1 00:00:00,226 --> 00:00:02,420 - Let say that I have two different gases 2 00:00:02,420 --> 00:00:03,593 at two different temperatures 3 00:00:03,593 --> 00:00:06,252 that just got in contact with each other. 4 00:00:06,252 --> 00:00:08,922 So, this is my magenta gas. 5 00:00:08,922 --> 00:00:11,290 Right over here, let me draw a bunch of 6 00:00:11,290 --> 00:00:14,390 the molecules of my 7 00:00:14,390 --> 00:00:17,757 magenta gas right over here. 8 00:00:17,757 --> 00:00:19,754 And it's just, this system over here, 9 00:00:19,754 --> 00:00:21,217 I guess, whatever you want to call it, 10 00:00:21,217 --> 00:00:24,474 has just come in contact with this blue gas. 11 00:00:24,851 --> 00:00:28,874 This blue gas, right over here. 12 00:00:30,017 --> 00:00:31,921 And lets say that right where when we're starting 13 00:00:31,921 --> 00:00:33,953 our simulation, our experiment, 14 00:00:33,953 --> 00:00:37,837 that this magenta gas has a higher temperature. 15 00:00:38,376 --> 00:00:41,134 Higher temperature. 16 00:00:41,546 --> 00:00:43,770 And our blue gas has a lower temperature. 17 00:00:44,286 --> 00:00:46,939 Lower temperature. 18 00:00:47,154 --> 00:00:50,114 So lets just remind ourselves what temperature is. 19 00:00:50,114 --> 00:00:52,517 Specially if we think about it on a molecular scale. 20 00:00:52,517 --> 00:00:54,990 So higher temperature, lower temperature. 21 00:00:54,990 --> 00:00:59,193 Temperature is proportional to average kinetic energy. 22 00:00:59,193 --> 00:01:02,316 So these molecules, they're gonna be vibrating around, 23 00:01:02,316 --> 00:01:03,849 they're gonna be bumping around. 24 00:01:03,849 --> 00:01:04,661 They're gonna have, 25 00:01:04,661 --> 00:01:06,356 each of them are gonna have kinetic energy 26 00:01:06,356 --> 00:01:07,320 and if you average them, 27 00:01:07,320 --> 00:01:09,549 that's gonna be proportional to temperature. 28 00:01:09,549 --> 00:01:12,219 So let me depict each of this individual 29 00:01:12,219 --> 00:01:13,659 molecule's kinetic energy. 30 00:01:13,659 --> 00:01:15,087 Maybe this one is doing that. 31 00:01:15,087 --> 00:01:16,922 Maybe this one is doing that. 32 00:01:16,922 --> 00:01:18,617 Maybe this one is going in this direction. 33 00:01:18,617 --> 00:01:20,288 Maybe that one is going that direction. 34 00:01:20,288 --> 00:01:22,617 That one is going in that direction. 35 00:01:22,617 --> 00:01:24,570 This is going in that direction. 36 00:01:24,570 --> 00:01:26,221 That is going in that direction. 37 00:01:26,221 --> 00:01:27,203 So, 38 00:01:27,626 --> 00:01:29,623 notice, they all have different directions. 39 00:01:29,623 --> 00:01:32,421 And the magnitude of their velocity can be different. 40 00:01:32,421 --> 00:01:34,273 They all have different speeds. 41 00:01:34,627 --> 00:01:37,501 They all might have different speeds. 42 00:01:38,226 --> 00:01:39,886 So they have different speeds right over here. 43 00:01:39,886 --> 00:01:42,359 And they're all bumping into each other. 44 00:01:42,359 --> 00:01:43,857 Transferring their kinetic energy, 45 00:01:43,857 --> 00:01:45,157 transferring their momentum 46 00:01:45,157 --> 00:01:47,328 to from one particle to another. 