all right. let's begin. last time, we talked about, hey, what's gonna happen to a hole if you heat it up? will the hole get bigger, smaller or stay the same? and we all tried it with our kid sister's ring and we found out what, gang? bigger. smaller.

you see, right now, it goes through. with a little effort, it goes through, yeah? i'm not gonna take the time to heat up the ball. i'd waste your time because you know the ball would get bigger and no way it would go through. i'm gonna heat, instead, the ring. you get that ring nice and hot. is the ring expanding? yep. yeah. yeah, it's expanding. how about the hole? is the hole expanding? see, a lot of people thing this. a lot of people reason that the hole is gonna-- it's gonna expand this way and expand in this way 'cause the ring is gonna get fatter. will that ring get fatter? it turns out it will. so they think this part would expand out here and this part in here, and the hole gets smaller. but lo and behold, it's appreciably bigger. so the hole does get bigger, gang.

did that kind of confirm your reasoning anyway? yeah. did you find out-- how many people said in their homework, "yeah, the hole does get bigger"? show of hands. how many people are a little bit unhappy today? they kind of said something else? can we still be friends? can we? can we? here's the way i look at that, gang. oh, man. no friends today. no friends today? sometimes it's not good to give the answers, huh? if we take our ring and we cut it up into four pieces, gang, four pieces, and if i put even any one of those pieces in the oven, would it get bigger? yeah. would it expand and maybe look like this? no way. it's gonna expand and look like that. and this side here is longer in an expanded state than like this. so take them apart and look at it and you can kind of see it. or take your ring and open it up like this. is it gonna get fatter? yes, it's gonna get fatter.

so it expands up to here maybe, out to here maybe, yeah? but that's not the end of the story. you see, we're so busy thinking about what happens like this, huh, that we forget it's expanding this way, too. and it expands lengthwise as well. and lengthwise, we'll see it expands out to here. and i submit to you that this dotted line on the top is longer than the solid line below. and that dotted line on the top represents the new inner circumference. so every part of the ring, the thickness, the width, circumference, outer, inner, everything expands by the same rate if the heating is uniform and it pretty well was here. so the hole gets larger, okay? cool. yeah. you've got a jar of peanut butter, you can't open it up and you give to a muscle brawn-type. he still can't open it up. so what do you do and if it's handed to you? you take it, and put it in a stove upside down momentarily or put it under the hot water. doesn't that metal lid expand?

does it make it tighter against the glass or looser? looser. and you--hey, mind, mi. hey. brawn one thing, wonderful, but the mind, too. see? so you think and you can open it up by just eating it up. it turns out the whole darn thing will expand whole and all. sure, you know, they're kind of nice thing, too. you have thermostats at home? here's a piece of metal, two kinds: brass, steel. does everything expand the same rate, gang? no. let's see. the steel is on the top. the brass is on the bottom. which part expanded more? the brass. the brass expanded more and pushed the steel right into a curve. you see that? and that's the basis of a thermostat. thermostat acts like that. you know what i'm saying? notice we have a wooden handle on here. why do we have a wooden handle on this demonstration apparatus?

this metal here, i can just barely hold it there. why is it i just didn't have the metal and hold that in there? why, gang? because the metal part would get hot and the wooden part wouldn't. and that wooden part wouldn't. but why--and what are we gonna talk about today? heat transfer. heat transfer. and the first kind of heat transfer begins with a c and what is it? conduction. conduction. and it turns out that heat would have been conducted to my hand had i held it here and it would have burned me, that's because metal is a very good conductor of heat. how come i get a wooden handle, gang? because the wood is a very poor conductor of heat. but let's not be so negative. instead of saying poor conductor, we can say good insulator. see, see, you can see it the other way around, okay? it turns out in the metal, all metals got a lose electrons. and those electrons, when you start to heat this up, you--how many people say, "oh, when you put that in a flame, those electrons just kind of hang loose on it?" those electrons are bopping around and those electrons-- hit their neighbors--

and that energy cascades all down the middle. so it turns out that things with lose electrons are good heat conductors. later on, we're gonna learn things with good-- with lose electrons are good electrical conductors. and how many say, "oh, that's probably a coincidence"? it's not a coincidence. it's a good electrical conductor for the same reason. those lose electctrons can flowight through a piece of metal, gang. and when that--now, we're talking about electricity. we'll be talking about that soon, yeah? but for now, those metal-- those little electrons will bop, bop, bop, bop, bop, which are se in metals and carry that energy righalong. so somthings conduct better than other things. piece of wood, very, very good insulator. this wood is a good insulator at any tempetu. this wood is a good insulator even if it's very, very hot. yep. diyou ever reach in a hot oven and grabbed on to a frying fan that's about mayb400 degrees fahrenheit? you're gonna cook some food or something,

