Lesson 10 - Atmospheric Pressure - Properties of Gases - Demonstrations in Physics

how do you do ladies and gentlemen and boys and girls I'm Julia Sumner Miller and physics is my business and our business today has more than ordinary and gentlement and I will go at once to it consider the following I have a tin can with some water in it under which I have put a burner so that the water has now been boiling for a time as evidenced by the water vapor seen here condensed and we call it steam so that what remains in the vessel now is some water less than before and some water vapor now I'm going to take the burner away and stop her up the can and we will witness an astonishing and absolutely astonishing thing if we listen very carefully we will hear a certain flexing of the can what is happening some of the water vapor is condensing the pressure is getting less in the can and the atmosphere is taking hold an atmospheric pressure is an astonishing thing I will hasten the process by pouring some cold water on it and this is an experiment you can do with absolute enchantment yourself here it is I am going to take the burner away I'm going to stop of this up I'm going to put it down here listen oh oh but watch it I'll hasten the process oh there it is let me turn it around there it is and I say that is something to bear witness to that is fantastic the push of the air staggering in its consequences for the human race now listen listen the water is boiling inside that can at reduced pressure at a much lower temperature than 100 degrees centigrade because the pressure inside is less than one atmosphere this is fantastic fantastic let me show it to you another way here I have another tin can and I'm going to connect this tin can to a vacuum pump which permits me to pump out some of the air and just a little of it I could never pump it all out if I pump for a million years I couldn't get it all out why too many molecules in there but could be there it is there it is there it is the push of the air staggering to witness staggering Shore to you another way take a look at another piece of apparatus I have a funnel a funnel and across the top of it is a sheet of rubber tubing as from an automobile inner tubing the air inside is at atmospheric pressure I'm going to connect the vacuum pump here and reduce the pressure inside from atmospheric less and less and less and less and the rubber sheet suffering the forces the push of the air will be depressed inside deeper and deeper and deeper and indeed if it can endure the reduction in pressure and the push of the atmosphere I shall be surprised very likely it will be B be torn apart here it is there's the rubber tube I am going to connect this ah now I'm going to reduce the pressure there it is getting pushed in there it is getting pushed in oh man this is something I'm going to stop it a minute let's get a tight shot if you will of that and notice notice the enormous push of the atmosphere fantastic now I'm going to evacuate more and more and let the push of the air push that more and more and I'm going to step aside because when this does indeed burst it has a detonation which could well unseat this building that'd be very good oh I like to do this once in a while because it adds to the drama of the matter it adds to the drama oh there it went there it went I hadn't expected it really but notice the enormous forces of the air how enormous well only about 15 pounds per square inch at sea level let me remind you of the consequences the ordinary human being has about 3,000 square inches of surface 3000 square inches of surface 3000 square inches if you skinned an ordinary human being laid out the skin and measured the projected area be about that on every square inch that's about 15 pounds 45,000 pounds is the push of the air on me 40 over 20 tons is there any wonder why at the end of the day I fell it goes around all worn out carrying that load of air now to show you further the consequences consider these things which we need to look at sharply acutely circumspectly because what is said about them is always wrong they are called suction cups suction cups and I must advise you to put the word suction out of your vocabulary it is unacceptable why because there is no suction no such force what do we do let me squeeze them together face upon face and what have I done I have squeezed the air out from between them now why are they held together they are held together because of the push of the air I could compute it very readily if I let us say I have squeezed out all the air inside from between them all I need to do is measure their area PI R square is the area of a circle and that area in square inches times 15 pounds per square inch you need it turns out that about 300 pounds is necessary to pull these apart tap about 300 pounds necessary to pull them apart of course look at the for us look at that indeed regarding the usefulness the next time you watch people who change big panes of glass in department store windows you will notice the pane of glass weighs half a ton and 3/4 men carry it how by by fixing a so-called forgive me for the use of the language so-called suction cups and then they carry it around