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qitara
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Hi guys
Is there any thing such as Negative Pressure, or is there only total vacuum 0 and nothing below that ?
Is there any thing such as Negative Pressure, or is there only total vacuum 0 and nothing below that ?
qitara said:isn't a vacuum and negative pressure the same thing ?.
Nugatory said:No, a vacuum is zero pressure and because you can't have a pressure smaller than that of a perfect vacuum, there are no negative pressures.
What's confusing you is that people often say "pressure" when they mean "difference from atmospheric pressure" - by that definition any "pressure" that is less than atmospheric is negative.
abitslow said:In almost all areas of Engineering and Chemistry, yes its relative pressure and it can be negative.
In some areas of Physics, it has an entirely different meaning. This is the second post I've answered in the last 10 minutes where the poster didn't explain in what context the term was being used in. *sigh* Unless you are dealing with quantum field theory, you can NOT have negative absolute pressure and you CAN have negative relative pressure (also known as "gauge pressure").
Nugatory said:No, a vacuum is zero pressure and because you can't have a pressure smaller than that of a perfect vacuum
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Why do you think that?Varun Bhardwaj said:There is no any perfect vacuum because if there is perfect vacuum than it will exert infinite force to fill it.
Sure there are:Nugatory said:No, a vacuum is zero pressure and because you can't have a pressure smaller than that of a perfect vacuum, there are no negative pressures.
When attractive forces (e.g., van der Waals forces) between the particles of a fluid exceeds repulsive forces due to thermal motion. These forces explain ascent of sap in tall plants. Negative pressure must exist at the top of any tree taller than 10 m, which is the pressure head of water that balances the atmospheric pressure. Van der Waals forces maintain cohesion of columns of sap that run continuously in xylem from the roots to the top leaves.
This is just negative gage pressure (relative to 1 atm.). It is not negative absolute pressure.A.T. said:Sure there are:
http://en.wikipedia.org/wiki/Pressure#Negative_pressures
This here is absolute negative pressure:Chestermiller said:This is just negative gage pressure (relative to 1 atm.). It is not negative absolute pressure.
When attractive forces (e.g., van der Waals forces) between the particles of a fluid exceeds repulsive forces due to thermal motion. These forces explain ascent of sap in tall plants. Negative pressure must exist at the top of any tree taller than 10 m, which is the pressure head of water that balances the atmospheric pressure. Van der Waals forces maintain cohesion of columns of sap that run continuously in xylem from the roots to the top leaves.
A.T. said:This here is absolute negative pressure:
Pure water can have negative absolute pressure too:Chestermiller said:Tree sap, which is a polymer solution, exhibits viscoelastic rheological behavior, which is a combination of viscous-fluid and solid behavior. So tree sap can also exhibit, to some degree, what would be defined as negative pressure. But ordinary Newtonian viscous gases and liquids are not viscoelastic, and do not exhibit negative pressure.
No, it's not (just) capillary action (attraction by the walls), but actually negative absolute pressure (attractive forces between the water molecules).Nugatory said:That wikipedia article is stretching things a bit by characterizing capillary action (which is what the tree is using to lift its sap above 10 meters) as negative absolute pressure.
That article is amazing, if correct. I've never heard of this type of thing before. Thanks for calling it to my attention. Maybe you can help me. What is it the enables the water to maintain this metastable state, and prevents it from cavitating? Is it somehow related to the small dimensions of the channel?A.T. said:Pure water can have negative absolute pressure too:
http://www.nature.com/nature/journal/v455/n7210/full/nature07226.html
Chestermiller said:That article is amazing, if correct. I've never heard of this type of thing before. Thanks for calling it to my attention. Maybe you can help me. What is it the enables the water to maintain this metastable state, and prevents it from cavitating? Is it somehow related to the small dimensions of the channel?
Chet
Here it is:dauto said:The article is correct. There is an excellent Veritasium video on youtube called "The Most Amazing Thing About Trees" about that topic. It's worth watching.
Chestermiller said:Maybe you can help me. What is it the enables the water to maintain this metastable state, and prevents it from cavitating? Is it somehow related to the small dimensions of the channel?
Varun Bhardwaj said:There is no any perfect vacuum because if there is perfect vacuum than it will exert infinite force to fill it.
Negative pressure is a condition in which the pressure of a system or space is lower than the surrounding atmospheric pressure. This results in a pressure gradient that causes air or other substances to move from areas of higher pressure towards areas of lower pressure.
No, total vacuum and negative pressure are not the same. Total vacuum refers to a complete absence of matter in a space, while negative pressure refers to a lower pressure compared to the surrounding environment. Negative pressure can exist in a system without reaching a total vacuum state.
Negative pressure can be created in a system through various methods such as using a pump, creating a sealed or enclosed space, or through temperature changes. It can also be a natural occurrence in some environments.
Negative pressure is used in a variety of applications, including HVAC systems, vacuum cleaners, medical devices, and industrial processes. It can also be used to create clean rooms and control air flow in buildings.
While negative pressure can be useful in many applications, it can also pose risks if not properly managed. In some cases, it can lead to implosions or damage to structures. It can also affect air quality and cause health issues, so proper ventilation and monitoring are important when working with negative pressure systems.