Air pressure and opening a can of juice

[Recovering Engineer]

Hi there, I have a question about opening a can of juice (for instance, pineapple juice) by puncturing the top end on with a knife.

 

While I think I can guess why you have to make 2 holes on opposite sides of the lid, instead of just one (because two holes will allow opposing air & liquid / current flows, create an imbalance, finally pushing the juice out...

 

1) what exactly happens ?

 

2) how come making, for instance, two inch-long slits side by side on the lid, works too? Wouldn't the juice be coming up against air pressure pushing down in almost exactly the same direction? How come this works but having just one single ridiculous looking 2-inch slit will never let the juice out?

 

3) I'm asking this in relation to a room in our house that has a 2-ft wide x 2.5ft high window. It feels as though no air comes through the window, no matter the season. I am wondering whether, just like with the juice can, instead of the 2-ft window, they should have made two side by side 1-ft wide windows instead? Would that have created more flow?(The other 3 walls lead to other rooms so no chance for extra windows there).

[swansont]

You have to displace the liquid with air. A single slit would work as long as it is not completely covered by the fluid when you pour it.

[Recovering Engineer]

So if I waited a while (to let some air in) before pouring the liquid, would a single slit work even if the slit is fully covered with liquid? (sorry, im confused)

[Sisyphus]

You have to be letting air in AS you're letting liquid out. When liquid leaves, the volume it formerly occupied has to be replaced with something. With two holes, air can flow in one as liquid flows out the other. With only one hole, you have air and liquid trying to pass each other in different directions, and there is no smooth flow.

[ewmon]

... a room in our house that has a 2-ft wide × 2.5ft high window. It feels as though no air comes through the window, no matter the season. (The other 3 walls lead to other rooms so no chance for extra windows there).

Like any fluid, air needs a place to go, and it more easily flows through a room (that is, into and out of it), rather than only into it.

 

Your window is small, which limits your options (without installing a larger window in its place or installing another window elsewhere in the room).

 

So, buy a small window fan (8- to 12-inch blade diameter). If it has an adjustable width, adjust the width to its narrowest or maybe remove the adjustable “wings” altogether. Open the sash and install the fan on one side of the window, leaving the other side open.

 

See if you can tell if the wind outside tends to blow right or left. Try putting the fan in the “downwind” side of the window ... the side toward which the wind blows. With this extra effort, the wind will tend to blow away the exhausted room air. If you install the fan in the “upwind” side, the wind will tend to blow the exhausted air across the open part of window, and it will be drawn in again.

 

To make sure the fresh air doesn’t go right back out again, adjust the fan’s vanes to shoot the air away from the open side of the window or, between the fan and the opening, install a straight partition or bent or curved deflector (even cardboard helps) which extends into the room several inches to a foot.

[Recovering Engineer]

You have to displace the liquid with air. A single slit would work as long as it is not completely covered by the fluid when you pour it.

 

Okay now Im wondering - why is the air so powerful, when liquid is so much denser? Even if it's the liquid in one can of juice vs. all the air in the room, still, theres space between each air molecule, how come the liquid molecules can't displace those spaces?

 

 

Also, now I'm also wondering about how, for instance, scuba divers can enter a submerged ship / underwater station without taking all the water in the ocean with them into the ship. How do you build up enough air pressure in a single vessel, however big it might be, to match the water pressure of the entire ocean.?

 

To ewmon:

 

About the tiny window - which started this whole thread - I think I will actually install a fan in the window now. Thanks for this!!!

I neglected to mention the room has a high ceiling, and someone also suggested that making a small hole, at least 8x8 inches, near the

ceiling, would help let the hot air out because otherwise, the hot air that has risen to the top has no way to go but down into the room again. I think it makes sense, but do you reckon this would make a difference?

Edited October 10, 2010 by swansont
fix quote tag

[swansont]

The air has a pressure of 10^5 Pa (14.7 PSI). Normally this has no effect on motion, because the pressure is the same all around. Any liquid that escapes must create a void in the container, unless that volume is replaced by the air, meaning there is now an upward force on the liquid. The air is powerful because there is a column of it kilometers high, all under the influence of gravity, available to push on the container.

[Recovering Engineer]

meaning there is now an upward force on the liquid.

 

 

pls check whether i'm understanding this or not.

by upward force do you mean the air that has entered the can, helping to push the liquid out? so it's all clear enough now (i think) but

am just curious..

 

is this scenario possible: a void being created in the can while it's being poured?

 

 

 

 

 

 

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Sorry for the change of name, it was a bad call but I cant change it back now. I've emerged from illness with a mushy mind. But to be honest I dont know if I understood this stuff even pre-insanity days.

[swansont]

When you create the void, the air is pushing the liquid into the container. If air can get into the container, gravity is unopposed, and the liquid can fall (pour) out.

[Recovering Engineer]

About air pressure:

In all this , should air pressure be assumed as an upward force or a downward force? eg.

 

when the can with one slit in it, is standing on the table, air pressure is pushing down on it - ?

when the can (with just one slit in it) is being tilted downwards to try to pour, is air pressure pushing upwards onto the slit, stopping the juice from coming out?

Edited October 13, 2010 by Recovering Engineer