you got a frying p in there that's all iron. ever be reaching there with your bare hand and grabbed the iron pan? if you ever had, you can show that today, you'd be tattooed, all right? you're gonna buryourself. isn't that right? but could you reach into that oven and grab a pan if it had a woodenandle and grab it momentarily? you can do that. you can do that. you know why? because t--that wood is hot. that's the same temperature as the metal. but not very much energy is gonna conduct from the wo to your hand, so you can safely hold it. you cado that. ever see these people walking at hot coals after they've paid more than $300 for some sort of a course that teaches you how to have self-confidence in yourself? and then the test of that self-confidence is they say, "we are gna violate the laws of physics "and show you that mind ov matter. "that $300 you've ent for that 15-minute session "is gonna pay off "becse wre gonna apply those chniques "and we' gonna show that you can walk with bare feet on hot coals." and so the people do that. they take off theishoes, bare feet, they step on the hot coals.

they walk across and they think they have violated physics. gang, those are hot coals of what? wood. wood. and that wood is verhot, isn't it? how much of that heat in that wood is gonna get to the feet? what does it depend up? not only the temrature of the wood but something else. begin with a c, end with onductivity. see if you can put it together. what is it, ng? conductivity. it's conductivity. and that conductivity of the wood is good or not so good? not so good. not so good. so when you st with your barefoot on tt hot coal, the is a heat transfer. but a lot or a little? what the answebegin with, gang? l. okay, l. okay. so it's a little l, right? and so only it makes sense, little, little heat. only a little bit of heat will get to yo foot and you can-- , you're not gon stand on one place there, okay? you're not gonna reach in that oven and you're hanging on that wooden handle. honey, you're gonna be hurt. but you can grab it anpull it out. boom. and you can step, step, step, step across red hot coals of wood without harm. it's even better if your feet are wet.

once--remember when you were a little kid and you wanted to know if the iron was hot and your mother says, "touch it and see"? now,f your motr doesn't care vermuch of yo she says, "touch it and e." but if your mother really loves you, she says, "before you touch it, beetting your hand." [makes sous] rit? [makes sous] that littlsound? [makes sounds] what's that little sound? we're gettg a chapter ahead. you guys don't know about this. it turns to steam. how many say, "oh, don't take any energy to turn to steam. it just happens to do it." come on. the energy it took to turn that to that little puff of steam is ergy that did not go to your ha. so it's better if you walk across those coals and you got wet feet beuse then it'll be even safer. so a homework assignment this weekend. every ontake off their shoes and sos, get some hot coals, pour theall over your fnt lawn and get out there and do it, all right? okay? actually, me people say, "anything i can do for extra credit?" there you are. and take a photograph and 'll put the photo in the next book, okay, becausi need to get a new photo

of someone doing that, all right? you're sitting in that tables. rt of your table is metal and partf your table is wood. i wa y to all put your hand on the woodepa. oh, go ahead. it's all right. all right? now, take the hand off and touch the metal part underneath, the metal part of your table, okay? now which is hotter, the wood part or the metal part? wood. that is kind of an ambiguous question, gang. is it not? which has the higher temperature, the wood or the metal? try it again. same. the same. yeah, the same. but if you just feel it, you'd say what? oh, the metal is colder. is the metal colder? it fls colder because it's a betteconductor. your body temperature is higher than the temperature of everything else in this room.

and temperature is highe over your hand than that which you touch. gang, there's a heat flow, an ergy flow that we call heat. it goes from high temperature to low. and when you did tt, enou touched the wood, there's a heat difrence but conductivity not so much. how much energy flow from your hand when you touched the wood? a lot or a little? a little. how much flowed from your ha when you touch the metal? a lot or a little? a lot. the other l, right? a lot. and it has to do with the conductivity. it kinda makes sense, that stuff, doesn't it, condtivity? kinda neat stuff. hey, i'll show you something kinda nice forhat. here's a piece of iron. good conductor? yeah here's a piece of wood. good conductor? no. no. it turns out wood is a very, very poor conductor as wve talked about. i'm goa put a piece of paper around this iron. i'm gonna light up my torch again.

