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Some basic questions about pressure!

by Vikram
Tags: basic, pressure
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Vikram
#1
May1-14, 07:54 PM
P: 2
I have some really basic questions regarding pressure. I hope they aren't too trivial. Here I go:

1) We know air pressure at a point develops because of the weight of all air present above that point. So when I stand beneath something, say a loft, now there's a very little air above me, so there should be a sharp fall in air pressure, but it doesn't happen. Why?

2) When we keep a container open in air, and then close it. We treat the pressure of air inside the container as equal to the atmospheric pressure. Why is that so?

Please help me as I have these really basic questions. Thanks! :)
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phinds
#2
May1-14, 08:43 PM
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Quote Quote by Vikram View Post
I have some really basic doubts regarding pressure. I hope they aren't too trivial. Here I go:
Just FYI, what you have are questions, not doubts. The two don't mean the same thing in English even though you have been taught that they do.

http://www.physicsforums.com/showthread.php?t=607274


1) We know air pressure at a point develops because of the weight of all air present above that point. So when I stand beneath something, say a loft, now there's a very little air above me, so there should be a sharp fall in air pressure, but it doesn't happen. Why?
You are forgetting that air is a fluid.

2) When we keep a container open in air, and then close it. We treat the pressure of air inside the container as equal to the atmospheric pressure. Why is that so?
Why would it not be so? Why would the pressure of that gas change just because you put a lid on the jar?

It is the same until you move the container further up or further down in the atmosphere, in which case it would be different because it stays the same but you would be moving it to somewhere that had a different pressure.
Vikram
#3
May1-14, 08:48 PM
P: 2
Quote Quote by phinds View Post
Just FYI, what you have are questions, not doubts. The two don't mean the same thing in English even though you have been taught that they do.

http://www.physicsforums.com/showthread.php?t=607274



You are forgetting that air is a fluid.


Why would it not be so? Why would the pressure of that gas change just because you put a lid on the jar?

It is the same until you move the container further up or further down in the atmosphere, in which case it would be different because it stays the same but you would be moving it to somewhere that had a different pressure.
First of all, thanks for pointing out my mistake! I won't commit it again! (Question edited)
And secondly, can you please explain your points more elaborately? What does air being a fluid imply?

paisiello2
#4
May1-14, 09:27 PM
P: 558
Some basic questions about pressure!

1) Let's say your hypothesis was correct and that there actually was "very little air" above you. And when you stand away from the so called "loft" the pressure is then higher. But high pressure will always move into areas of low pressure. Therefore, there can't be a fall in pressure just because you are standing underneath something. It will always equilibrate to be the same pressure everywhere until the weight of the gas/fluid above prevents the high pressure from moving upwards.

2) Atmospheric pressure is caused by the molecules of air moving around randomly and colliding with each other and with other objects at a given rate. It is these large number of collisions over a specific time interval which give the force that you feel. If you increase the number of molecules in a given volume then statistically there will be more collisions and therefore the pressure will increase as well. Since gravity is the main force acting on these moving molecules it will tend to attract more molecules to lower elevations until the pressure (collisions) builds up enough to equilibrate the push down from the weight of the molecules above.

When you close the container, all you have done is capture a fixed number of air molecules at a given pressure. They are still colliding with each other and with the container walls at a given rate. Nothing has really changed from when the container was opened and therefore the pressure is the same.
phinds
#5
May1-14, 09:33 PM
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Quote Quote by Vikram View Post
First of all, thanks for pointing out my mistake! I won't commit it again! (Question edited)
And secondly, can you please explain your points more elaborately? What does air being a fluid imply?
paisiello2 has give a good explanation and I'll just add this: fluids exert equal pressure throughout the volume in which they are contained. Using your reasoning, a balloon wouldn't work because there would be no pressure on the top of the balloon, only on the bottom, but that's not how fluid pressure works.


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