Thought experiment about pressure (intro physics)

[jenzao]

Thought experiment about pressure (intro physics) ..you have a U-shaped tube?
that is open to the atmos on both the left and right side
you fill it with water so the water level is the same on both sides.

then you put a rubber stopper in one end (..the right end)
(also the remaining air between stopper and water surface was somehow sucked out after you put the stopper in)

next you somehow suck out some of the water from left side, so now the levels on each side are uneven. (right side remains right up to the stopper, and left side is halfway down)

So... the pressure at the surface of the left side is 1atm right?

But what is the pressure at the SAME LEVEL on the right side? If it is also 1atm please explain why, because its totally confusing me. thanks alot!

 

[mgb_phys]

The pressure down at the other side is the weight of water above that point.
Since this is probably less than 1 atmoshere (unless you have a tube 9.8m tall) the other side is actually in compression, the weight of the atmosphere is forcing the water in the other side up - this is what most people think of as the vacuum 'sucking' the water in that side up.

Once you get to a column more than 9.8m the weight of the water is greater than the weight of the atmosphere and you will get a (partial) vacuum at the top of the sealed water column.
Then you have a barometer - the height of the water column tells you the air pressure. Generally you use mercury since 760mm is a lot easier to fit on your wall than 9.8m!

 

[astrokreng]

This thought experiment is a great way to explore the concept of pressure in a simple and visual way. In this scenario, we have a U-shaped tube filled with water and open to the atmosphere on both sides. When the water level is the same on both sides, the pressure at the surface of the water is equal to the atmospheric pressure, which is 1 atm.

When we add a rubber stopper to one end of the tube and remove some of the water from the other side, the water level becomes uneven. The right side, where the stopper is, remains at the same level as before, but the left side is now halfway down. This means that the pressure at the surface of the left side is still 1 atm, as it is still open to the atmosphere.

Now, the question is, what is the pressure at the same level on the right side? The answer is also 1 atm. This may seem confusing, but let's break it down. The pressure at any given point in a fluid (such as water) is determined by the weight of the fluid above it. In this case, the water level on the right side is the same as before, so the weight of the water above it has not changed. This means that the pressure at that level is also the same, which is 1 atm.

In conclusion, the pressure at the same level on both sides of the U-shaped tube is 1 atm. This is because the water level on the right side has not changed, and the pressure at any given point in a fluid is determined by the weight of the fluid above it. I hope this explanation helps to clarify any confusion.