Okay, what are we studying about today . Atmospheric pressure in the air, right . What if we werent studying about atmospheric pressure . Isnt it common to say, oh, there is nothing in the glass. Then you take the glass and you say, there is nothing. And you can show a little ball, floats. Look at the air in the glass gang. Can you see evidence of the air in the glass . See that. There is a little ball, its floating on the surface and the surface of the water goes do because something pushed the surface of the water down. Begin with an a, end with a r, try it. The air, and there is air in the glass. Yeah, there is air in the glass gang. Any air in this room . Isnt it nice that there is air in the room. What if there is no air in the room . Well not survive. You would be trouble. Have you wondered about that like youre breathing and you really need the air in your nighttime, youre breathing all of a sudden you think, wait a minute, if all the air molecules that are right here all of a sudden goes somewhere else and i go to take a breath and there is nothing there. Honey, youre gonna go out, youre gonna check out. Arent you glad the air is all around how come the air stays near where your mouth is . Because its being pushed by a pressure and that pressure is due to the weight of air above us. Air has any weight . It turns out air has weight, a lot of weight if you talk about a lot of air, okay . Whats it called here . One cubic meter of air, its not like this, huh, its like a lot like this. Bambambam, one cubic meter of air, like a little small truckload gang, has one and a quarter kilograms, okay . Thats about 23 4 pounds. So try this tonight when you go home. Open up the fridge. Now your fridge is about threequarters of a cubic meter. Ask the people at the house, hey, the air inside there have any weight . What are they going to say . Yeah, its got some weight. See that big grapefruit sitting there on the shelf, does that have any weight . Yeah. Say which weight is more, the grapefruit or the air . Take a guess. Youve got about 2 pounds of air in there. And if that grapefruit is less than a 2pounder, then the weight of air in that refrigerator is greater than the weight of so greater than the weight of a dozen of eggs, dozen of eggs weigh less than the air in an ordinarysized refrigerator. Now youre not noticing that weight of air. Let me ask you a similar question. Does water have a weight . Yeah, water kind of weighs a lot, doesnt it . If i handed you a great big bag of water and i say, here, take this. And i put it in your lap, vrooom. And i ask you, does water have any weight . What are you gonna say . You are gonna say, yeah, its got weight. Now lets suppose we go in a Swimming Pool and youre under the water. And under the water i take that baggy and i hand you that baggy of water and you grab it and i say to you, does the water have any weight . And youre gonna say, if it does, i cant feel it. What does a fish know about the weight of water . Nothing. Fish knows nothing about water fish doesnt know what its like not to have water, okay . Were the same way. We walk through the air all our lives, and we just take it for granted. We dont realize its there, and we dont realize its heavy. In your kid sisters room the air in your kid sisters room has more mass than your kid sister does, if she has an ordinarysized bedroom. Air is quite heavy, one and a quarter kilos for every cubic meter. Got quite a few cubic meters, youve got more than a ton of air in an airplane to keep it pressurized. So air is kinda heavy, relatively heavy. And we dont notice the weight because why . Were in it. Its like the fish doesnt know the weight of water. Now air has air has pressure too and air has pressure because of the weight. Just look at the world. Here is the whole world here, okay . Im going to draw the top of the atmosphere, how high should i go . You know what gang, the top of thetmosphere is within the line that i drew. Because the air only goes up about 30 kilometers, and this is over 6,000 kilometers. So, 30 parts in 6,000. Gang, thats withi the thickness of the truck, all right . So it turns out there is a very, very thin, thin vapor of air hugging the world and thats our atmosphere. Its not inexhaustible. Take a pool ball and put it in a fridge, huh. Let it cool. Open up the fridge and say huh, breathe on it. You get a little mist on it, right . You know how thick that mist is . About the proportion of the thickness of our atmosphere. Weve a very very thin vapor 30 kilometers, not very much, you know, compared to the world 6,000 kilometers radially. So it turns out that that thin vapor of air nevertheless has a weight and pushes down on us. It pushes down on us with a pressure. Ill show you an example of atmospheric pressure. See these cans . I put a little water in them, the water is boiling. And so, are there fewer molecules in there now than before or more . Well, you can see em coming out, stuff