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Physics in Action: Pressure Changes in a Bell Jar


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About this Lesson

  • Type: Video Tutorial
  • Length: 8:02
  • Media: Video/mp4
  • Use: Watch Online & Download
  • Access Period: Unrestricted
  • Download: MP4 (iPod compatible)
  • Size: 86 MB
  • Posted: 07/01/2009

This lesson is part of the following series:

Physics (147 lessons, $198.00)
Physics: Fluids (13 lessons, $13.86)
Physics: Fluid Statics (9 lessons, $6.93)

This lesson was selected from a broader, comprehensive course, Physics I. This course and others are available from Thinkwell, Inc. The full course can be found at The full course covers kinematics, dynamics, energy, momentum, the physics of extended objects, gravity, fluids, relativity, oscillatory motion, waves, and more. The course features two renowned professors: Steven Pollock, an associate professor of Physics at he University of Colorado at Boulder and Ephraim Fischbach, a professor of physics at Purdue University.

Steven Pollock earned a Bachelor of Science in physics from the Massachusetts Institute of Technology and a Ph.D. from Stanford University. Prof. Pollock wears two research hats: he studies theoretical nuclear physics, and does physics education research. Currently, his research activities focus on questions of replication and sustainability of reformed teaching techniques in (very) large introductory courses. He received an Alfred P. Sloan Research Fellowship in 1994 and a Boulder Faculty Assembly (CU campus-wide) Teaching Excellence Award in 1998. He is the author of two Teaching Company video courses: “Particle Physics for Non-Physicists: a Tour of the Microcosmos” and “The Great Ideas of Classical Physics”. Prof. Pollock regularly gives public presentations in which he brings physics alive at conferences, seminars, colloquia, and for community audiences.

Ephraim Fischbach earned a B.A. in physics from Columbia University and a Ph.D. from the University of Pennsylvania. In Thinkwell Physics I, he delivers the "Physics in Action" video lectures and demonstrates numerous laboratory techniques and real-world applications. As part of his mission to encourage an interest in physics wherever he goes, Prof. Fischbach coordinates Physics on the Road, an Outreach/Funfest program. He is the author or coauthor of more than 180 publications including a recent book, “The Search for Non-Newtonian Gravity”, and was made a Fellow of the American Physical Society in 2001. He also serves as a referee for a number of journals including “Physical Review” and “Physical Review Letters”.

About this Author

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Consider this, we live in an atmosphere of air, which is exerting a pressure of 14.7 pounds-per-square inch on us all of the time. This is the size of a square inch, and 14.7 pounds is even more than the weight of this whole bowling ball. Imagine the weight of this bowling ball exerted over this very small area; that happens to us all of the time. Now why don't we feel the air? Why do we take it for granted? That is because the pressure inside of our bodies is usually more or less the same as the pressure outside of our bodies so they balance each other and we don't feel anything significant. The same is true for fish who swim in the ocean.
Now one way of demonstrating this and showing the dramatic effects of the air around us is to remove the air and see what happens. And this bunch of demonstrations will show exactly that.
This over here is a marshmallow man, a stick figure made up of marshmallows. Marshmallows just like this. MMM... Good. Now this marshmallow man is going to be put in this bell jar. This bell jar is nothing but a very finely designed piece of glass, which will keep all of the air out that is outside and whatever we want inside, inside. Now notice that the marshmallow man is sitting inside this bell jar, the air on the outside of the marshmallows is the pressure of the air is balanced by the pressure inside, that is why marshmallows look the way that they look. I am going to fasten this down. What I am going to do is attach the marshmallow man inside to a vacuum pump through this tube over here; and when I turn on the pump it is going to draw out all of the air. And you know that is happening when you hear "chugga chugga, chug" That is the sign that the vacuum pump is pumping out all of the air. Lets see what happens, but before we do, I want you to think, imagine in your mind what is going to happen-think before-what is going to happen when we draw out all of the air. Think and let's see what happens.
Ok, we have marshmallow man in here and we are going to open up all of the valves, turn on the pump and let's watch. Notice what is happening: our marshmallow man is getting much, much bigger, now why does that happen. What has happened is that marshmallows look the way that they look in part because they are fluffed up with a lot of air inside. This is now a situation what we have created here where the air inside is no longer balanced by the air outside, and for that reason, the air inside the marshmallow swells up and the marshmallow man looks much bigger than he was before.
Now the interesting question is going to be "what happens when we open this valve and let the air back in? Before I do it I will give you a second of two to let you think about what is going to happen. We are going to let the air back in. Is the marshmallow man going to go back to the way he was before? Let's see what happens.
Here the air going back in, he didn't go back to exactly to where he was before; he shrunk down. Now he looks kind of like Dracula out of some late night horror movie. What has happened in the process is several things: for one we have now drawn out all of the air that was inside before, balancing the atmospheric air on the outside. We have also torn up some of the sugar strands in there so the molecules aren't as strong as they were before; and this is what is left. This is an example of the dramatic effect of what happens when we remove the air. Let's thank our marshmallow man, remove him, and do something else.
Let's replace the marshmallow man with Mr. and Mrs. Grump, your unfriendly neighbors, and add in a few of their kids, just for good measure. And now we are going to do the same thing. We are going to pump out the air (put them inside this bell jar) and let's see what happens. And we don't like Mr. and Mrs. Grump too much, so we will turn on the pump, notice what is happening to these balloons, this again a situation where we see the effects of the air by removing it. The air inside the balloons is not in equilibrium with the air outside, so the balloons expand. In fact all of the Grump family is now gone, what about this last person getting bigger and bigger and bigger... Is he going to make it? We don't allow betting in this program. He's tough, he is outlasting everyone else. And there we go.
Now this is not just a fun demonstration, this is what would happen to a human being let's say an astronaut out in empty space, if he or she went in empty space without the benefit of a space suit. The air inside their bodies would expand because there is no counter pressure to keep the air equilibrium. Literally if an astronaut were somehow to end up in space without a space suit, they would literally explode, exactly as over here.
For our final demonstration of the effects of air, let's let the air back in. We have over here a little bottle and we are going to fill this bottle with ordinary shaving cream. Now to make it easy to see what is happening, let's add a few drops of food coloring. We have a little bit of red over here... oops, and a little bit of green. Okay, maybe a little more than a little... Let's put this under the bell jar. Now, close the valve and let's see what happens again. Now we know that shaving cream is certainly full of air that is why we use it. Let's turn on the pump and see what happens. It's a gift that keeps on giving, more and more and more. This is a way of saving money... you invest in a small amount of shaving cream in a large pump and look how much shaving cream you end up with. There you have it; now what is going to happen if we open up the valve and we let the air back in. Think about it again for a second. Here we go. And there it is.
Again, the principle is this, we live in an atmosphere of air, which we normally take for granted because the air inside and the air outside balance each other. The pressure inside and the pressure outside normally are more or less the same. When we change that situation by keeping the air inside more or less the same, but removing the air outside, we see dramatic effects. And one way in which we actually use this information, is in the design of space suits and, in fact, deep sea diving suits as well where we try to maintain the pressure inside and outside so that the individuals affected remain alive.
Fluid Statics
Physics in Action: Pressure Changes in a Bell Jar Page [2 of 2]

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