paper doesn't ghon fe, gang. but if the paper is taken out like this, and it's flamin'. put it against the iron, it goes out. it's charred. that part lit up. oops. ll the fire marshal. but what's going ohere, gang? well, let's try it with wo let's try it with wood. our test. be checkin' the neighbor and see if the neighboknows why that paper didn't light. did you ever read the book by ray bradbury call fahrenheit 451? yes. that 451 is what? that's the temperature in frenheidegrees at which paper burns. that was about a book-burning book, yeah? watch this, gang. fire city. at thing is burning up. look at that all right. okay.

why with the woo but not with the metal? neighbor time, neighbor time. okay. what would be the answer, gang? heat transfer is-- most of the energy here went to what, the paper or the metal? metal. the metal. how much was left in the paper? none. and before i can get the paper up to 451, i've got to make that very, very good conductor back there at 451, too. and that takes a lot and lot of energy to do. anso you didn't see thpaper ignite. it never got to 451. this is considerably more than 451 degrees, considerably more. when i wrapped it around the wood, look the wood is all scarred now. look at that. you saw the paper light up. why? because i didn't have to heat up all the wood to 451, just the surface. see? just the surface. and it--right up easily. but around--on a piece of metal, all the heat is conducted all throh here.

let's try something similar. this time, i've got-- oh, i'll take a paper cup rst. paper cup, some wate okay, my flame aga. there we go. water in the cup. then, you know, fire and all, no flames at all, gang. [laughter] except the edge, okay? and yet, if i take a cup like this--oy. any fire marshals around? okay. you nda get the ea ain't that nice? and this over here, it turns out just the edge you see there, the edge does t in touch with the water. but the same type thing is happening here. not so much

beuse of the conductivity of the water. water turns out to be not a good conduor. but something else is going on here, gang. and what is it? transfer. oh, i can read your minds. i can read your minds. [laughter] you're all saying, "but, gang, yore taking a fluid which has an enormous specific heat, hewitt." and if you wanna increase the temperature of that, honey, yogotta put a lot of calories. and you got hardly left any leftover for the water-- i mean, for the-- what's the outside made of? beginning with a p? per. okay. so it turns out, yeah, most of my energy is going-- being absorbed by the water. and it's a little bit warmer but not very much. i'll show you something really neat. i'm gonna put an ice cube at the bottom of this. oops. i hope i can get an ice cube in there. oh, gein there, son of a gun. uhh.kay. see that hunof ice down there, gang? and i'm gonna wedge that ice in there with this paper clip 'cause i'm gonna pour some water in there. and i don't want the ice to float

and i got the ice wedged down there. now i'm gonna fill it up with some water. [laughter] okay, that was the cup that i didn't have the water in, yeah? okay. now what i'm gonnao is i'm gonna get this going again. and-- [laughter] oh, please. [laughter] i'm gonna bring the top to a boil. and the ice cube at the ttom is still intact. look at that, gang. the water is boiling. downelow-- do you see the piece of ice?

the e is still in there. ain't that rightgang? do you see that? so that shows that water is a pretty poor conductor. remember we talked last time about the idea of the four-degree water at the bottom of the lake? and the summertime ces, why doesn't that wle lake all heat right up? it turns out that water will not conduct the energy, that sunlight down very far at all. water isot a good conductor of heat. it's a poor conductor. and you saw that here. i boiled the top, and that heat did not conduct downward to melthe ice. [makes sound] interesting, interesting, interesting this world we live in, ye? hmm. it turns out that air is a gd conductor or a poor conductor? poor. it turns out it's a poor nductor. and aren't you glad? because what if air were a very, very good conductor? how would you feel all the te? it begins with a c, ends with old. put it together, gang. cold.

you would feel cold. let's suppose the air was as good condting as like a piece of metal. do you ever stand on a night at home and lean up against a metal door? it's cold to the touch. but that mal door has the same temperature as the air arounyou. soe glad that air is a poor conductor and a good insulator. in fact, when you nna keep yourself warm whenou're gonna go campin', that sort of thing, don't you get a down-filled slping bag? what's all the dowfor? it's holding air in one place. it keeps the air from circulating. and that down will just hold the-- how about animals? u wonder about the anils. how do they make it up on the snow-covered mountains? they're covered with fur. and that fur puffs up and what-- guess what it holds insi? it begins with "a," ends with r. air. and at air is a very, vy good insulator. so most of the things that keep you very, very warm in a cold climate like tha which insulates like styrofoam, yeah? or spun glass or things like that. they're all what? they are things that hold air. see these thermal underwear they have now? the thermal underwear-- you look at a thermal underwear, and there's like a net, li fishnet.