must be going out. So it must be that the pressure inside there is now realized by fewer molecules moving faster. Does that make sense . A few molecules moving very fast can exert as much pressure as a greater number of molecules moving slowly. So im reducing the number of molecules in there. What if i cool this very suddenly . Shall we try it again . What happened to these cans, gang . Lets try one more time. Look at these cans. Some people say, oh, the cans are sucked in. Cans aint sucked in. Whats happening to those cans . Check your neighbor, whats happening to those cans . Now when you try that tonight, gang, youre gonna want some explanation. A nice explanation will wait until we get up to heat and get up to condensation. When we get up to the idea that when steam condenses, okay, it loses volume. But for now, a first order explanation would just be that in here there is heated air. And heated air means not so many molecules. But the pressure inside the can is actually a little bit more than the pressure outside. Can you see evidence of that . See the steam coming out. So the pressure is building up because theyre moving fast, fast, fast, yeah . What i do is i remove it from the heat, put it over here and cool it. And when i cool it what happens . The pressure inside goes down. How about the pressure outside . Some people say, oh, the pressure outside must have all of a sudden increased. And you would say, no, it didnt all of a sudden increase, its what it is all the time. And that humongous pressure pushed the can in because there wasnt the same pressure inside to hold it out again. Were gonna come back to these cans again when we talk about condensation, water changing to vapor and vapor changing back to water, thatthose ideas. But nevertheless, there is an enormous pressure due to the atmosphere and i think you can see that now. Isnt nice that you guys have air inside you . What happens all of a sudden someone takes the air outside of you . What are you what are you goi to look like . Here you are, here gang. Isnt it nice youve got the air inside you too, yeah, okay . Okay. If i had a bamboo pole, a bamboo pole right here and it went up about 30 kilometers and that bamboo pole was one square centimeter in area. And i considered the mass of all the air that would fill up that bamboo pole, yoknow, what all of that mass would be . It would be one kilogram, one square centimeter up about 30 kilometers will be one kilogram of air. So that 1 kilogram pushes down with a weight. And it pushes down with a weight of 10 newtons. So it turns out the pressure due to the atmosphere here at sea level is 10 neons for every square centimeter. And were talking about si units, we talk about square meters. Its a little unwieldy, but it turns out its about 10 to the fifth square ceimeters and one square meter, and it turns out if you have one square meter, think of a great big sewer pipe about one square meter in crosssection. And that sewer pipe goes up 30 kilometers, you have about 10 to the fifth newtons pushing down and thats the atmospheric pressure, 10 to the fifth newtons per square meter and thats due to the weight of air above. That being the case, ive got a question that you can answer. If you climb the top of a mountain and then measure the atmospheric pressure, would it be less, the same or a little bit more . Youll be checking the neighbor. Hey gang, lets suppose you had a pet lobster and the pet lobsters way down there in the deep brine there and the pet lobster says, you know what . Its just too much water pressure. And someone says, well, why dont you climb that ledge. When you get closer to the surface, the water pressure will be a little bit less. Will that be a true statement or a false statement . True. It would be a true statement. And where do we live in . We live in an ocean of air. So weve got a pressure acting down on us. So when we climb a mountain top, are we getting closer to the surface . Now there isnt a sharp sharp surface with the air like there is with the water. It kind of peters up and peters out, doesnt it . In fact, you guys know if you go about 3. 5 kilometers high, is it 3. 5 kilometers high . Its in the textbook, i forget the figures. Yeah, 5. 