Pressure and Density

4 Key excerpts on "Pressure and Density"

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  eBook - PDF

  Science of Diving

  Concepts and Applications

  • Bruce Wienke(Author)
  • 2015(Publication Date)
  • CRC Press

    (Publisher)

  And then the structure of these components might be further divided. Both Pressure and Density are intuitive, fundamental concepts, elucidated and measured at early times in our scientific history by the Greeks, Romans, Babylonians, Egyptians, and probably others, well before atomic hypotheses. Pressure, P , is simply the force, F , per unit area, A , that is, P = F A and is equal in all directions (scalar quantity, while force itself is formally a vector quantity). As will be seen, pressure in gases results from molecular collisions with surroundings. Pressure from extended matter results from the collective forces applied across boundaries of fluids and solids. Density Density ρ , similarly is mass, m , per unit volume, V , ρ = m V and suggests how tightly packed matter can exist. Weight density, ρ g , is weight per unit volume, differing from mass density by the acceleration of gravity, g . Both are used interchangeably in applications. Objects denser than a fluid will sink in that fluid, and objects less dense than a fluid will float. Sinking objects have negative buoyancy, while floating objects have positive buoyancy. Objects with the same density as the fluid have neutral buoyancy, and can be moved about without sinking or rising. Relative buoyancy obviously depends on fluid and object densities. Table 7 list densities of known, naturally occurring, elements as function of atomic number, Z , and atomic mass, A . Part 2 : Pressure , Density , and Bubbles 85 Table 7.

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  Introduction to Physics

  • John D. Cutnell, Kenneth W. Johnson, David Young, Shane Stadler(Authors)
  • 2015(Publication Date)
  • Wiley

    (Publisher)

  11.1 | Mass Density Fluids are materials that can flow, and they include both gases and liquids. Air is the most common gas and flows from place to place as wind. Water is the most familiar liquid, and flowing water has many uses, from generating hydroelectric power to white-water rafting. The mass density of a liquid or gas is one of the important factors that determine its be- havior as a fluid. As indicated below, the mass density is the mass per unit volume and is denoted by the Greek letter rho (r). Definition of Mass Density The mass density r is the mass m of a substance divided by its volume V: r 5 m V (11.1) SI Unit of Mass Density: kg/m 3 Equal volumes of different substances generally have different masses, so the density depends on the nature of the material, as Table 11.1 indicates. Gases have the smallest den- sities because gas molecules are relatively far apart and a gas contains a large fraction of empty space. In contrast, the molecules are much more tightly packed in liquids and solids, and the tighter packing leads to larger densities. The densities of gases are very sensitive to changes in temperature and pressure. However, for the range of temperatures and pressures encountered in this text, the densities of liquids and solids do not differ much from the values in Table 11.1. It is the mass of a substance, not its weight, that enters into the definition of density. In situations where weight is needed, it can be calculated from the mass density, the volume, and the acceleration due to gravity, as Example 1 illustrates. The air is a fluid, and this chapter examines the forces and pressures that fluids exert when they are at rest and when they are in motion. In a tornado the air is moving very rapidly, and as we will see, moving air has a lower pressure than stationary air. This difference in air pressure is one of the reasons that tornadoes, such as the one in this photograph, are so destructive.

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  The Characterization of Chemical Purity

  Organic Compounds

  • L. A. K. Staveley(Author)
  • 2016(Publication Date)
  • Butterworth-Heinemann

    (Publisher)

  DENSITY MEASUREMENTS TOMASZ PLEBANSKI Division of Physico-Chemical Metrology, National Board for Quality Control and Measures (CUJiM), Warsaw, Poland 1. INTRODUCTION Specific mass, p (t, p), commonly called density, is a strictly defined property of physical bodies, namely the mass per unit volume at a definite temperature t and pressure p. Since this discussion will be simplified by confining it, with few exceptions, to liquids, the small influence of atmospheric pressure variations will be neglected, and the absolute density at temperature t will be denoted p t , or simply p if it is clear that all densities under consideration are referred to the same temperature. For pure compounds p can only be predicted approximately from various semi-quantitative correlations between density and chemical structure. We shall therefore consider density as a quantity determined by experiment, and we shall be interested primarily in the relation between density and the composition of the material in question. This relation may be used as a purity criterion whenever material consists of a main component and a small amount of contamination. Under favourable conditions, when the average density of impurities differs from that of the main component by 0-1-1 g/cm 3 , the accuracy and sensitivity of this criterion may be as high as 10~ 2 and 10 -3 per cent by weight, respectively, provided density differences as low as 10 -6 g/cm 3 are measurable. Generally, however, impurity concentrations which may usefully be discussed are of the order of 0*1-1 per cent by weight. The examination of purity by density measurements is always a matter of comparison of the density />* of the material in question with a reference density pr) associated with a definite purity of the same material. Hence the accuracy of purity determination by this method depends very much on the critical use of reference materials and reference numerical data.

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  Engineering Fluid Mechanics

  • Donald F. Elger, Barbara A. LeBret, Clayton T. Crowe, John A. Roberson(Authors)
  • 2022(Publication Date)
  • Wiley

    (Publisher)

  • Explain the constant density assumption and make decisions about whether or not this assumption is valid. • Determine changes in the density of water corresponding to a pressure change or a temperature change. STRESS (§2.4) • Define stress, pressure, and shear stress. • Explain how to relate stress and force. • Describe each of the seven common fluid forces. THE VISCOSITY EQUATION (§2.5) • Define the velocity gradient and find values of the velocity gradient. • Describe the no-slip condition. • Explain the main ideas of the viscosity equation. • Solve problems that involve the viscosity equation. • Describe a Newtonian and non-Newtonian fluid. SURFACE TENSION (§2.6) • Know the main ideas about surface tension. • Solve problems that involve surface tension. VAPOR PRESSURE (§2.7) • Explain the main ideas of the vapor pressure curve. • Find the pressure at which water will boil. Chapter 2: Fluid Properties 38 EXAMPLE Suppose an engineer is analyzing the air flow from a tank being used by a SCUBA diver. As shown in Fig. 2.1, the engineer might select a system comprised of the tank and the regulator. For this system, everything that is external to the tank and regulator is the surroundings. Notice that the system is defined with a sketch because this is sound profes- sional practice. If you make a wise choice when you select a system, you increase your probability of get- ting an accurate solution, and you minimize the amount of work you need to do. Although the choice of system must fit the problem at hand, there are often multiple possibilities for which system to select. This topic will be revisited throughout this textbook as various kinds of sys- tems are introduced and applied. Systems are described by specifying numbers that characterize the system. The numbers are called properties. A property is a characteristic of a system that depends only on the pres- ent conditions within the system.

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