we can also relate this to variations in Earth's atmosphere and weather are also related to pressure and gases and we're just just we're thinking about breathing I think about wind where does wind come from yeah it's uneven heating of the Earth's surface which causes differences in pressure of the air so if we look at variations in the atmospheric pressure I just said that when we breathe we reduce the pressure in our lungs and the external pressure is constant well it's relatively constant so if you watch weather reports they'll often pull up maps like this and they'll talk about high pressure and low pressure systems right so high pressure an elevated atmospheric pressure is usually associated with clear weather and low pressure is usually associated with unstable weather and so it's these differences in pressure in different parts of the earth that caused a lot of our weather and cause wind we've got this little digital weather station here in the classroom if you're you know come up here and look at it today the pressure is thirty point two nine inches of mercury and so that is kind of medium but then it'll give us a little here it's got sort of a partly cloudy to cloudy little icons here well how does it predict how does this little thing predict the wetter weather is it talking to a guy at Casey 24 now it's looking at changes in barometric pressure atmospheric pressure and so if the pressure is high it's gonna predict that things are gonna be less stable and if it's low it's going to predict more likely you know sunny we also see variations in the atmospheric pressure as you move above the Earth's surface so the number of gas particles in a given volume the concentration of the gas if you will decreases with increasing altitude we're talking about gases we usually talk about pressure rather than a concentration but those things are related to each other as you go up the pressure the atmospheric pressure decreases so the pressure of a gas is related to how many gas particles are in a given volume we just got done talking about molarity right the moles of a substance per liter of solution so if we look at these two jars with gases in them could we calculate the molarity of the gas how many moles of gas in a liter of the gas we don't usually think of it that way but that's really what we're talking about this has more gas particles in the same volume so this is a higher concentration the result is also a higher pressure because each of these gas particles is hitting the walls of the container sometimes I like to think about gas particles as being a bit like kindergarten children if the teachers not libram they tend to kind of run around and bump into things right if we have a few children in a room there's going to be a certain number of collisions if we increase the number of students in that same area volume of the room there are going to be more collisions right and that results in more pressure for a gas rummy yes they are directly proportional another real-life effect of gas pressure and atmospheric pressure is what happens in your ears when you go up in an airplane or when you go up in the mountains so I took like we took the kids up to the snow a couple of weeks ago when there still was snow and as you're driving up that hill to Grant Grove your ears pop right what why do they do that has to do with gas pressure so here's an illustration of your middle ear you have an eardrum and then on the inside there's also a cavity that has air in it and that is connected into the back of your throat by this little tube here called the eustachian tube so normally the pressure inside and the inner ear and your middle ear is equal and so your eardrums all happy and it vibrates and you can hear very nicely so as you go up the hill and the air pressure outside decreases the internal pressure of your ear does not change as easily because that eustachian tube is small and you know really do you want soup getting up into your ear no you don't want stuff getting up there so it's good that it's that way but then the pressure inside is higher and the pressure outside is lower and that causes your eardrum to bulge out which normally is just sort of a mild the uncomfortable feeling and then you yawn or swallow and you hear that popping sound which is air leaving your inner ear and then everything's okay again now if you've got a bad cold and you're really congested or if you have an ear infection in your ear hurts anyway that pressure imbalance can be excruciating and you're probably aware that babies tend to cry on airplanes which is extra frustrating because you're all in this like sardine can and you're stuck with somebody else's screaming baby right babies cry because their ears hurt not because they're trying to make you miserable babies have various even smaller eustachian tubes and so it's harder for their ears to balance when you come back down the reverse happens now the pressure outside becomes greater and it causes your eardrum to bulge the other way and then again you have to get your ear to pop and let gas let air into your inner ear we call that ears popping because it's kind of what it sounds like anybody have any questions about that the reason that chewing gum are yawning or swallowing can help is because that that movement in your neck area helps to open that up and let the air go in or out how do we measure pressure well we have a lot of modern tools for measuring pressure but we're going to look at the original pressure measurement device the mercury barometer so the original mercury barometer was a very simple device it was a long glass tube filled with liquid mercury that was inverted into a dish of mercury now if this tube was open at the end the mercury in here of course would run out but this is closed and if you played with water and cups in the bathtub as a child you know that if you have a cup of water you have a cup under the surface of the water and you turn it upside down and you lift it up without letting the lip come out of the water the water stays in the cup right this is kind of cool as a child you're like ooh this is fun and then when you lift it up out of the water all the water rushes out well you were essentially making a very crude barometer in the bathtub so the reason that liquid mercury is used is because it is much more dense than water when you played with this in the bathtub you didn't see any what appears to be air forming up