6 kilometers high gang. Youve got half the air underneath you. And you go up to what is it, 30 kilometers high, okay . Thats about 19 miles, 30 kilometers high, 99 of all of the air is underneath you. So you went flying in an airplane and the airplane pilot says, hey, were at 40 kilometers high. Honey, he has made a mistake. Youve got nothing to ride on up there, right . The wings arent going to ride in the air. Youll run out of air. The air doesnt go up that far. This is kind of a neat little deal here. This is called the unicorn experiment. Little bottle, piece of paper. Match. Light the paper on fire, fire burning, i cut off the oxygen and when i do that. Follow me gang, ill make your crops grow. Whats going on here, gang . Oh gosh, well, this is my new style. Ill see you guys down the wave tonight, all right . [laughter] a little tattoo there. Whats going on here, gang . Ahah. The heated air cooled and when the heated air cooled what happened to the the pressure inside . It went down. How about the pressure outside. Same. But why does it hold to my head . Why . How many people say, well, it has something to do with air, but there is probably no reason for that, huh. Its sort of like this. One of these devices here. You know what this is, this is a physics experiment, right . How come its sticking, gang . It looks like we all at times gonna have gonnhave one of these, gang. Are you looking whats going on there, huh . Why . How many say, oh, there is probably no ason for that. Isnt the air pressure pushing against the side there . Isnt the air pressure inside pushing back out . But not as much. Because i squashed some of the air out, and when i squashed the air out, it starts ce back, it reduces the pressure inside. So i have reduced pressure inside, but atmospheric pressure outside. So i have more pressure pushing here than in there and so it stays. Isnt that neat . You could explain this to someone. Everyone knows this will happen. Someone said, oh, its because of suction. All right, okay, its because of suction. Now explain the suction. And we can do it, cant we . More pressure on one side than the other end, huh . Yeah. So a toilet plunger makes a nice suction cup gang, but you dont have to have a toilet plunger because this is like a suction cup too. You know what this is . Its just a regular piece of rubber, a piece of floppy rubber and ive got a hook on it. Im going to put it on the stool, im going to lift it up, im going to lift it up. Would you lift it up, please . Lift it up by the hook. Yeahhh, all right, like a suction cup gang. Whats happening there . When this pulls up, can you be explaining such a thing to your friend . Yeahhh. Glass filled with water, yeah . Okay, where is my cards . Take a card, uhhuh. Okay, lookt that. No probl, no problem. Still no problem. Somebody said, wait a minute, int that wate aint that water pushing down with the pressure. Yeah. Well, dont you know that pressure pushed the card away . No and your neighbor says say, if there was a little air bubble on that, and it still holds . Yeah, what if there is air in there, gang . Does that help it to stay or not help it to stay . Not helps it to stay. Lets try it with the like air in that, okay. When it fills with water, yeah. Now, some people would say, why doesnt the water pressure push the card away . And your neighbor said, because the air pressure pushing up on the card. See, lets look at this on the board. There is water pressure pushing down like that, but there is an air pressure pushing here, air pressure pushing here, air pressure pushing here. Im denoting pressure with these forces that would really be the force due to the air pressure, you kind of see that. But there is more up than down. Because in here that water pressure is not pushing down as hard as 30 kilometers of air. Thirty kilometers of air pushing down a lot harder on that card than the water. Now i would have to make the water taller. Some of you people know, how tall a water column would it have to be to push down just as hard as 30 kilometers of air. Some of you people know because youve read the text. Check the neighbor and see if your neighbor knows. How many meters high of water will push down just as hard as 30 kilometers of air . Okay, gang. Whats the answer . How tall a water column would you have to have to push down just as hard as the atmospheric pressure is pushing down . 10. 3. 10. 3 meters, thats right. 10. 3 meters, thats about 30 feet, about 30 feet high. If you have a 30foot high column or 10. 3meter column of water thats going to push down just as hard and that card will not stay there, itll push away. Or if i have some air pressure back here, if i have atmospheric pressure back here, that atmospheric pressure plus this will push that away. And once this starts to come down and reduces the pressure enough such that it wont sometimes you can get an air column and a liquid in there. Thats where the card starts to pucker, air pressure reduces and yostill have less pressure pushing down than you have here. Then, of course, there is a little surface tension that will help you in there too. But its kind of neat, how that works. Now if you want to make an instrument to measure air pressure, you could have one of these 10meter high things of water. But better than use water, its better to use something more dense, so your height wouldnt have to be so high. And whats the densest liquid we know of, gang . Mercury. Mercury is 13. 6 times as dense as water. That means the column of mercury would only have to be 1 13. 6 as high. And 1 13. 6 of 10. 3 meters turns out to be 0. 