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, mind.

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 cay that eney ght along. so somthings conduct betr than other things. piece of wood, very, very good insulator. this wood is a good insulator at any tempetu. this wood is good insulator even if it's very, very hot. yep. did you ever reach in a hot oven and grabbed on to a frying fan that's about maybe 400 degrees fahrenheit? you're gonna cook someood or something, you got a frying pan in there that's all in. ever be reaching there with your bare hand angrabbed the iron pan?

if you ever had, you can show that today, you'd be tattooed, all right? you're gonna burn yourself. isn't that rht? but could you reach into that oven and grab a pan if it had a wooden handle 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 wood to your hand, so you can safely hold it. you can do 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 over matter. "that $300 you've spent for that 15-minute session "is gonna pay off ecause wre gonna apply those techniques "and we' gonna show that you can walwith bare feet on hot coals." and so the people do that. they take off their shoes, 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 very hot, isn't it? w much of that heat in that wood is gonna get to thfeet? what does it depenup? not only the temperature of the wood but somethinelse. begin with a c, end with onductivity. see if you can put it together. what is it, gang? 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 step with your barefoot on that 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, a little, little heat. on a little bit of heat will get to your foo and yocan-- no, you're not gonna stand on one place there, okay? you're not gonna reach in that oven and you're hanging on that wooden hane. honey, you're gonna be hurt. but you can grab it and pull it out. boom. and you can step, step, step, step across red hot coals of wood without harm. it's even bett if ur 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, if your mother doesn't care vermuch of you, she says, "touch it and see." but if your mother really loves you, she says, "before you touch it, be wetting your hand." [makes sounds] right? [makes sounds] that little sound? [makes sounds] what's that little sound? we're gettg a chapter ahead. you guys don't know about this. it tur to steam. how ma say, "oh,on'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 energy that did not go to your ha. so it'better if you walk across those coals and you got wet feet beuse then it'll be even fer. so a homework assignment this weekend. every one take off their shoes and socks, get some hot coals, pour theall over your front lawn and get out there and do it, all right? okay? actually, me peoe 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 tt, all right?

you're sitting in that tables. of your table is metal and part of your table is wood. i want you to all put your hand on the wooden pa. 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 better conductor. your body temperature is highe than the temperature of everything se in this room. and temperature is hher over your hand than that which you touch.

ng, there's a heat flo an energy flow that we call heat. it goes from high temperature to low. and when you did that, when you touched the wood, there's a heat difference 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 nd when you touch the metal? a lot or a little? a lot. the other l, right? a lot. and has to do with the conductivity. it kinda makes sense, that stuff, doesn't it, conductivity? kinda neat stuff. hey, il show you something kinda nice forhat. here's a piece oiron. 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 gonna put a piece of paper around this iron. i'm gonna light up my torch again.

paper doesn't light on 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. call the fire marshal. but what's gng on here, gang? well, let's try it with od let's try it with wood. our test. be checkin' the neighbor and see if the neighboknow why that paper didn't light. did yoever read the book by ray bradbury call fahrenheit 451? yes. that 451 is what? that's the temperature in frenheideees at which paper burns. that was about a book-burning book, yeah? watch this, gang. fire city. that thing iburning up. look at that all right.kay.

why with the wood but not withhe 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 ch was lt in the paper? none. and before i can get the paper up to 451, i've got to make that very, very good conductor ba there at 451, too. and that takes a lot and lot of energy to do. and so you didn't see the paper ignite. it never got to 451. this is considerably more than 451 degrees, considerably more. when i wrapped it around the wood, lookhe wood is all scarred now. look at that. you saw the paper light up. why? because i dn'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 meta all the heat is conducted all throh here. les 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 a all, no flames at all, gang. [laughter] except the edge, okay? but i can st about bring that to a boil. and yet, if i take a cup like this--oy. any fire mshals around? okay. you kia get the idea. ain't that nice? anthis over here, it turns out just the edge you see there, the edge does not in touch wi the water. but the same type thing is happening here. not so much beuse of the conductivity of the water. water rns out to be not a good conductor.

but something else is going on here, gang. and what is it? transfer. oh, i can read your minds. i can re your minds. [laughter] you're all saying, "but, gang, you're taking a fluid which has an enormous specic heat, hewitt." and if you wanna increase the temperature of that, honey, you gotta 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? paper. 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, get in there, son of a gun. uh-oh. okay. see that hunof ice down there, gang? and i'm goa 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 gonna do is i'm gonna get this going again. and-- [laughter] oh, please. [laughter] i'm nna bring the top to a boil. and the ice cube at the ttom is still intact. look at that, gang. the water is boiling. down below-- do you see the piece of ice? the ice is still in there. ain'that rht, ga? 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 comes, 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 is not 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 melt the ice. [makes sound] interesting, interesting, interesting this world we live in, ye? hmm. it turns out that air is a good conductor or a poor conductor? poor. it turns out it's a poor conductor. and aren't you glad? because what if air were a very, very good conductor? how would you feelll the time? 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 condti