and you put that on and that's gonna keep you warm? but over that fisht underwear, you put a regular t-shirt. and now what's trapped between your skin and that t-shirt? air. it begins with a "a." air. all right. it ends with r. air. air. okay. and n that air circulate? now if the air warmed up and just went away, then you get cold again, see? but it's all helthere by these little netting. that's what it is. when that stuff first came out, i looked at that and said, "what?" 'cause, you know, "this is gonna keep warm, honey." and they show you something with all these holes. [laughter] all these holes? that doesn't sound right. but i--wait a minute, wait a minute. i said, "i know wh that is." it begins with a p. i mean, begins with an f." what is it? physics. oh, you don't be kwin'? physics. [laughte oh, yeah. yeah. let's talk about convection. convection, the other form of heatransfer. first, conduction then convection. convection is what, gang? convection is the moving of fluids. for example, if you at part of your thtub up, pretty soon this part is hot, too.

a little conduction going on there, but mostly the water is kinda moving around in currents. and so when you carry heat from one place to another by virtue of currents in fluid, we just have a name for that, convecon. and it has an enormous-- a lot to do with the climates of the world. isn't that true? convection so we get convection currents. you he that term a lot. it turns out that warm air, when warm air rises, warm air rises-- somethin' happens to the warm aiwhen it rises. do y know, guys, do you know why that warm does re, by the way? if you're painti a ceiling and you're all the way at the top paintin', you' find out it's a lot warmer at the top of that ladder than if you're laying tile on the floor. then someone says, "how come it's so warm up there?" "oh, it's warm up there because warm air rises." somebody say to you, "well, how come warm air rises?" and you, "well, it's--warm air is less densthan the other air so it rises." well, somebody say, "how come it's less dee, it rises?" "well, because it's less dense." [laughter]

"well, come on, come on. why does less dense aigo up?" "well, there's probably no reasofor that. it's just a rule, man. it's a rule, okay?" less--why does less dense air rise, gang? it expands. and you know why and why? because-- if a bag of air goes up and-- take air anput it in a great, big plastic baggie. now can you see it? where's the greater air pressure? in theottom of that baggie or on the top? top. bottom. on the bottom. and if that air is expanded, what happens to its density? less dse. so a greater density air from below will push on that pocket of harder than the air above, and it will be buoyed, right? it should go up, up, up, up. but, you kno we can understand this from another point of view, and that's thinking small. and let's think small. you got one molecule. and let's suppose that molecule is hot and it stays hot. to say that one molecule is hot is to sa it's moving around faster th the molecules around it. now, here's a thought experiment if you have a molecule that moves faster than all the others and you better go here--

[makes sound] ain't gonna hit all these other atoms, right? it gonna hit. why would start to migrate upward? you know it would. before you know it, it will keep going up, up, , up and all this hits, hits, hits. it keeps going up, up, up. it doesn't go down, it only goes up. doesn't only go up, but on the average it goes up. can u be thinking of a reason why? check ur neighr. see if your neigor can think of a reason why. okay, gang. let's look athis, thinking sma, why is it that one molecule moving faster than any other d, by the way, a heliumtom will do that. if you take a helium atom and let it go in this room, it'll continually go faster than any other molecule. does anyone know w? why would helium moles move faster? you guys know any helium in atmosphe find its way at the top? 's going faster. helium is the fastest moving molecule of any group of molecules around.

in fact, helium stays ithe atom form, just one atom. don't gang up into molecules, and a helium atom is very fast. and let's see if we can see why. let's suppose we have some helium gas in e room here. wod that have the same temperature as the air in the room? let's have a helium balloon here d it's in contact with the air in the room. won't itome to the same temperature? yes, it ll. e air in this room all s the me temperature. that doesn't mean every mocule is going exactly as fast as the other. it means on the avage they are. the average kinetic energy r molecule is the same for everything that has the samteerature. so helium atoms move faster on the average than others because they are... smaller. let's use the equation to guide our thinking. remember we talked about kinetic energy before, gang. let's get in a little physics here. now, this is kinda-- this is a little deep physics. that's t expression, kinetic energy.

that is also proportional to temperature. to say two volumes of gas have the same temperature is to say those two volumes of gas have the same kinetic energy for their molecules. so the kinetic energy is the same. now, a helium atom has a very, very tiny mass and compared to the other maes ke thexygen and nitrogen they have large mass. so i could put the oxygen and nitrogen like that. and if that has the same energy as the helium, but the helium has only a little mass, so what must be the end of the story, gang? check your neighbor. what's gonna happen? if this is gonna be the same as this, what's the v gonna be? so little things that have the same kinetic energy as big things must necessarily be going faster. i can put it to you this way. let's suppose there's a mouse and an elephant running down the street and i tell you that the mouse and the elephant have the same kinetic energy.

