Atomspheric pressure

[Lizwi]

Why atmospheric pressure measure at the bottom of this fluid because atm is exerted by atmosphere and the atmosphere is at the top of the fluid?

I though atmospheric pressure should be measured at P₀

[Endy0816]

By atmospheric pressure they mean the pressure of the Earth's atmosphere at the lowest point.

I like to imagine a column of air above me weighing me down.

Weight(force) / Area = Pressure

[studiot]

5 hours ago, Lizwi said:

Why atmospheric pressure measure at the bottom of this fluid because atm is exerted by atmosphere and the atmosphere is at the top of the fluid?

I though atmospheric pressure should be measured at P₀

 

Perhaps a little more detail is needed here ?

What is the fluid represented by the blue lines?

I am guessing the following:

Your text is about the rate of flow or speed of flow of something like water out of a hole in the bottom of the tank (called orifice discharge), using Bernoulli's theorem and the continuity equation.

I expect that somewhere your text indicates that atmospheric pressure is sensibly the same at the top and bottom of the tank, because 'h' will be very small compared to the height of the air column causing p₀ , the pressure at A1.

So the discharge will depend only on the head (h) and density of the fluid in the tank.

 

Edited August 25, 2019 by studiot

[swansont]

8 hours ago, Lizwi said:

Why atmospheric pressure measure at the bottom of this fluid because atm is exerted by atmosphere and the atmosphere is at the top of the fluid?

I though atmospheric pressure should be measured at P₀

The diagram does not say the pressure above the fluid is at atmosphere. It says it’s P₀

One can conclude that it’s at a lower pressure above the fluid.

[studiot]

6 minutes ago, swansont said:

The diagram does not say the pressure above the fluid is at atmosphere. It says it’s P₀

One can conclude that it’s at a lower pressure above the fluid.

Good point, it could also be much higher that atmospheric if the tank is part of an old fashioned 'gasometer'.

 

So more information please Liz.

[swansont]

13 minutes ago, studiot said:

Good point, it could also be much higher that atmospheric if the tank is part of an old fashioned 'gasometer'.

 

So more information please Liz.

Right. I was assuming it was static,  but giving the areas suggests that flow might be involved.

[Lizwi]

7 hours ago, Endy0816 said:

By atmospheric pressure they mean the pressure of the Earth's atmosphere at the lowest point.

I like to imagine a column of air above me weighing me down.

Weight(force) / Area = Pressure

Thanks, does that mean the pressure at the bottom of any fluid is atmospheric pressure?

[studiot]

7 minutes ago, Lizwi said:

Thanks, does that mean the pressure at the bottom of any fluid is atmospheric pressure?

 

No

It is not even properly true of the atmosphere!

Atmospheric pressure is the pressure of the atmosphere.

Which is true but useless since the pressure depends upon location and temperature and even whether or not a wind is blowing.

So it is necessary to specify more detail.

 

How about more detail?

Edited August 25, 2019 by studiot

[Endy0816]

3 hours ago, Lizwi said:

Thanks, does that mean the pressure at the bottom of any fluid is atmospheric pressure?

No, there is an unseen outlet in this case. Honestly is a confusing way to draw/label things.

Your original instinct would be correct in the case of a lake or body of water.

Would need more information on the tank(vented or non-vented) and pressure at the top of it to say what exactly would happen next.

Edited August 25, 2019 by Endy0816

[jajrussel]

On 8/25/2019 at 12:42 PM, swansont said:

Right. I was assuming it was static,  but giving the areas suggests that flow might be involved.

I’m probably entirely wrong but if the diagram is the whole of the subject wouldn’t  the system be static if there is no vent to allow for the change in pressure the arrows may simply indicate the direction of flow for the working system. Don’t know anything about the subject. Just puzzled and wondering? Trying to learn something.

[swansont]

12 hours ago, jajrussel said:

I’m probably entirely wrong but if the diagram is the whole of the subject wouldn’t  the system be static if there is no vent to allow for the change in pressure the arrows may simply indicate the direction of flow for the working system. Don’t know anything about the subject. Just puzzled and wondering? Trying to learn something.

If there's no vent the area above will decrease in pressure and will end up at vacuum (or close, as there will be some trace water vapor), assuming air can't come in the nozzle 

[J.C.MacSwell]

14 hours ago, swansont said:

If there's no vent the area above will decrease in pressure and will end up at vacuum (or close, as there will be some trace water vapor), assuming air can't come in the nozzle 

That would of course depend on "h" the height of the fluid, but certainly true if h is more than 32 feet in height.