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 around you. so be glad that ai is a poor conductor and a good insulator. in fact, when you wanna keep yourself warm whenou're gonna go campin', that sort of thing, don't you get a down-filled sleeping 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-- hoabout animals? you wonder about the anils. how do they ke it up on the snow-covered mountains? they're coved with fur. and that fur puf up and what-- guess what iholds insi? it begins with "a," ends with r. air. and that air is a very, vy good insulator. so most of the things that keep you very, very warm in a cold climate like that 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, like 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 can that air circulate? now if the air warmed up and just went away, then you get cold again, see? but it's all held there by these little netting. that's what it is. when that stuff first came out, i looked at that and sd, "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 what that is." it begins with a p. i mean, begins with an f." what is it? physics. oh, you don't be knowin'? physics. [laughter] oh, yeah. yeah. let's talk about convection. convection, the other form of heat transfer. first, conduction then convection. convection is what, gang? convection is the moving of fluids. foexample, if you heat part of your thtub up, pretty soon this part is hot, too. a ttle 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, convection. and it has an enormous-- a lot to do with the climates of the world. isn't that true? convection so we get convecon currents. you hear that term a lot. it turns outhat warm air, when warm air rises, warm air rises-- somethin' happens to the warm air when it rises. do you know, guys, do you know why atarm does rise, by the way? if you're paintin' a ceiling and you're all the way at the top paintin', you'll find out it's a lot warme 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 wa up there because warm air rises." somebody say to you, "well, how come warm air rises?" anyou, "well, it's--warm air is less dense than 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 air go 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 a why? because-- if a bag of air goes up and-- take air and put it in a great, g plastic baggie. now can you see it? where's the greater air pressure? in the bottom of that baggie or on the top? top. bottom. on the bottom. and if that air is expanded, what happens to its density? less dense. so a greater density air from below will push on that pocket of ai hard than the air abov and it will be buoyed, right? it should go up, up, up, up. but, you know, we can understand this froanother point of view, and that's thinking small. and let's think small. you got one molecu. and let's suppose that molecule is hot and it stays hot. to say that one molecule is hot is to say it's moving around faster th the molecules around it. now, here's a thought experiment. if you have molecule that moves fasr than all the others and you better go here-- [makes sound] ain't gonna hit all these other atoms, right? it's gonna hit.

whwould start to migrate upward? you know it would. bere 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 you be thinking of a reason why? check your neighr. see if your neighbor can think of a rson why. okay, gang. let's look at this, thinking small, why is it that one molecule moving faster than any other and, by the way, a helium atom will do that. if you take a helium atom and let it go in this room, it'll continually go faster than a other molecule. does anyone know why? why uld helium molules move faster? you guys know any helium in atmosphere find its way at the top? it's going faster. heli is the fastest moving molecule of any group of molecules around. in fact, helium stays in the 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. would that have the same tempeture as the air in the room? let's have a helium balloohere and 'sn contact with their in the room. won't it come to the same temperature? yes, it will. e air in this room all has the same tperature. that doesn't mean every molecule is going exactly as fast as the other. it means on the average they are. the average kinetic energy per molecule is the same for everything that has the samtemperature. so helium atoms move faster on the avera 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 the 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 the oxygen and nitrogen, they have large masses. 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

is to 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 engh 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 to do 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, theye going at escape speed. they escape the earth. and what helium there is in the very, very top, it's all gone. it goes out. the helium you 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 helm comes from. it comes from the radioactive cay of elements ke uranium and radium. that's what its, that's t 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 outrue. that's right. that's what it is. yeah, alpha particles, slowed down. yeah. - yeah, really? - yeah, really. yeah. it turns out whenhat warm air rises-- and it rises, by the way, because--i didn't mention that. itises because it's 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 quitso many as down. so when it's banging-- [makes sound] won't it finally bumble to the regions of less 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 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 se. [laughter] and that drunk is out of it and you wanna go find that drunk-- and you 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'd 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 slodown. and when they're going no faster 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 got more speed. that me 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 thbuoyancy is bigger than the weight 'cause it's very, very low density, see? that's 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 thaabove. but so does in every other kind of atom, but this kind of atom is moving soast it migrates more tn others and--find itself out there very, very quick. i wanna show you guys something really dramatic. do you ever wonder when you take your friends, you go to the motaintops and you go to the mountainps and up there the temperature is what? - cold. - it's cold. now, the mountaintops are closer to the sun. so your friend says, "gee, i sewe're oser to the sun. we shoulbe a little warmer." and the hier you climb, the colder it gets. did you ever wonder why?

is there anyone here that ever went up in the motains and they get cold and didn't wonder y? i wanna see what you look like. stanup. everybody wonderhy? yeah, we wonder how come it's cold 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 less 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 a doesn expand, i want you to blow. a little disruptn right there. okay nobring your mouth down really tight, so the air expands when it cos 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? watcthis. here's some steam here, huh. would i dare to take this glove away? yeah. yeah, easy, easy saying that.