Atmospheric Pressure at Spout of Water Tank

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In summary, the pressure at the spout of an open cylindrical tank is equal to atmospheric pressure due to the effects of gravity. As the water flows out of the spout, it experiences a decrease in pressure due to expansion. This decrease is significantly larger for water than for gases, like CO2, because water is essentially incompressible. The pressure at the spout is a result of the weight of the water column above it, and as the water flows out, the pressure is converted to velocity pressure according to Bernoulli's equation. In real cases, pressure loss due to friction must also be taken into account. Ultimately, the pressure at the spout is due to gravity and can be compared to standing on a chair in a
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Gabe805
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Picture an open cylindrical tank with a spout at the bottom from which water is flowing. Why is the pressure at the spout equal to the atmospheric pressure? I thought pressure increases with depth (i.e. rho*g*h). The explanation I have been given is that fluid that is exposed to the atmosphere has atmospheric pressure. What happened to the pressure from the inside?
 
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The pressure falls as the water expands going down and emerging from the spout.

Consider what happens with a gas such as the C02 used in some fire extinguishers. The Gas expands a lot as it goes from the high pressure inside the tank to the low pressure outside. As it does so it will try to absorb energy from it's surroundings. If that can't happen fast enough the gas becomes colder. (See youtube videos on making dry ice using a C02 fire extinguisher)

Unlike a gas water isn't easily compressed - so it only has to expand a tiny bit for the pressure to drop a lot. You are very unlikely to notice such a small expansion as the water emerges.
 
  • #3
Sorry, cwatters, but that really isn't correct -- because as you said, water is essentially incompressible.

In the ideal case of no loss, all of the [hydro]static pressure difference between the tank and atmosphere is converted to velocity pressure as the flow field constricts (not expands) and accelerates to enter the nozzle. You use Bernoulli's equation to solve for V.

In the real case, we just need to subtract the pressure loss due to friction and unstable flow before solving for velocity. For a poor transition, it can be as much as half the velocity pressure.
 
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Ok, I guess I am just confused where the pressure goes from inside the spout. What causes the water to come out of the spout? Is it gravity or pressure or a mixture of both?
 
  • #5
Gabe805 said:
Ok, I guess I am just confused where the pressure goes from inside the spout. What causes the water to come out of the spout? Is it gravity or pressure or a mixture of both?
Since its an open tank the pressure above and below is the same i.e. atmospheric.
The water particle at the bottom of the tank experiences a load of the water column above it which is PRESSURE. The load is there because all of the water is being pulled by gravity but the bottom particle has nowhere to go, its being squished by a pressure.
As the spout is opened the particle can now fall freely and is essentially a falling drop.
Lets say YOU (water column) are standing on a CHAIR (bottom water particle) in a helicopter(tank bottom). The chair is experiencing the pressure of ur weight.
Now what happens if suddenly under the chair there develops a HOLE( spout)? Both the chair and you will fall. Will the chair still experience your weight i.e. pressure? NO. Both of you are freefalling.

I can't make it any simpler. The PRESSURE is due to GRAVITY!
 
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Sorry I missed the follow-up:
Gabe805 said:
Ok, I guess I am just confused where the pressure goes from inside the spout. What causes the water to come out of the spout? Is it gravity or pressure or a mixture of both?
Gravity creates the pressure. It's a type of pressure called hydrostatic pressure:
http://en.wikipedia.org/wiki/Hydrostatics

Bernoulli's principle tells how different forms of pressure are related and can be converted from one form to another:
http://en.wikipedia.org/wiki/Bernoulli's_principle

Imagine you have a plastic bag full of water, with a hole in it. Water might pour out slowly. Now step on the bag. Water shoots out, right? You've just simulated what would happen if a lot of bags of water were piled on top of each other -- or if they were all in one container with a hole at the bottom.
 
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Rippetherocker said:
Since its an open tank the pressure above and below is the same i.e. atmospheric.
The water particle at the bottom of the tank experiences a load of the water column above it which is PRESSURE. The load is there because all of the water is being pulled by gravity but the bottom particle has nowhere to go, its being squished by a pressure.
As the spout is opened the particle can now fall freely and is essentially a falling drop.
Lets say YOU (water column) are standing on a CHAIR (bottom water particle) in a helicopter(tank bottom). The chair is experiencing the pressure of ur weight.
Now what happens if suddenly under the chair there develops a HOLE( spout)? Both the chair and you will fall. Will the chair still experience your weight i.e. pressure? NO. Both of you are freefalling.

I can't make it any simpler. The PRESSURE is due to GRAVITY!
Thank you. I understand now.
 
  • #8
russ_watters said:
Sorry I missed the follow-up:

Gravity creates the pressure. It's a type of pressure called hydrostatic pressure:
http://en.wikipedia.org/wiki/Hydrostatics

Bernoulli's principle tells how different forms of pressure are related and can be converted from one form to another:
http://en.wikipedia.org/wiki/Bernoulli's_principle

Imagine you have a plastic bag full of water, with a hole in it. Water might pour out slowly. Now step on the bag. Water shoots out, right? You've just simulated what would happen if a lot of bags of water were piled on top of each other -- or if they were all in one container with a hole at the bottom.
Makes sense. Thank you for explaining it to me!
 

Related to Atmospheric Pressure at Spout of Water Tank

1. What is atmospheric pressure?

Atmospheric pressure, also known as air pressure, is the force exerted by the weight of the atmosphere on the Earth's surface. It is caused by the gravitational pull of the Earth on the air molecules in the atmosphere.

2. How does atmospheric pressure affect the spout of a water tank?

Atmospheric pressure can affect the spout of a water tank by pushing down on the water inside the tank, causing it to flow out of the spout. The higher the atmospheric pressure, the more force it will exert on the water, resulting in a stronger flow from the spout.

3. Can the atmospheric pressure at the spout of a water tank change?

Yes, the atmospheric pressure at the spout of a water tank can change due to various factors such as weather patterns, altitude, and temperature. Changes in atmospheric pressure can also affect the flow rate of water from the spout.

4. How is atmospheric pressure measured at the spout of a water tank?

Atmospheric pressure at the spout of a water tank can be measured using a barometer, which measures the weight of the atmosphere pressing down on a column of mercury or aneroid cell. The measurement is typically recorded in units of pressure, such as millibars or inches of mercury.

5. How does the height of the water tank affect atmospheric pressure at the spout?

The height of the water tank has a direct impact on the atmospheric pressure at the spout. The higher the water tank is, the greater the atmospheric pressure at the spout will be due to the increased weight of the water pushing down. This can result in a stronger flow from the spout compared to a lower water tank.

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