, besides direct our 526 00:36:16,208 --> 00:36:19,869 compasses. what the magnetic field does, is that it 527 00:36:19,869 --> 00:36:25,257 interacts with that solar wind of charged particles that are constantly being 528 00:36:25,257 --> 00:36:29,810 emitted by the sun. And what happens to these charged 529 00:36:29,810 --> 00:36:33,369 particles, when they hit the earth's magnetic field 530 00:36:33,369 --> 00:36:38,758 is that they go into circular motion in a direction perpendicular to the field. 531 00:36:38,758 --> 00:36:43,505 And essentially that means that they are trapped along the field lines. 532 00:36:43,505 --> 00:36:49,121 They can slide along the field lines and rather than penetrate through the field 533 00:36:49,121 --> 00:36:50,918 and and pinch upon the earth. 534 00:36:50,918 --> 00:36:55,168 They are trapped along the field lines and they travel along the field lines in 535 00:36:55,168 --> 00:36:58,567 a sort of spiraling around them in, in the helices is that we saw. 536 00:36:58,567 --> 00:37:02,924 And the next calculation that I couldn't show you with that demo, shows that 537 00:37:02,924 --> 00:37:06,217 regions of strong magnetic field actually repel the particles. 538 00:37:06,217 --> 00:37:10,520 What that means, is that if you look at the field lines the particles in pinching 539 00:37:10,520 --> 00:37:14,664 from the sun are trapped along the field lines, they propagate along the field 540 00:37:14,664 --> 00:37:17,480 lines until new the poles with the field intensifies. 541 00:37:17,480 --> 00:37:22,011 They are repelled, they bounce back. So they end up bouncing back and forth 542 00:37:22,011 --> 00:37:25,257 and creating the so called Van Allen radiation belts. 543 00:37:25,257 --> 00:37:30,463 These are regions around the Earth where the magnetic field has trapped particles 544 00:37:30,463 --> 00:37:34,260 from the solar wind. And this prevents this intense flux of 545 00:37:34,260 --> 00:37:37,200 charged particles from impinging upon the Earth. 546 00:37:37,200 --> 00:37:41,132 Now, the solar wind actually it plies a pressure to this. 547 00:37:41,132 --> 00:37:46,663 We often talk about charged particles we saw what a magnetic field does to charged 548 00:37:46,663 --> 00:37:50,184 particles. Charged particles in turn influence a 549 00:37:50,184 --> 00:37:55,126 magnetic field when they're moving. we saw that for example with our 550 00:37:55,126 --> 00:38:00,598 electrons which were moving in a circle they themselves constituted a motion of 551 00:38:00,598 --> 00:38:05,390 charges in circles which would generate a magnetic field and as you can predict 552 00:38:05,390 --> 00:38:08,744 from stability. That magnetic field actually opposes the 553 00:38:08,744 --> 00:38:12,697 magnetic field in the cause that caused them to move in the helix. 554 00:38:12,697 --> 00:38:16,830 what that means is that when you have a magnetic field that is 555 00:38:16,830 --> 00:38:21,690 trapping a flux of charged particles. Those, the, that the, momentum of those 556 00:38:21,690 --> 00:38:26,117 charged particles is absorbed by the magnetic field and essentially pushes it. 557 00:38:26,117 --> 00:38:30,658 You can imagine sort of a hydrodynamics problem where the charged particles are 558 00:38:30,658 --> 00:38:35,142 just pushing on the fields and will often apply this sort of picture, where the 559 00:38:35,142 --> 00:38:39,002 particles push on the field. What this shows you is what the earth's 560 00:38:39,002 --> 00:38:43,770 magnetosphere actually looks like it, the earth is trying to make that nice apple 561 00:38:43,770 --> 00:38:47,857 shaped symmetric magnetic field. But the sun here on the left is pushing 562 00:38:47,857 --> 00:38:50,922 that field out. So that the field is compressed on the 563 00:38:50,922 --> 00:38:55,089 left hand side, and then extends way off to the right and this is the D-4 564 00:38:55,089 --> 00:38:58,492 magnetosphere. And what we see is that the flux from the 565 00:38:58,492 --> 00:39:02,991 sun is not constant in the course of a magnetic storm there's a dynamics here. 566 00:39:02,991 --> 00:39:07,490 And when the magnetics storm, when the stream of particles from the sun has 567 00:39:07,490 --> 00:39:10,893 enough flux then some of them break through the atmosphere. 568 00:39:10,893 --> 00:39:15,392 And they break through the atmosphere where the magnetic field line enter the 569 00:39:15,392 --> 00:39:18,598 atmosphere that would be near the Earth's poles, and 570 00:39:18,598 --> 00:39:21,817 then they impinge upon the gases in Earth's atmosphere. 571 00:39:21,817 --> 00:39:24,450 And as they did in the tube, they ionize them. 572 00:39:24,450 --> 00:39:28,743 And this gives rise to the beautiful light show we call the Aurora. 573 00:39:28,743 --> 00:39:32,140 In this case the Aurora Australis over Antarctica. 574 00:39:32,140 --> 00:39:35,714 So the charged particles get trapped along the field lines. 575 00:39:35,714 --> 00:39:39,635 When they penetrate, they enter the atmosphere along the field lines, 576 00:39:39,635 --> 00:39:44,420 which enter the atmosphere near the poles and this is why we have Auroras near the 577 00:39:44,420 --> 00:39:47,130 poles. And that's why the intensity of northern 578 00:39:47,130 --> 00:39:51,742 or southern lights is dependent upon the intensity of solar activity, as we will 579 00:39:51,742 --> 00:39:55,312 see next week. So let's summarize what Earth has taught 580 00:39:55,312 --> 00:39:58,546 us. we learned how to identify the benchmarks 581 00:39:58,546 --> 00:40:02,050 of tectonics. Looking, look for ridge-like structures, 582 00:40:02,050 --> 00:40:07,036 those linear mountain structures. Geological activity will be reflected in 583 00:40:07,036 --> 00:40:09,694 volcanoes. we learned the importance of an 584 00:40:09,694 --> 00:40:13,040 atmosphere and what that does to the temperature structure. 585 00:40:13,040 --> 00:40:17,520 We learned how to compute the equilibrium temperature of a planet, about a star. 586 00:40:17,520 --> 00:40:22,652 And we learn that the existence of a solid and a liquid core of conducting 587 00:40:22,652 --> 00:40:27,375 material generates a magnetic field, which allows protection, leads to 588 00:40:27,375 --> 00:40:31,686 protection from the solar wind, this will become very important. 589 00:40:31,686 --> 00:40:36,409 And the last thing we learned is that the air stressed is very young. 590 00:40:36,409 --> 00:40:41,884 So as a way to test our understanding of the very early solar system, Earth is a 591 00:40:41,884 --> 00:40:46,880 poor place to do the study because the atmospherics, the erosion, geologic, 592 00:40:46,880 --> 00:40:50,850 tectonic activity erases the past rather quickly on Earth. 593 00:40:50,850 --> 00:40:53,420 Maybe you can find a better place to study that.