76 meters, turns out to be 76 centimeters, turns out to be 760 millimeters. And so when youre talking about atmospheric pressure in terms of millimeters of mercury thats what youre talking about. See because if you look at the if you have like all you have to do is take a dish of mercury and that mercury will glug right out there and when that mercury column. Is 760 millimeters tall, its about like this, huh, okay . When its that tall, thats going to push down on the surface with just as much pressure as the atmosphere outside and so itll stand right there. Now let me ask you this commonsense question. What if the atmospheric pressure increases, whats going to happen to the mercury . Its gonna push it right up further, isnt it . Its going to push it up further until its pushing down with the same pressure that the atmospheric pressure is exerting, isnt that true . And so what would happen if you took this barometer, this is a barometer, now right . Lets suppose you put it in an elevator and took a ride to the top of a skyscraper. Whats going to happen to the reading . Its going to do down. You guys ever feeling some of these skyscrapers going up, you can feel the reduced pressure, you really can and that barometer would pick it up. And so the barometer would fall the higher you got because there is less air up there, less air pressure. It kind of makes sense, doesnt it . Whats above the column of mercury . Question. Whats above the column of mercury . Begins with a z. Zip. Zip, nothing, a vacuum. Maybe a little bit of mercury vapor, but in the sense nothing. There is no air up there. See, because you would make this thing by filling it up completely with air. I should have mentioned that. I mean, completely you would make this by filling it complete with mercury, dip it over and the mercury will gogogogogogoglug out and whats on the top, nothing, a void. So that would be like a vacuum, maybe a little mercury vapor, but certainly not air. Because how does the air get there . Can you see that, gang . I mean, i could take this. Suppose this was 10. 3 meters tall, i bring up like that and all of a sudden it would start to drop. Seesuppose 10. 3 meters here, then there would be nothing there. So itll be a void. Maybe a little water vapor, yeah, but there woulbe nothing, wouldnt be like this air there, there is nothing oh, i am uhhuh. You see that, yeah. This is kind of nice, youve seen people do this. Is this a big deal that i can pour water from one container to the other . How many people say, wow, look at that. Oh man, poured the water from one container, am i am i glad i came today, huh . Thats no big deal. Do you want to see something this kind of a big deal gang . Here is what i want you to do tonight. Take a glass of water, tip it over, take a glass of air and pour the air into the water and aint that nice, huhhuh . Hey, you like that . Youre glad you came, huh . Is physics fun . Physics, yumyum or yukyuk. It can be yukyuk, but it can be yumyum too, yeah. Do you ever wonder about how they build how they pour concrete down under the river to make the bridge . Do you ever wonder about that . I mean, you see these bridges in the water, right . And down its all con how do they get the concrete underneath there . They have workmen go down there, do you know what they do . They have a great big thing like this, okay . Just little tiny guys like this, and they bring it down, down, down like that, right down to the bottom and they are the guys who are down there working. But you know what, as i keep pushing this down, down, down, it turns out the pressure gets squashed, not the pressure, the air gets squashed and it starts to get more and more compressed. This is not deep enough that you can really see the difference. But if i push it 10. 3 meters down, then the water pressure would be just equal to the air pressure at the top, so youll have twice the total pressure, 10. 3 meters under and thats going to squash the air up to half size. And that brings us up to boyles law. Boyles law named after a dude by the name of robert j. Law, okay, were learning these things, all right . And boyles law just says that pressure multiplied by volume at any one point will equal pressure times volume at another point. So get twice the pressure, youll get twice the volume. Youll have half the volume. Yeah, what i told twice the pressure will be half the volume. Very good, lee, sometimes i goof a rooney a little bit, right gang, but does that make sense gang . So you get twice you get twice the pressure thats going to squash that stuff up. So the volume will be half as much, huh . What if you pushso youve got three times the pressure, then the volume will get squashed up to how much . A third. How abouti dont know if you can do this. Let me try, seven times the pressure, okay, okay, now i shouldnt say seven, now lets say five times the pressure, a fifth right . How about nine times how aboutall right, here is for the a students, 7. 9 times the pressure. Got you, got you, got you, got you, got