I’ve done this so many times I can’t count. But suddenly, I’m playing with it and wondering. Put some water in a glass tumbler. Cover the mouth with a piece of paper (I’ve found that photographic print paper works very nicely). Invert the glass. The water stays in. We always say that this illustrates the fact that air has pressure (in all directions), and that the pressure exerted by the atmosphere up on the paper is greater than the weight of the water inside the glass. Ergo, it holds the card up. Now, I notice that it makes no difference how much water is in the glass. It can be full. But, it can’t be empty. And I can’t exactly see why. It works with just a tiny amount of water. The explanation seems to be the same. But why doesn’t atmospheric air pressure hold a piece of paper to a glass with NO water in it? I have speculated that the weight of the water in the glass under the trapped air pulls down on it, expanding it, and causing it to exert slightly less than atmospheric pressure. This causes the imbalance that allows the paper to stay attached to the glass. But it also works with no air at all in the glass. I’m overlooking something here. dopers? xo, C.
WAG - With just air a good seal cannot be achieved. Try this - fill glass with a bit of warm water and place paper on top. As the water cools it should pull the paper into a concave curve. If it doesn’t try it with a piece of plastic wrap (that will give a better seal).
You could also try to smear some greasy substance around the rim of the glass before putting on the paper.
Why doesn’t the outside air pressure just push it up against the glass? At 15 lbs/sq. in., it should be able to. With just a tiny amount of water it does.
How does this help understand the phenomenon?
Yes, but isn’t that the same as sticking it there with glue? I’m not helped enough by your efforts, I fear.
I have to think that surface tension is involved somehow, but I’m not going to suppose to explain how.
I don’t think it has to do with the “seal” or with surface tension. When there is no water, the air inside and outside the container is pressurized equally. With no pressure difference, gravity will act on the paper and there is no opposing force. With some air and water inside the container, let’s imagine what would happen if the water were to flow out of the container: air would bubble up through the water. Some of the water is actually moving up - the force exerted on it by gravity is less than the force exerted by the pressure difference created between air inside and outside the container.
In order for water to leave the container, the air inside it now occupies a greater volume and hence is at a lower pressure. With the paper over the opening, the water + paper will “fall” just enough to introduce a small pressure drop in the air inside the container, and the force of the air acting upward on the paper prevents enough of a gap at the edges for air to leak in and bubble up. To test this explanation you could try filling the container entirely with water and putting paper over the opening. I would do it over a sink, though, because the predicted result is that you will get very wet. I imagine that if you were somewhere with very low atmospheric pressure, this experiment would not work so well with only a tiny amount of water.
I am not an expert, but I think this is the explanation.
I’m going to be thinking about the rest of your answer, but as for the above, as I said in my original post, it works just fine with a completely full glass of water - no air in the glass.
I guess, now that I ponder a little longer, I’m wondering how even the slightest amount of water in the glass makes a differential in the inside/outside pressure. No water - no sticking paper. Full of water - stick. 1/2 full - stick. 1/4 full - stick. 1/20 - stick. None - no stick.
Because the air inside is at the same pressure. There’s no net force on the paper because there is no pressure differential across it.
I suspect the weight of the water causes the middle of the paper to bulge downwards. This increases the volume, therefore reducing pressure inside. And it does help that wet paper creates a much better seal than dry paper.
You’re right. I don’t know what I was thinking with that part. The water will experience the same effect as air with regards to the pressure decrease inside the container.
Bingo! And when there’s more water in the glass, it weighs a good bit and pulls down on the air in the glass, expanding it, and causing it to exert less pressure on that side, and then, the atmosphere can overcome the weight of the water, now unbalanced by the air above it. And push back. Very much the same way that with the end of a syringe sealed, it’s almost impossible to draw the plunger out. And, yes, a better seal makes a difference so that the air can’t just sneak back in and begin to rebalance the system. Oh, man, I love this bbs. Something cleared up for me that had puzzled me for a long time - and I have taught this a zillion times. But I always finessed a part of the explanation, and recently just discovered the nature of my puzzlement. And, of course, that’s the reason that water or mercury only go so high in a barometer. Thank you, thank you. xo, C.