what is the meaning of the negative pressure?

Answer 1

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.


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

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.

Answer 2

Negative pressure is a term used to describe a pressure less than that of a surrounding fluid.

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.

Answer 3

The other poster is correct, but if you're not looking for a scientific explanation, negative pressure is basically just suction (like a vacuum).

This is because pressure flows from areas of high pressure to low pressure (pressure gradient force).