Why do air rush into vacuum

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Why do air "rush" into vacuum

I was reading the quite enjoyable "Physics for Superheroes" By J. Kakalios. At some point he explains what is entropy and give an example of air molecules rushing into a previously sealed and empty room (no air either) It was explained how the molecules would get there and why bit I couldn't understand what's the rush?

You encounter it in every day life, even if the air seems still when you open a window or open a door there is "wind" why is air coming into the room so fast and not seeping through the cracks? It also seems that the smaller the crack the faster the air will go to the previously less pressurized area, Why is that?

tl;tr - Why is air rushing through cracks instead of gently seeping inside.
 

Answers and Replies

  • #2
K^2
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Because the air molecules are flying about at a very high speed, comparable to the speed of sound. They don't rush specifically towards vacuum either. Some molecules just happen to be flying that way, and there is nothing going in reverse, since there is nothing there but vacuum. The net flow is therefore from filled room towards the empty one.
 
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But why can you feel this net flow while you don't feel any kind of movement prior of opening this theoretical room?
 
  • #4
Vanadium 50
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Because as many air molecules strike you on one side as on the other, so the effect cancels.
 
  • #5
RonL
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I was reading the quite enjoyable "Physics for Superheroes" By J. Kakalios. At some point he explains what is entropy and give an example of air molecules rushing into a previously sealed and empty room (no air either) It was explained how the molecules would get there and why bit I couldn't understand what's the rush?

You encounter it in every day life, even if the air seems still when you open a window or open a door there is "wind" why is air coming into the room so fast and not seeping through the cracks? It also seems that the smaller the crack the faster the air will go to the previously less pressurized area, Why is that?

tl;tr - Why is air rushing through cracks instead of gently seeping inside.
Homes that are pretty well sealed will generally have an internal positive pressure if an AC system is working. Air movement over and around a home (external) will create very small pressure changes inside, both positive and negative, depending on cracks or openings anywhere in the house.
An expierment you might play with that can show small pressure change that might not be sensed physically......go to a bathroom that has a perfectly motionless water level in the comode bowl (best if you can find an angle that has a light reflection across the surface) have someone open and close an exterior door, you should see a movement in the water.
This will not work using water in a sink.

There will be some that will understand this at first read, and others that will just say "so what"? Life is full of these normally unnoticed events, observing and then understanding them is of key importance.

I'll see if anyone responds with an answer of what and why.

Ron
 

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