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I’ve a question about pressure

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DivingIndex

New Member
Oct 31, 2005
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I’ve a question about pressure. I know it looks like a kid’s homework, but it’s the case that my friend and I have argued about



Assume that in the picture is a Car under the water. (10M Depth)
The arrow is the vector of the pressure that acts with the air.

In car #1: we put the tube that completely sealed in the hole of the car.
In car #2: we do nothing to the hole.


My question is whether the air pressure in Room A, B and C are the same or not.

IMO, the air pressure in room A and B is still the same (1ATM) because of the tube that share the pressure between room A and B

But in room C, it should have more pressure depend on the depth.

Am I wrong?

Thx..
 
In both cases, the air will escape in a while completely. There will be no stable state where the air pressure would be in equilibrium with the water pressure. As for the dynamical state, the air pressure would be about the same in both cases (same as the one of the environmental water minus a small gradient due to the opening), but the air would escape easier and quicker with the tube - due to less resistance of water surface tension at creation of the bubbles.

You can easily verify it with two identical plastic containers (one with a hole, another with a tube) submerged into a bathing tub.
 
Thank you Trux

So in both cars B and C should have a pressure more than 1 ATM.

In the car C if it goes deep to 30 M with some small hole, the pressure in the car should be very close to 4 ATM? Am I right?
 
DivingIndex said:
Thank you Trux

So in both cars B and C should have a pressure more than 1 ATM.

In the car C if it goes deep to 30 M with some small hole, the pressure in the car should be very close to 4 ATM? Am I right?
Click to expand...
Simplified told, yes, but in reality there will be no constant air pressure, but rather a dynamic gradient of pressure until all air escapes. The gradient will be the more significant the bigger the hole (hence also faster air escaping). And as I wrote, in the first case the air will escape easier, since there is less resistance / no need for breaking the surface water tension during the bubble creation.
 
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PS: the above claim assumes that there are enough openings in the car to let water in. If it is perfectly sealed, no water can enter except in the second case through the small hole, and assuming the car construction is strong enough to withhold the pressure (which is not likely in 30m), then in the first case the atmospheric pressure would remain inside the car, of course - that's the way some military tanks or other armored vehicles can cross some rivers.
 
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