Negative pressure
From Free net encyclopedia
- Absolute negative pressures occur in some theories in physics. See dark energy, equation of state (cosmology) and negative mass.
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.
Except in special circumstances, the lowest possible pressure is a vacuum, which corresponds to −1 atm gauge pressure (measured relative to atmospheric pressure; vacuum is also 0 atm absolute pressure). Actually what is meant is that there is a pressure difference between a region of high pressure and another region of low 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.
Some examples of negative 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.
Negative pressure is the opposite of positive pressure. But pressure is due to the force that pushes in on all sides when an object is submerged in a fluid (liquid or gas). That external pressure would drop to zero in a complete vacuum. So negative pressure would then actually mean that there is an outward pull on an object when it is surrounded by some other rather odd material. This makes for great science fiction but not science fact.
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.