Negative pressure

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Absolute negative pressures occur in some theories in physics. See dark energy, equation of state (cosmology) and negative mass.

Pressure is defined in terms of a force applied over an area. In many physics problems we consider idealised situations; typically a single, positive ("pushing") force acting on a surface. For example, an ideal gas is modelled as particles (eg. air molecules) that strike (and elastically bounce away from) the walls of its container. In many idealised contexts like this, pressure cannot be negative: the lowest pressure is zero (eg. in a vacuum, when no gas molecules are present to strike the container walls).

However, the term "pressure" is used in a much wider variety of situations. It may be used to refer to the difference in forces on two sides of a surface, for example, or in a model where a surface is being "pulled" rather than "pushed". In cases like these, the pressure difference may be negative, or the system may behave in a way most simply described as having negative pressure.

In theoretical physics "pressure" may refer to more abstract ideas that emerge from the use of mathematical equations. If an equation for a particular system resembles that for an ideal gas, for example, then physicists may identify the term that takes the role of pressure in the ideal gas formula and refer to that term (and to whatever physical idea it represents) as "pressure". Doing this helps physicists use their intuitive understanding of everyday objects (like gas filled balloons) to help work with the complex mathematics involved in physical processes outside normal experience (like the evolution of the universe over billions of years). And, again, the term identified as "pressure" may take on negative values in these cases.

So while "negative pressure" makes little sense in idealised situations (common in both physics textbooks and as close approximations to many real life problems), it can be a useful in many other contexts.

[edit] Negative gauge pressure

Negative pressure is a term used to describe a pressure less than that of a surrounding fluid (such as the air). The term comes from gauge pressure pressure gauges, which measured a pressure against air pressure. These were extensively used in steam engines because the pressure was relative to air pressure, which was the final pressure of the exhaust steam. The standard pressure gauge design (Bourdon tube) measures gauge pressure by default, making this an easy design choice. Negative pressure meant that the pipe or vessel being measured was below atmospheric pressure.

Negative pressure is the opposite of positive pressure.

[edit] Some examples of negative pressure

A room with a lower pressure than its surroundings will cause air to flow into the room from the outside whenever a door or window is opened. It is often used in a hospital setting to quarantine people afflicted with highly contagious, deadly, airborne diseases. Some player pianos used vacuum to drive the mechanism and measured things in negative inches of water pressure.

Large pressure differences are also often found in sub-sea pipelines where the depth of water produces such a high external pressure that any leak would actually be from the sea water into the pipeline which is a good way to avoid oilspills. This is due to difference in densities between crude oil (800 kg/m³) and water (1000 kg/m³) resulting in a difference of pressure of 2 kPa/m depth, or a difference in pressure of 20 atmospheres 1 km down.

[edit] Negative absolute pressure

In normal circumstances, the lowest possible pressure is a vacuum, which corresponds to 0 absolute pressure (or −1 atm gauge pressure measured relative to atmospheric pressure).

However, if the fluid has a high surface tension, such as is the case with water, it is possible to make higher negative pressures than a pure vacuum. What is actually happening here is the water's surface exerts a force on anything contained within the water by sticking to the surface. If the water has gases dissolved some will form bubbles and relieve the force, but if the water has had all the gas removed it is very hard for the water to form a new surface and so a pressure below 0 absolute is possible. This is crucial to the transpirational pull which allows leaves to pull water hundreds of meters up from the roots.