What is Pressure?

autoclave/pressure sterilizer

Pressure is the force per unit area applied to the surface of an object in a vertical direction (i.e. perpendicular to the surface). Gauge pressure is related to the local atmospheric or ambient pressure. Pressure is a force is applied on the surface of that object measured per unit area of surface.

Pressure is represented by the symbol p or P. The SI unit for pressure is the pascal (Pa) and is equal to one newton per square meter (N/m2 or kg/m.s2 - prior to 1971 the unit was simply known as N/m2). Non-SI measures in common use are pounds per square inch (psi) and bar. In cgs units the pressure is expressed as the barye (ba). One barye is equal to 1 dyn/cm2. Sometimes pressure is also expressed as grams-force/cm2, or as kgf/cm2, where a kilogramme-force is the weight exerted by a kilogramme at the earth’s surface.

The same force applied to a smaller area will result in a larger pressure. This is the principle behind any sharp object such as a knife or a pin. The pressure applied to the large handle of the knife is applied through the much smaller area of the knife blade. Similarly the point of a pin can exert great pressure because of its small area, enough to penetrate an object unless it is made of a stronger material than the pin itself.

Atmospheric pressure is defined as the force from the weight of the atmosphere that pushes down on a unit area. When we experience changes in pressure with changing altitude it can cause our ears to pop as the internal and external pressures equalize. This is even more noticeable in a liquid such as sea water, where even a small change in depth can be felt. If the pressure is decreased too rapidly by surfacing quickly after diving, it can cause gas bubbles to form in the blood resulting in decompression sickness.

Blood pressure is defined as the force that the blood exerts on a unit area in our artery walls due to the pumping of the heart. If the blood pressure is too low there will not be enough force to push the blood around the body. A high blood pressure is a warning sign of a variety of impending health problems. Blood pressure is measured using a special kind of manometer called a sphygmomanometer.

The motion of fluids is greatly dependent on pressure. The increased pressure in front of a moving object causes the motion of the air which moves around the object. If the object moves too fast for the pressure field to travel (which it does at the speed of sound) the air molecules are unable to move out of the way and a shock wave forms. These shock waves are a feature of supersonic flow, for example around fighter aircraft.

Pressure differences can be measured using a manometer, which is a graduated U-tube containing a liquid. A device to measure atmospheric pressure is called a barometer, which can be a kind of manometer, or it may consist of some kind of sealed cell which will be compressed to a greater or lesser degree by the changing pressure of the surrounding atmosphere (aneroid barometer). Barometers are widely used to predict weather conditions because of the connection between atmospheric pressure and air movement which is the driving force behind weather systems.

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