the push of the atmosphere now I use the vacuum pump in this and you should read about Otto von Guericke e GU er IC ke who in the 16th century invented the vacuum pump and I want to show you his first one if we can get a tight shot of that there we are notice here is a pump with a piston and two men are drawing on it and he reports in his report to the Society's and scientific societies that there was first a slight hissing as by bees and birds and then bang the cast collapsed here is another one showing the same fantastic those two hemispheres put together the original Magdeburg hemispheres and here is a picture of von Guericke II so we should pay him tribute we should pay him tribute now I have spoken the Magdeburg hemispheres let's take a look at them here they are in replica two hemispheres made of steel which I put closely face to face and then I connect the vacuum pump and the valve is open and I take out some of the air I take out some never all could never take it all out then I'm going to close the valve shut off the pump now I'm going to try to pull them apart never not one man alone indeed two could hardly do it so I'm going to do something obviously the inside is too low the push of the atmosphere outside is too big I'm going to do something watch listen listen go and open the valve instantaneously there now what did I do Oh some say you let some of the vacuum out oh that murders me as Macbeth doth murder sleep didn't let any of the vacuum out I let some air in still too much let more of the vacuum out oh oh now I let so much out that all of its out and so we have the Magdeburg hemispheres a classical demonstration historically all that miss farish pressure one more very dramatic one on an earlier program I showed this when I talked about inertia but I want to do this now because it is a demonstration of atmospheric pressure you see ladies and gentlemen and boys and girls what I'm trying to do is not teach any physics in these recitations but rather stir your interest by events which are dramatic for the soul here is a sheet of paper about 20 inches one way in 30 the other 600 square inches on each square inch 15 pounds of air 600 times 15 9,000 pounds of air and you know as I did before I'll do it once again I cannot move the weight of that air there it is and I say that is staggering to contemplate the push of the air of what usefulness is this oh well without air birds couldn't fly and airplanes couldn't fly indeed talking about flying here is an instrument called an altimeter are now Tomita what does it consist of it consists of a little chamber which is highly evacuated I'll try and draw that little chamber which is highly evacuated here it is highly evacuated and it has a leave a system on it with a scale now if we go high into the atmosphere the air inside expands little the cover lifts up and pushes the lever down so we get a reading of the altitude in terms of the pressure of the air or if you wish another use made of the atmosphere which has so much to do with the events of weather here is an aneroid barometer which has inside it exactly the same thing the response of the chamber to increase and diminution of pressure gives rise to a reading and I advise you invite you to look up the word aneroid because what does it mean it means without a meaning not and near us wet without wetness so it is not a mercury-in-glass barometer but a dry barometer and talking about a mercury in glass thermometer here's something you can do take a long glass tube about a meter long and fill it with mercury it's closed at one end closed at one end fill it with mercury then take a vessel in which some mercury resides turn the tube upside down of course with your finger across the open end immerse submerge the open end in this vessel of mercury and take your finger away remember the tube is filled with mercury what would you expect you would expect all the mercury to fall out no it does not it falls to a certain level and notice the shape of the meniscus the shape of the surface it falls how far so that this length is about 76 centimeters or about 30 inches and that is one atmosphere which sustains this column of mercury and here is a region highly highly empty of everything except a little mercury vapour perhaps so one of the best vacuums we can get is produced by such a performance of a mercury column in a glass tube now let's see what else we could show what else could we look at this isn't this fantastic indeed ohh-ho-ho another one I have another one and since it is chance me I'm going to do it this was boiling over there here it is notice notice I'm in a hurry listen there it goes notice I thought I have not boiled the water long enough because if there was any occluded air in there we would have trouble and so are you not agreed that atmospheric pressure is a staggering thing to witness and I thank you for listening you

Channel: Professor Julius Sumner Miller

Views: 117,401

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Keywords: science, julius, sumner, miller, dramatic, demonstrations, physics, professor, Demonstrations In Physics (TV Program)

Id: P3qcAZrNC18

Length: 14min 29sec (869 seconds)

Published: Mon Feb 03 2014