47 00:01:47,328 --> 00:01:48,408 But when we talk about temperature 48 00:01:48,408 --> 00:01:49,987 we talk about the average kinetic energy 49 00:01:49,987 --> 00:01:51,055 or what's proportional to the 50 00:01:51,055 --> 00:01:52,837 average kinetic energy of the system. 51 00:01:53,121 --> 00:01:54,654 Well this one, each of these molecules 52 00:01:54,654 --> 00:01:57,025 are also gonna have some kinetic energy. 53 00:01:57,405 --> 00:01:59,193 But on average it's gonna be lower. 54 00:01:59,193 --> 00:02:01,156 Maybe this one is doing something like this. 55 00:02:01,156 --> 00:02:03,222 This one is doing something like this. 56 00:02:03,222 --> 00:02:05,161 This is doing something like this. 57 00:02:05,161 --> 00:02:07,431 This is doing something like this. 58 00:02:07,646 --> 00:02:08,957 So they're different, 59 00:02:08,957 --> 00:02:11,111 but on average they're gonna be lower. 60 00:02:11,152 --> 00:02:12,046 So, 61 00:02:12,046 --> 00:02:14,019 hopefully you see that this magenta arrows 62 00:02:14,019 --> 00:02:17,491 are bigger than these blue arrows 63 00:02:17,491 --> 00:02:18,739 that I'm doing. 64 00:02:19,047 --> 00:02:20,080 And they don't all have to be, 65 00:02:20,080 --> 00:02:21,322 for example this one 66 00:02:21,322 --> 00:02:22,622 might have a lot of kinetic energy. 67 00:02:22,622 --> 00:02:23,760 But when you average it out, 68 00:02:23,760 --> 00:02:26,309 the average here is gonna be lower than the average here. 69 00:02:26,790 --> 00:02:28,863 So, just like that. 70 00:02:29,112 --> 00:02:30,761 Now, if this is our initial state, 71 00:02:30,761 --> 00:02:33,629 what do we think is going to start happening? 72 00:02:33,629 --> 00:02:36,717 Well, before our different groups of gases 73 00:02:36,717 --> 00:02:39,724 were colliding with itself 74 00:02:39,724 --> 00:02:41,454 or the magenta was colliding with the magenta. 75 00:02:41,454 --> 00:02:42,684 The blue is colliding with the blue. 76 00:02:42,684 --> 00:02:44,821 But now they're gonna start colliding with each other. 77 00:02:44,821 --> 00:02:46,423 And so you can imagine when 78 00:02:46,423 --> 00:02:48,524 this molecule right over here, 79 00:02:48,524 --> 00:02:50,092 collides with this molecule, 80 00:02:50,092 --> 00:02:52,390 it's gonna transfer some kinetic energy to it. 81 00:02:52,390 --> 00:02:56,256 So after the collision, this one might be going. 82 00:02:56,256 --> 00:02:58,469 After the collision. 83 00:02:58,555 --> 00:03:00,088 So lets just say they just bounced in to. 84 00:03:00,088 --> 00:03:01,353 So this is right before, 85 00:03:01,353 --> 00:03:03,670 and lets say they just finished bouncing into each other. 86 00:03:03,954 --> 00:03:06,520 So right after they finish bouncing into each other, 87 00:03:06,520 --> 00:03:08,476 this one might ricochet off. 88 00:03:08,656 --> 00:03:11,634 So this one is going to go this way. 89 00:03:11,721 --> 00:03:13,074 Let me do this in a different color. 90 00:03:13,486 --> 00:03:16,214 So it might hit this one, bounce off, 91 00:03:16,214 --> 00:03:18,048 and then transfer some of its kinetic energy 92 00:03:18,048 --> 00:03:20,521 and then it bounces off in this direction. 93 00:03:20,521 --> 00:03:23,586 While this one, after the collision, 94 00:03:23,586 --> 00:03:27,099 after the collision is going to 95 00:03:27,220 --> 00:03:29,357 is maybe going to move 96 00:03:29,357 --> 00:03:31,853 much faster in this direction. 