Physics
Pressure
Pressure is the force applied perpendicular to the surface of an object per unit area. It is a measure of how much force is distributed over a given area and is calculated by dividing the force applied by the area over which it is distributed. In physics, pressure is a fundamental concept used to describe various phenomena, such as fluid dynamics and mechanical systems.
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9 Key excerpts on "Pressure"
eBook - PDF- Charles Thomas(Author)
- 2015(Publication Date)
- Cengage Learning EMEA(Publisher)
Pressure —force or weight per unit area (Force 4 Area 5 Pressure); measured in pounds per square inch. Temperature —the hotness or coldness of a substance. 4.1 Basic Principles of Pressure Pressure is defined as force or weight per unit area (Force 4 Area 5 Pressure). The term Pressure is typically applied to gases or liquids. Pressure is measured in pounds per square inch (psi). Atmospheric Pressure is produced by the weight of the atmosphere as it presses down on an object resting on the surface of the earth. “The earth is surrounded by a fluid consisting of 78% nitrogen and 21% oxygen. Pressure at the top of this fluid, “air” is measured at zero psia and 14.7 psia (1.3 kPa) at sea level. Figure 4–1 illustrates this point. The higher the atmosphere, or gas, or liquid, the greater the Pressure at the bottom. In a liquid, Pressure is not dependent upon the shape or size of the vessel or pipe. Figure 4–2 illus-trates this point. Pressure is equal to the force divided by the area. A simple equation can be used to calculate Pressure in a process system. Height 3 .433 3 specific gravity 5 Pressure. Any additional Pressure or force above the liquid must be added to the answer. Vapor Pressure, nitrogen blankets, or control Pressures are examples of variables that should be added into the total Pressure. The factor of .433 was developed using the equation P 5 F 4 A. Figure 4–3 illustrates how this factor was developed. Specific gravity for a substance is also calculated using the water standard. For example: 1 gallon of water weighs approximately 8.33 pounds. 8.33 4 8.33 5 1. The spe-cific gravity of water is 1. Other substances have different weights. For example, if 1 gallon of a substance weighs 6.5 pounds, it’s specific gravity can be calculated by dividing 6.5 pounds by 8.33 pounds 5 .78. The specific gravity of this new substance is .78. Using this simple approach the specific gravity of any substances can be calculated.
eBook - PDFPhysics of Continuous Matter
Exotic and Everyday Phenomena in the Macroscopic World
- B. Lautrup(Author)
- 2011(Publication Date)
- CRC Press(Publisher)
The component acting tangentially to the surface is called a shear force or a traction force . Fluids in motion, and solids at rest or in motion, are able to sustain shear forces, whereas fluids at rest cannot. Should shear forces arise in a fluid at rest, it will begin to flow until it again reaches mechanical equilibrium without shear forces. In this chapter we shall first investigate the basic properties of Pressure, and afterward develop the mathematical formalism that permits us to analyze hydrostatic equilibrium in the sea and the atmosphere. Along the way we shall recapitulate some basic rules of thermody-namics. In the following chapters we shall continue to study the implications of hydrostatic equilibrium for balloons and ships, and the shapes of large fluid bodies subject only to gravity and small fluid bodies subject mainly to surface tension. 2.1 What is Pressure? Pressure is defined as normal force per unit of area . The SI–unit for Pressure is accordingly newton per square meter, but was in 1971 given the name pascal and the special symbol Pa D N m 2 . Earlier units for Pressure were the bar ( 1 bar D 10 5 Pa) and the standard atmosphere ( 1 atm D 101; 325 Pa), which is close to the average air Pressure at sea level. Modern television weather forecasters are now abandoning the older units and tend to quote air Pressure in hectopascals rather than in millibars, even if they are exactly the same ( 1 hPa D 100 Pa D 10 3 bar D 1 millibar). 22 PHYSICS OF CONTINUOUS MATTER Case: The incompressible sea A p 0 p h 6 pA ? p 0 A r ? Mg 0 A column of sea water. The pres-sure difference between bottom and top must carry the weight of the water in the box. Notice that the sum of the forces vanishes. Before presenting a formal definition of the Pressure field, we use simple arguments to cal-culate it in the sea. In the first approximation, water is incompressible and has everywhere the same mass density 0 .