now, to say they have the same kinetic energy iso say when they hit the barn door, they'll do the same work, the same damage. from the fact that an elephant and a mouse, honey, running down, doing the same damage and hit the barn door, is that enough information for you to say which is going faster? now, which one's going faster? that mouse is a bullet, okay? it'll have to be like a bullet too the same damage. so it turns out little things will go faster than big things if they have the same energy. that tells you a lot, gang. that tells you why there's no helium. why you go-- you don't get helium. all the helium atoms are going so fast, they're going at escape speed. they escape the earth. and what helium there is in the very, very top, 's all gone. it goes out. the heliumou get in your balloons, that comes from under the ground when they' mining gas, they get helium in there too. later on, we're gonna learn about where that helium comes from. it comes from the radioactive decay of elements ke uranium and radium. that's what it is, that's the alpha particles. so next timeou see a kid walking down the street

with a helium-filled balloon, say, "hey, radioactive decay waste." true or false? ends out true. that's right. that's what is. yeah, alpha particles, slowed down. yeah. - yeah, really? - yeahreally. yeah. it turns out whenhat warm air rises-- and it rises, by the way, because--i didn't mention that. it rises because it in migrations. every time it happens to be going down, does it see a lot of atoms a little bit? a lot to bounce off. when it happens to be going up, does it see a lot or a little? not quite so many as down. so when it's banging-- [makes sound] won't it finally bumble to the regions of le pressure? and then go a little further-- [makes sound] bounce and comes back down, there's more underneath. so zillions and zillions bounces per second, you know, [mes sound] find its way right to the top. just like if you had a whole lot of people in the room and they're all dancing around to something and they're all crowed to one end of the room and you put some drunk in the middle who moves faster than anyone else. [laughter]

and that drunk is out of it and you wanna go find that drunk-- anyou look in the part of the room with a very, very few people. that drunk will just bumble right to the region of less opposition, very, very improbable he'd bumble to the-- where it's crowded 'cause every time he bump into a bunch of persons, they'd go further this way. and he'd keep, keep finding-- bounding, bounding, back, back, same with the mocule in the room. you know-- [makes sound] finally find itself at the top if it's faster moving than the others, see? but air doesn't do that and r cools off. when you light smoke, you light a fire, you see the smoke go up, yeah? but the smoke esn't keep going forever. what's the smoke finally do? it finally settles off. and what's going on there? the smoke molecules have lost their energy. they've bumped into the other things. and all those bumping, they kind of slow down. and when they're going no faer than the other there, they'll just take on with that air, see? but helium will never slow down as slow as the other atoms because it's got the same energy. it's g more speed. that make sense? you got some physics today, huh? so that's why helium is something that--huh? okay. that's not, by the way,

why a helium-filled balloon goes up. the helium-filled balloon, it's all trapped. the helium-filled balloon goes up for a different reason: 'cause there's more pressure in the bottom than the top and the buoyancy is bigger than the weight 'cause it's very, very low density, see? that a different idea, see? we're talking about now one little atom. doesn't make any sense to say one little atom being void, okay? it does get hit more from the below than above. but so does in every other kind of am, but this kind of atom is moving so fast it migtes more than others and--fd itself out there very, very quick. i wanna show you guys something really dramatic. do you ever wonder when you takyour friends, you go to the motaintops and you go to the mountainps and up the the temperature is what? - cold. - it's cold. now, the mountaintops are closer to the sun. so your friend says, "gee, i sewe're closer to the sun. we shoulbe a little warmer

and the higher you climb, the colder it gets. did you ever wonder why? is tre anyone here that ev went up in the mountains and they get cold and didn't wonder why? i waa see what you look like. stand up. everybody wonderhy? yeah, we wonder how come it'sold up there, okay, and why is that? it's because when that rising air goes up, what ds it do when it gets to regions of ls and less pressure? cools. cools, but why does it cool? begin with a x. - expand. - expands. it expands just like a heliumilled balloon would expand, wouldn't it? and so whethe air expands, then the air cools. hc i got a demonstration foyou all to try right there at your seats. are you ready? everyone have a hand?

put yourand in front of your mouth and with your mouth open, so when you breath-- so the air doesn expand, i want you to blow. a little disruption right there. okay now bring your mouth down really tight, so the air expands when it comes out. notice any difference in temperature? yeah. try it again, gang. open mouth. open mouth. now close mouth, so the air expands. okay, you wanna see that more dramatically? watch this. here's some steam here, huh.