97 00:03:31,853 --> 00:03:34,082 And so notice, you have a transfer of energy. 98 00:03:34,082 --> 00:03:35,823 Just with that one collision you had 99 00:03:35,823 --> 00:03:37,251 a transfer of kinetic energy 100 00:03:37,251 --> 00:03:39,457 from this molecule to that molecule. 101 00:03:39,457 --> 00:03:41,791 And this is going to happen throughout the system. 102 00:03:41,791 --> 00:03:44,624 That the faster molecules, the ones with more kinetic energy 103 00:03:44,624 --> 00:03:48,118 as they collide you're gonna have transfer of energy. 104 00:03:48,118 --> 00:03:49,953 So you're gonna have a 105 00:03:49,953 --> 00:03:52,217 transfer of energy from the higher temperature 106 00:03:52,217 --> 00:03:53,639 to the lower temperature. 107 00:03:53,819 --> 00:03:59,096 Transfer, transfer of energy. 108 00:03:59,357 --> 00:04:01,121 And this transfer of energy. 109 00:04:01,121 --> 00:04:03,525 And you can consider this transfer of thermal energy, 110 00:04:03,525 --> 00:04:04,918 we're talking about temperature here. 111 00:04:04,918 --> 00:04:07,716 So, the things that are related to temperature, 112 00:04:07,716 --> 00:04:08,726 we would say thermal. 113 00:04:08,726 --> 00:04:11,971 So these, this is transfer of thermal energy. 114 00:04:12,592 --> 00:04:14,426 Transfer of thermal energy. 115 00:04:14,426 --> 00:04:15,959 The amount, so you're gonna have, 116 00:04:15,959 --> 00:04:18,954 if you're gonna start with higher energy here. 117 00:04:18,954 --> 00:04:20,382 You have higher average kinetic energy. 118 00:04:20,382 --> 00:04:22,356 You have lower average kinetic energy here. 119 00:04:22,356 --> 00:04:25,421 But this is going to transfer energy 120 00:04:25,421 --> 00:04:27,790 from the magenta to the blue. 121 00:04:27,790 --> 00:04:29,183 It's gonna go from higher temperature 122 00:04:29,183 --> 00:04:30,692 to the lower temperature. 123 00:04:30,692 --> 00:04:33,153 And that energy that's being transferred, 124 00:04:33,153 --> 00:04:35,324 that energy that's being transferred, 125 00:04:35,324 --> 00:04:37,124 we call that, and this is a word that you 126 00:04:37,124 --> 00:04:39,585 have probably heard many times in your life, 127 00:04:39,585 --> 00:04:41,861 we call that energy that's being transferred, 128 00:04:41,861 --> 00:04:43,609 we call that heat. 129 00:04:43,684 --> 00:04:46,992 That literally this hotter gas over here 130 00:04:46,992 --> 00:04:48,351 is heating up. 131 00:04:48,351 --> 00:04:51,358 Is heating up this cooler gas. 132 00:04:51,358 --> 00:04:53,552 And the way that this transfer of 133 00:04:53,552 --> 00:04:55,154 thermal energy is happening 134 00:04:55,154 --> 00:04:57,627 where it's through the collision of the particles, 135 00:04:57,627 --> 00:05:01,354 the transfer of kinetic energy to the collision of particles 136 00:05:01,354 --> 00:05:02,724 that's transferred momentum, 137 00:05:02,724 --> 00:05:04,541 we call this conduction. 138 00:05:05,220 --> 00:05:07,728 We call this thermal conduction, 139 00:05:07,728 --> 00:05:09,406 or I'll just call it conduction. 140 00:05:09,852 --> 00:05:11,972 Let me write thermal conduction. 141 00:05:12,453 --> 00:05:13,736 I'll do it in a new color. 142 00:05:14,055 --> 00:05:19,240 So this that is being described is thermal conduction. 