- Ravi Doddannavar, Andries Barnard, Jayaraman Ganesh(Authors)
- 2005(Publication Date)
- Newnes(Publisher)
These molecules move throughout the volume of the fluid colliding with each other and with the walls of the container as a result of which the molecules undergo a change in momentum. Now, let us consider a surface within the fluid which is impacted by a large number of molecules. This results in a transfer in momentum from the molecules to the surface. The change in momentum transferred per second by these molecules on the surface gives the average force on the surface, while the normal force exerted by the fluid per unit area of the surface is known as fluid Pressure. 2.2.2 Pressure at a point in a liquid The Pressure at any point in a fluid at rest, is given by the Hydrostatic law, which states that the rate of increase of Pressure in a vertically downward direction must be equal to the specific weight of the fluid at that point. The vertical height of the free surface above any point in a liquid at rest is known as the Pressure head. This implies that the Pressure (called head Pressure) at any point in a liquid is given by the equation: = P gh ρ Where ρ is the density of the liquid h is the free height of the liquid above the point and g is the acceleration due to gravity. Thus, the Pressure at any point in a liquid is dependent on three factors: 1. Depth of the point from the free surface 2. Density of the liquid 3. Acceleration due to gravity. 18 Practical Hydraulic Systems 2.2.3 Atmospheric, absolute, gage Pressure and vacuum Atmospheric Pressure The earth is surrounded by an envelope of air called the atmosphere, which extends upwards from the surface of the earth. Air has mass and due to the effect of gravity exerts a force called weight. The force per unit area is called Pressure. This Pressure exerted on the earth’s surface is known as atmospheric Pressure. Gage Pressure Most Pressure-measuring instruments measure the difference between the Pressure of a fluid and the atmospheric Pressure.
No longer available |Learn more- (Author)
- 2014(Publication Date)
- Learning Press(Publisher)
If we enclose the gas within a container, we detect a Pressure in the gas from the molecules colliding with the walls of our container. We can put the walls of our container anywhere inside the gas, and the force per unit area (the Pressure) is the same. We can shrink the size of our container down to an infinitely small point, and the Pressure has a single value at that point. Therefore, Pressure is a scalar quantity, not a vector quantity. It has magnitude but no direction sense associated with it. Pressure acts in all directions at a point inside a gas. At the surface of a gas, the Pressure force acts perpendicular (at right angle) to the surface. ________________________ WORLD TECHNOLOGIES ________________________ A closely related quantity is the stress tensor σ , which relates the vector force F to the vector area A via This tensor may be divided up into a scalar part (Pressure) and a traceless tensor part shear. The shear tensor gives the force in directions parallel to the surface, usually due to viscous or frictional forces. The stress tensor is sometimes called the Pressure tensor, but in the following, the term Pressure will refer only to the scalar Pressure. Types Explosion or deflagration Pressures Explosion or deflagration Pressures are the result of the ignition of explosive gases, mists, dust/air suspensions, in unconfined and confined spaces. Negative Pressures While Pressures are generally positive, there are several situations in which negative Pressures may be encountered: • When dealing in relative (gauge) Pressures. For instance, an absolute Pressure of 80 kPa may be described as a gauge Pressure of -21 kPa (i.e., 21 kPa below an atmospheric Pressure of 101 kPa). • When attractive forces (e.g., van der Waals forces) between the particles of a fluid exceed repulsive forces. Such scenarios are generally unstable since the particles will move closer together until repulsive forces balance attractive forces.
