Need help understanding negative pressure

In summary, gauge pressure and absolute pressure are two different ways of quantifying air pressure. Gauge pressure can be negative, while absolute pressure cannot. Differential pressure is a third type of pressure measurement that compares the pressure difference between two points.
  • #1
deathlucky
10
1
so i understand possitive pressure eg 14.7psi is about twice atmospheric pressure so if you were to fill a 1L bottle with air to 14.7psi it would have 2L of air in it.

question is if you were to have a 1L bottle and suck air out of it until it read -14.7 psi how much air would be in the bottle?

another question what is the vacuum of space
 
Physics news on Phys.org
  • #2
Well if you sort of suck out the air from the bottle, the bottle might become crushed.
 
  • #3
rock.freak667 said:
Well if you sort of suck out the air from the bottle, the bottle might become crushed.

i was just using bottle as an example let's say the bottle can not be crushed
 
  • #4
How is your gauge calibrated? Zero psi would be a total vacuum unless you have your instrument zeroed to some other reference. Negative numbers would be impossible in that case. :confused:
I think that you mean internal vs. external pressure differential, but one has to be sure of such things.
 
Last edited:
  • #5
Danger said:
How is your gauge calibrated? Zero psi would be a total vacuum unless you have your instrument zeroed to some other reference. Negative numbers would be impossible in that case. :confused:
I think that you mean internal vs. external pressure differential, but one has to be sure of such things.

gauge is 0psi at sea level

im trying to work out how much air is going into my car when when at idle and coasting
 
  • #6
A bottle at 14.7 PSI is at 1 atmospheric pressure. Now, if there's a pressure difference[i/]of 14.7PSI, then yes, the bottle would have a PSI of 29.4PSI and have 2L of air normally at 1atm in it.
 
  • #7
Perhaps I'm missing something, but why don't you just multiply your swept volume by your rpm's? That's how you work out the required cfm for a carb.
 
  • #8
You can't have negative air pressure. There's no way. Inflationary cosmology contains the concept of negative pressure being the cause of outward expansion.
 
  • #9
deathlucky said:
gauge is 0psi at sea level

Then your gauge is calibrated to read 14.7 psi less than the true pressure. So if your gauge reads -5 psi, you know the absolute pressure is 9.7 psi.

ernestpworrel said:
Inflationary cosmology contains the concept of negative pressure being the cause of outward expansion.

Let's answer the OP's question and not go off into cosmology.
 
  • #10
deathlucky said:
question is if you were to have a 1L bottle and suck air out of it until it read -14.7 psi how much air would be in the bottle?


So anyway, this question has no answer. You can't have negative air inside a container.
 
  • #11
ernestpworrel said:
deathlucky said:
question is if you were to have a 1L bottle and suck air out of it until it read -14.7 psi how much air would be in the bottle?
So anyway, this question has no answer. You can't have negative air inside a container.
That is wrong, Ernest. The original post obviously is talking about gauge pressure. You are misconstruing this to mean absolute pressure.

The answer to the original post is "very little", with very little being relative to the amount of air at ambient conditions. How much (or how little) depends on the quality of the vacuum system used to evacuate the container. Even the container itself needs special treatment to create an ultra-high vacuum.

Note well: Scientists use absolute pressure rather than gauge pressure to measure the quality of a vacuum.
 
  • #12
D H said:
Note well: Scientists use absolute pressure rather than gauge pressure to measure the quality of a vacuum.


But that's not even air pressure though. I don't understand why you believe that I am saying that absolute pressure cannot be negative. But we both know that air pressure can never be negative. Wow, would I be in trouble in your physics class or what?
 
  • #13
ernestpworrel said:
But that's not even air pressure though. I don't understand why you believe that I am saying that absolute pressure cannot be negative. But we both know that air pressure can never be negative. Wow, would I be in trouble in your physics class or what?

Don't forget the cassimer effect, isn't that a kind of universal pressure?
 
  • #14
ernestpworrel said:
But that's not even air pressure though. I don't understand why you believe that I am saying that absolute pressure cannot be negative. But we both know that air pressure can never be negative. Wow, would I be in trouble in your physics class or what?
This post makes very little sense. Do you understand and agree with the following two statements:

1. Gauge pressure and absolute pressure are two different ways of quantifying air pressure.
2. Gauge pressure can be negative, absolute pressure cannot.
 
  • #15
James Leighe said:
Don't forget the cassimer effect, isn't that a kind of universal pressure?
Maybe, but it doesn't have anything at all to do with air pressure.
 
  • #16
russ_watters said:
This post makes very little sense. Do you understand and agree with the following two statements:

1. Gauge pressure and absolute pressure are two different ways of quantifying air pressure.
2. Gauge pressure can be negative, absolute pressure cannot.


I'm okay with the first statement, but I have a problem with the second. If you're getting a negative gauge pressure, shouldn't your next step be to recalibrate your gauge? And a vacuum has negative pressure so am I to understand that negative pressure is not absolute pressure?
 
  • #17
Ernest, please google the phrase "gauge pressure."
 
  • #18
D H said:
Ernest, please google the phrase "gauge pressure."


Okay, now what? Is it absolute pressure or not?
 
Last edited:
  • #19
ernestpworrel said:
Okay, now what? Is it absolute pressure or not?
You said you agreed with my statement that said gauge pressure and absolute pressure are different! So you should already know the answer to that question is no.

Ok, I don't know what you found when you googled, but the first link is to the wiki on the subject. Please read the entire passage describing the difference between gauge and absolute pressure:
http://en.wikipedia.org/wiki/Pressu...e_and_differential_pressures_-_zero_reference

[edit] Btw, somehow I forgot about the third type, differential pressure (Danger used the word "differential"). It actually is probably redundant and the other two are subsets, nevertheless the way wiki describes it, gauge pressure usually has the negative sign dropped, whereas differential pressure doesn't. Regardless, it depends on context.
 

FAQ: Need help understanding negative pressure

What is negative pressure?

Negative pressure, also known as suction or vacuum, is a force that pulls objects towards a center point. It is the opposite of positive pressure, which pushes objects away. Negative pressure is commonly used in scientific experiments and in medical procedures such as wound drainage.

How is negative pressure measured?

Negative pressure is typically measured in units of pressure, such as atmospheres (atm) or pounds per square inch (psi). It can also be measured in units of force, such as newtons (N) or pounds (lb). In laboratory settings, negative pressure is often measured using a vacuum gauge or a manometer.

What are some examples of negative pressure in nature?

Negative pressure is a natural phenomenon that can be observed in various situations. Some examples include the formation of a tornado, the sucking action of a vacuum cleaner, the inhalation of air into the lungs, and the pulling of water up a tree through capillary action.

How does negative pressure affect gases and liquids?

Negative pressure can cause gases and liquids to expand or contract. This is because negative pressure creates a lower pressure environment compared to the surrounding atmosphere, causing the gas or liquid to move towards the area of lower pressure. This can result in the formation of bubbles in liquids or the expansion of gases in a container.

What are the practical applications of negative pressure?

Negative pressure has a wide range of applications in science, engineering, and medicine. It is commonly used in vacuum pumps, pneumatic systems, and filtration systems. In medicine, negative pressure is used in wound healing and to assist with respiratory problems. Negative pressure is also utilized in various manufacturing processes and in the preservation of food and other perishable items.

Back
Top