143 00:05:19,489 --> 00:05:21,056 Which is a way that many times 144 00:05:21,056 --> 00:05:24,121 you have experienced heat being transferred. 145 00:05:24,121 --> 00:05:26,014 For example, you probably have had 146 00:05:26,014 --> 00:05:28,057 the experience of, 147 00:05:28,057 --> 00:05:30,483 if you take a, I don't know, 148 00:05:30,483 --> 00:05:34,205 you take a pot. 149 00:05:35,023 --> 00:05:36,933 Lets say you take a pot like this. 150 00:05:37,217 --> 00:05:39,179 Lets say it's a cold pot at first. 151 00:05:39,179 --> 00:05:41,687 So its particles, the particles in the pot 152 00:05:41,687 --> 00:05:44,004 have a lower kinetic energy. 153 00:05:44,055 --> 00:05:45,536 So I'm not gonna be able to do all of them. 154 00:05:45,751 --> 00:05:48,061 And then you put it over a fire. 155 00:05:48,061 --> 00:05:49,971 You put it over a fire. 156 00:05:50,255 --> 00:05:51,573 Let me see if I can draw it. 157 00:05:52,693 --> 00:05:54,337 You put it over a fire. 158 00:05:54,992 --> 00:05:58,220 So this is the fire, this is the fire. 159 00:05:58,220 --> 00:05:58,793 And we're talking about really 160 00:05:58,793 --> 00:06:00,124 just heating up the metal of the pot, 161 00:06:00,124 --> 00:06:02,515 I'm not even concerned about what's in the pot right now. 162 00:06:02,515 --> 00:06:04,791 So this fire is going to heat up, 163 00:06:04,791 --> 00:06:07,473 it's gonna heat up the bottom of this pot first. 164 00:06:07,473 --> 00:06:09,818 And it's actually gonna do it primarily 165 00:06:09,818 --> 00:06:11,258 through thermal conduction, 166 00:06:11,258 --> 00:06:14,590 because fire is nothing but super hot air particles 167 00:06:14,590 --> 00:06:15,983 and those super hot air particles 168 00:06:15,983 --> 00:06:19,083 are gonna bump in to the metal particles of your pot. 169 00:06:19,083 --> 00:06:21,428 So these metal particles of this pot, 170 00:06:21,428 --> 00:06:25,085 they're kinetic energy is going to start going up. 171 00:06:25,085 --> 00:06:27,361 So this part of the pot is going to start heating up. 172 00:06:27,361 --> 00:06:30,995 And right when you turn your stove on, 173 00:06:30,995 --> 00:06:33,351 the top of the pot might still be cool, 174 00:06:33,351 --> 00:06:35,523 but the bottom is going to get hot very fast. 175 00:06:35,523 --> 00:06:37,183 But if you just wait a few minutes, 176 00:06:37,183 --> 00:06:40,283 these metal particles are gonna keep bouncing 177 00:06:40,283 --> 00:06:41,850 and vibrating into each other. 178 00:06:41,850 --> 00:06:44,323 And so eventually, the top over here 179 00:06:44,323 --> 00:06:48,259 is going to get quite hot. 180 00:06:48,259 --> 00:06:52,038 Is going to get quite hot. 181 00:06:52,287 --> 00:06:54,621 And the way that the top of this metal got hot 182 00:06:54,621 --> 00:06:56,920 it was through thermal conduction 183 00:06:56,920 --> 00:06:59,160 that the metal of the bottom got hot first, 184 00:06:59,160 --> 00:07:00,786 and then they bounced into their neighbors, 185 00:07:00,786 --> 00:07:02,284 or vibrated into their neighbors 186 00:07:02,284 --> 00:07:04,060 and transferred some of that kinetic energy. 187 00:07:04,060 --> 00:07:06,451 And so once again you see this transfer of heat 188 00:07:06,451 --> 00:07:10,608 from a higher temperature region, 189 00:07:10,608 --> 00:00:00,000 to a cooler temperature or lower temperature region.