eBook - PDFFundamentals of Physics
Mechanics, Relativity, and Thermodynamics
- R. Shankar(Author)
- 2014(Publication Date)
- Yale University Press(Publisher)
chapter 20 Fluids 20.1 Introduction to fluid dynamics and statics This is a relatively simple topic. If you took any kind of high school physics, you would have seen fluids. Whenever I say fluid, you are free to imagine water or oil. 20.1.1 Density and Pressure Let us begin with a basic property of the fluid, the density, denoted by ρ . The density of water is ρ w = 1, 000 kg / m 3 . The more subtle concept is the one of Pressure . If you dive down to the bottom of a swimming pool, the Pressure goes up. What is the formal definition of Pressure? That’s what I want to explain. If we pick a point in the fluid and say the Pressure there is such and such, we mean the following. Say you get into that fluid and you want to carve out a little space for yourself, maybe a glass cube, and you want to live inside that cube. The water is trying to push you in from all sides and compress this cube. You therefore have to push out on all the walls. If the force you exert on a wall is some F and the area of that wall is A , that ratio is the Pressure. The Pressure will not depend on which wall you choose, provided the cube you are in is infinitesimal. The Pressure is an intensive measure of how hard the water is trying to push in. Even if you don’t insert the cube, that Pressure is still there, but one way to measure the Pressure 335 336 Fluids Figure 20.1 A gas in a cylinder with a massless piston of area A on top. The mass m exerts a force F = mg and a Pressure P = mg A Pascals. is to try to go in there and push the fluid out and ask how hard it pushes back. The unit of Pressure is N / m 2 and is called a Pascal . Here’s another example of Pressure. You have a gas inside a cylinder as shown in Figure 20.1, with a massless piston at the top. The Pressure of the gas and the Pressure of the outside world are the same. But if you want to increase the Pressure in the gas, you can put some extra weights on the piston.
eBook - PDF- David Halliday, Robert Resnick, Jearl Walker(Authors)
- 2018(Publication Date)
- Wiley(Publisher)
C H A P T E R 1 4 Fluids 14-1 FLUIDS, DENSITY, AND Pressure Learning Objectives After reading this module, you should be able to . . . 14.01 Distinguish fluids from solids. 14.02 When mass is uniformly distributed, relate density to mass and volume. 14.03 Apply the relationship between hydrostatic pres- sure, force, and the surface area over which that force acts. ● The density of any material is defined as the material’s mass per unit volume: ρ = Δm ΔV . Usually, where a material sample is much larger than atomic dimensions, we can write this as ρ = m V . ● A fluid is a substance that can flow; it conforms to the boundaries of its container because it cannot with- stand shearing stress. It can, however, exert a force perpendicular to its surface. That force is described in terms of Pressure p: p = ΔF ΔA , in which ∆F is the force acting on a surface element of area ∆A. If the force is uniform over a flat area, this can be written as p = F A . ● The force resulting from fluid Pressure at a particular point in a fluid has the same magnitude in all directions. Key Ideas What Is Physics? The physics of fluids is the basis of hydraulic engineering, a branch of engineering that is applied in a great many fields. A nuclear engineer might study the fluid flow in the hydraulic system of an aging nuclear reactor, while a medical engineer might study the blood flow in the arteries of an aging patient. An environmental engineer might be concerned about the drainage from waste sites or the efficient irrigation of farmlands. A naval engineer might be concerned with the dangers faced by a deep-sea diver or with the possibility of a crew escaping from a downed submarine. An aeronautical engineer might design the hydraulic systems control- ling the wing flaps that allow a jet airplane to land. Hydraulic engineering is also applied in many Broadway and Las Vegas shows, where huge sets are quickly put up and brought down by hydraulic systems.
- David Halliday, Robert Resnick, Jearl Walker(Authors)
- 2023(Publication Date)
- Wiley(Publisher)
Physical quantities that we find useful, and in whose terms we express Newton’s laws, are mass and force. We might speak, for example, of a 3.6 kg block acted on by a 25 N force. With fluids, we are more interested in the extended substance and in proper- ties that can vary from point to point in that substance. It is more useful to speak of density and Pressure than of mass and force. Density To find the density of a fluid at any point, we isolate a small volume element ∆V around that point and measure the mass ∆m of the fluid contained within that element. The density is then ρ = Δ m _ ∆V . (14.1.1) In theory, the density at any point in a fluid is the limit of this ratio as the volume element ∆V at that point is made smaller and smaller. In practice, we assume that a fluid sample is large relative to atomic dimensions and thus is “smooth” (with uniform density), rather than “lumpy” with atoms. This assumption allows us to write the density in terms of the mass m and volume V of the sample: ρ = m _ V (uniform density). (14.1.2) Density is a scalar property; its SI unit is the kilogram per cubic meter. Table 14.1.1 shows the densities of some substances and the average densities of some objects. Note that the density of a gas (see Air in the table) varies con- siderably with Pressure, but the density of a liquid (see Water) does not; that is, gases are readily compressible but liquids are not. Pressure Let a small Pressure-sensing device be suspended inside a fluid-filled vessel, as in Fig. 14.1.1a. The sensor (Fig. 14.1.1b) consists of a piston of surface area ∆A riding in a close-fitting cylinder and resting against a spring. A readout arrangement allows us to record the amount by which the (calibrated) spring is compressed by the surrounding fluid, thus indicating the magnitude ∆F of the force that acts normal to the piston. We define the Pressure on the piston as p = ΔF _ ΔA .