here this is actually not air it's a vacuum and so here we have a column of mercury and why does it stay up there because atmospheric pressure is pushing down on the mercury on the outside atmospheric pressure pushing down supports the column of mercury here because in order for this to fall out this liquid has to come up and the atmospheric pressure is holding it down so Torricelli was the guy who first discovered this and he found that if he took this device and went up a hill with it mountain that the column of mercury would get shorter and shorter and then if he came back down it would go back up again and so he realized that this the height of the column of mercury was related to the pressure of the air and so this is measured it can be measured in inches 29.92 inches like the 30 was a 30 point it was 30 point 2 1 now inches of mercury that this barometer is giving us or 760 millimeters of mercury and that is literally measuring the height of this column of mercury with a ruler so the millimeter of mercury is a pressure unit average atmospheric pressure at sea level is 760 millimeters of mercury so as you go up in elevation the pressure will decrease the height of the column of mercury will decrease the unit millimetre of mercury has also been given the name tor in honor of Torah Shelly so one Torr is the same as a millimeter of mercury because the length of an element seems like a really weird pressure unit right and I kind of think they intended that tor would replace millimeters of mercury but instead we now just use both of them so that just added two things there of course our other pressure units atmosphere an atmosphere is average atomic pressure I'm sorry average is atmospheric pressure at sea level so an atmosphere is radiated ATM 1 atm is the same as 760 Torr so the way this table works is that all of these numbers these measurements here are equal to each other 1 Torr sorry 1 atmosphere is the same as 29.92 inches of mercury 760 Torr 14.7 PSI which is pounds per square inch force I'm sorry pressure equals force divided by area pounds per square inch makes sense when you think of it this way because a pound is actually the force due to gravity divided by square inches that's the area pounds per square inch the SI unit of pressure is the Pascal which is a Newton per square meter Esprit VA today a that's a much smaller unit so a hundred and one thousand three hundred 25 Pascal's is equal to one atmosphere the one that you should the ones that you should remember is the 760 Torr in one atmosphere I'll actually give that to you on an exam but we're going to do that conversion so many times that it'll save you a lot of time so 760 Torr or 760 millimeters of mercury is how high the column of mercury is and an atmosphere it's like it's one atmosphere of pressure right so are you gonna be able to walk down the street and find a place where there's three atmospheres of pressure no the variation in atmospheric pressure at a given elevation is really quite small so a lot of times we'll say about one atmosphere and that's close enough so let's look at how to convert pressure units and this is dimensional analysis again just with some different relationships so your local weather report announces that the barometric pressure is thirty point four four inches of mercury convert this pressure to psi the nice thing about how the pressure units are given to us in the textbook is that we can always do this in one step so I don't need to go from well these are next to each other but if I had an atmosphere I don't have to go through Pascal's and through PSI two inches of mercury these are all equal to each other so I just pick out the two that I need and I can do the conversion so thirty point four four inches of mercury convert to psi I need one term I want my new unit on the top and my old unit on the bottom and then I'm going to go to the table and this is how I'll give it to you on an exam and I'm just going to put the numbers in 29.92 inches of mercury is equal to fourteen point seven psi so my calculator as often as the case is giving me a whole mess of digits how many of those should I keep yeah three so this 14.7 is not an exact relationship the 760 is exact the rest of these will this 760 of course but 29.92 not exact 100 and 1325 not exactly so for sig figs three and four this should have three sig figs and so I should report this as 15 point zero psi it bothers me that the book uses fourteen point seven instead of fourteen point six nine I don't know why he did that any questions another simple device for measuring pressure is called the manometer if you look just look at the word the man o meter does this measure men no it doesn't I'm not sure where that came from but the manometer is used to measure the pressure of a gas inside of a container and again this is a very very low-tech way you don't need electricity or anything you just need some glass tubing and some mercury and a ruler so the manometer is this part right here here we have gas at a high pressure or inside this container and so we connect that to the manometer and there's liquid mercury in the manometer and so the pressure of the gas is pushing down on this side an atmospheric pressure is pushing down on this side if the inside was the same as the atmospheric pressure then the levels of the mercury would be equal but here this gas is pushing down this is at a higher pressure than atmospheric pressure this does not tell us directly the pressure inside but it gives us the pressure inside the container relative to outside so we can measure the difference between this column of mercury and that column of mercury H and that's the difference in pressure between inside and outside of the container if we know what atmospheric pressure is by using a barometer then we can figure out what the pressure is inside so why do we use mercury instead of water it's a lot more dense so if we go back to this barometer one an atmosphere of pressure will support a column of mercury that's 760 millimeters about this one about that long if I made this barometer with water it would be 30 feet tall that's not very practical is it wouldn't fit in their room so mercury is really useful for that fact only a few years ago each of the classrooms had a mercury barometer on the wall whose mercury in the classroom fine but you know people freaked out about it so let's take those down and now we have boring digital weather station