what is absolute pressure and gauge pressure ?obtain the expression for it also find the expression for the pressure at a some distant above the liquid surface
Answers
Explanation:
Pressure (P) = Thrust/ Area
The SI unit is ‘pascals (Pa)’. 1 Pa = 1N/m2
Example: It is easier to hammer a sharp pin than to hammer a blunt pin. This is because the area at the end of the sharp pin is smaller than the area at the end of a blunt pin. This leads to an increase in pressure leading to hammer the sharp pin easily.
Absolute pressure is the sum of gauge pressure and atmospheric pressure.
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In fact, atmospheric pressure does add to the pressure in any fluid not enclosed in a rigid container. This happens because of Pascal’s principle. The total pressure, or absolute pressure, is thus the sum of gauge pressure and atmospheric pressure:
Pabs=Pg+Patm(11.7.1)(11.7.1)Pabs=Pg+Patm
where PabsPabs is absolute pressure, PgPg is gauge pressure, and PatmPatm is atmospheric pressure. For example, if your tire gauge reads 34 psi (pounds per square inch), then the absolute pressure is 34 psi plus 14.7 psi (PatmPatm in psi), or 48.7 psi (equivalent to 336 kPa).
Definition: Absolute Pressure
Absolute pressure is the sum of gauge pressure and atmospheric pressure.
For reasons we will explore later, in most cases the absolute pressure in fluids cannot be negative. Fluids push rather than pull, so the smallest absolute pressure is zero. (A negative absolute pressure is a pull.) Thus the smallest possible gauge pressure is Pg=−PatmPg=−Patm (this makes \(P_{abs}|) zero).
There is no theoretical limit to how large a gauge pressure can be.
There are a host of devices for measuring pressure, ranging from tire gauges to blood pressure cuffs. Pascal’s principle is of major importance in these devices. The undiminished transmission of pressure through a fluid allows precise remote sensing of pressures. Remote sensing is often more convenient than putting a measuring device into a system, such as a person’s artery.
Figure shows one of the many types of mechanical pressure gauges in use today. In all mechanical pressure gauges, pressure results in a force that is converted (or transduced) into some type of readout.
Figure 11.7.111.7.1: This aneroid gauge utilizes flexible bellows connected to a mechanical indicator to measure pressure.
An entire class of gauges uses the property that pressure due to the weight of a fluid is given by P=hρgP=hρg.
Consider the U-shaped tube shown in Figure, for example. This simple tube is called a manometer. In Figure(a), both sides of the tube are open to the atmosphere. Atmospheric pressure therefore pushes down on each side equally so its effect cancels. If the fluid is deeper on one side, there is a greater pressure on the deeper side, and the fluid flows away from that side until the depths are equal.
Let us examine how a manometer is used to measure pressure. Suppose one side of the U-tube is connected to some source of pressure PabsPabs such as the toy balloon in Figure(b) or the vacuum-packed peanut jar shown in Figure(c). Pressure is transmitted undiminished to the manometer, and the fluid levels are no longer equal. In Figure(b), PabsPabs is greater than atmospheric pressure, whereas in Figure(c), PabsPabs is less than atmospheric pressure. In both cases, PabsPabs differs from atmospheric pressure by an amount hρghρg, where ρρ is the density of the fluid in the manometer. In Figure(b), PabsPabs can support a column of fluid of height hh, and so it must exert a pressure hρghρg greater than atmospheric pressure (the gauge pressure Pg