eBook - PDFThe Atmospheric Environment
Effects of Human Activity
- Michael B. Mcelroy(Author)
- 2021(Publication Date)
- Princeton University Press(Publisher)
A dif- ferential Pressure of this magnitude corresponds to a force of 10 5 dyn cm −2 , comparable to the force delivered by a pow- erful bomb. 2.9 The Perfect Gas Law The Pressure, density, and temperature of a gas are not inde- pendent quantities. They are related according to an equa- tion known as the equation of state. For a gas of relatively low density, a condition satisfied by the atmosphere, the equation of state assumes the simple form p = nkT , (2.16a) where n defines the number density of molecules, the num- ber of molecules per unit volume (in the cgs system, n is ex- pressed in molecules per cm 3 , abbreviated as cm –3 ). Expressed in the form of (2.16a), the equation of state is known as the perfect gas law. Chemists favor a different expression of the perfect gas law. They prefer to measure Pressure in units of atmospheres (atm), density in units of moles (mol) per liter (1) (a mole is equivalent to 6.02 × 10 23 molecules, where 6.02 × 10 23 is known as Avogadro’s number, discussed further in Section 3.3). It is no longer appropriate in this case to use k as the constant in the equation relating p, n, and T. The alternate, chemists’ expression for the perfect gas law is P = NRT , (2.16b) where P is given in atm, N in mol l −1 , and T in K. R, known as the universal gas constant, has the numerical value 0.08206, with units of atm l mol −1 K −1 . h P a P a P a P a Atmospheric Pressure Atmospheric Pressure Figure 2.4 Schematic view of a mercury barometer. Remember that gh = P a . The height (h) of the mercury in the column is proportional to the Pressure P a with which air presses down on the mercury outside of the column. We can rationalize the form of the perfect gas law by considering the momentum content of a gas confined in a container with elastic walls. Molecules are buzzing in all di- rections. From time to time they strike the walls.
- James W. Murdock(Author)
- 2018(Publication Date)
- CRC Press(Publisher)
Fluids will support small tensile forces due to the prop erty of su rfa ce tension (Section 1.17). Fluids can withstand compression forces, commonly called p re ssu re. Atmospheric Pressure The a c tu a l atmospheric Pressure is the weight per unit area of the air above a datum and varies with weather conditions. Since this Pressure is usually measured with a barometer, it is commonly called b a ro m etric p re ssu re. Standard Atmospheric Pressure By international agreement the standard atmospheric Pressure is defined as 101.325 kN/m2 (kPa). Converting this value into common units, we have 14.696 lbf/in.2 and 29.92 in. of mercury at 32°F. For most practical purposes 14.70 lbf/in.2 may be used for atmospheric Pressure. Numbers in brackets are those of references at the end of this chapter. 6 Chapter 1 Figure 1.4 Pressure relations. Observed Pressures Most Pressure-sensing devices (Section 2.5) (the barometer is an excep tion) indicate the difference between the Pressure to be measured and atmospheric Pressure. As shown in Figure 1.4, if the Pressure being sensed is g re a te r than atmospheric it is called g a g e p r e s s u r e , and if lo w er (neg ative gage) it is called a va cu u m . The algebraic sum of the instrument reading and the actual atmospheric Pressure is the true or absolute pres sure. Thus: p = pb + pi ( 1 . 1 ) where p is the absolute Pressure, p b the atmospheric (barometric) pres sure, and pi the instrument reading (positive for gage Pressure, negative for vacuum). A ll in stru m en t readin gs m u st be c o n v e rte d to a b so lu te p r e s sure b efo re th ey are u se d in calcu lation s. Conventional American engineering practice is to use the unit lbf/in.2 (psi) for Pressure. Gage Pressures are indicated by psig and absolute pres sures by psia. Vacuums are almost always reported in inches of mercury at 32°F. There is no equivalent of gage Pressure in the SI system, so that all Pressures are absolute unless gage is specified.
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