Pascal, definition

Pascal 3. The hydraulic lever. The hydraulic jack is a problem in fluid equilibrium, just as a pulley system is a problem in mechanical equilibrium (no accelerations involved). It s the static situation in which a small force on a small piston balances a large force on a large piston. No change of pressure need be involved here. A constant force on one piston slowly lifts a different piston with a constant force on it. At all times during this process the fluid is in near-equilibrium. This principle is no more than an application of the definition of pressure as F/A, the quotient of... 

In most vacuum work where a high degree of accuracy is not required, gas pressures are measured in Torr. Where published results call for the highest degree of accuracy, gas pressures measured in Torr can be coixected for temperature and gravity and converted to pascals using the definition... 

The SI unit of pressure is the pascal (Pa), defined as the pressure exerted by a force of one newton acting on an area of one square meter. By definition, one newton (N) is the force required to give a mass of one kilogram an acceleration of one meter per second per second. Symbolically we represent one newton as... 

The definition of a invariant with respect to positioning of the dividing surface can be worked out, if one analyzes trends in the/z)-pc(z) function within the discontinuity surface. The specified quantity has the same value in the bulk of both phases, equal to the negative external pressure (Fig. 1-4). Within the discontinuity surface, pressure p has a tensor nature, making Pascal s law invalid. Meanwhile, the concentration and pressure dependence of the surface energy density,/ given by eq. (1.1), is valid only in the regions where Pascal s law holds, i.e., where pressure is a scalar quantity (direct summation of a scalar and a tensor within the same equation is not permitted). 

Pressure is a force acting on a unit area. In mass spectrometry, a unit of pressure commonly used is the Torr, where 1 Torr is equivalent to approximately 1 mm of mercury, and 760 mm of mercury is 1 atmosphere (atm). A more accurate definition, which accounts for the variation in the density of mercury, is that a Torr is 1/760 of an atmosphere, which is itself defined as 101,325 pascals (Pa). There are a number of other units in use, and this can (and often does) cause confusion e.g., while Torr is typically used in the United States, millibar (1/1,000 atm) is used in Europe and pascal in Japan. 

The SI unit for pressure is a derived unit that reflects the definition of pressure. The SI units for force and area are the newton and the square meter, respectively. So pressure is measured in N m . This unit has also been named the Pascal, Pa. One newton is roughly the force exerted by gravity on a one-quarter-pound object. When spread out over a square meter, this force does not produce very much pressure, so the Pa is a relatively small quantity. Typical atmospheric pressures are on the order of 10 Pa. Thus a more practical unit is kPa, which is the unit used in weather forecasts in Canada and other metric countries. ... 

But the definition of the unit atm has since been changed to reflect SI units. It is now defined in terms of the pascal ... 

The basic definition of viscosity should be considered in terms of equation (8.3). Consider two 1-cm planes 1 cm apart imbedded in a liquid. If it takes 1 dyne of force to move one of the planes 1 cm/s relative to the other in a shearing motion, the liquid has a viscosity of 1 poise. Viscosity is also expressed in pascal-seconds, with IPa-s = 0.1 poise. 

Prior to 1982, the standard pressure was one atmosphere (1 atm), and this is still in common use. The difference in the two definitions is not great since 1 atm = 1.013 bar. The unit of pressure in the International System of Units (i.e., the SI system) is the pascal (Pa). The basic and derived units for the SI system are given in Appendix I for the most part, we will adhere to the SI system in this book. 

The situation with respect to pressure in SI units and thermodynamics is, however, not so happy. The SI pressure unit is the newton per square metre (N m ) or pascal (Pa) and the quantity previously called the standard atmosphere is equal to 101.325 kNm . The difficulty arises because for many purposes (for example in the definition of enthalpies of formation) the standard pressure has been chosen as the standard atmosphere. Thus not only is the number 101.325 involved between the old and new systems, but no universal factor can be applied to change the value of a thermodynamic quantity for a real vapour from a pressure of 101.325 kN m" to say 100kNm . Therefore it appears likely that for many years in the future thermodynamicists will have to head their tables, Standard pressure 101.325 kPa (formerly known as 1 standard atmosphere) . 

Units of pressure come from the definition. In the United States Customary System, if force is measured in pounds and area in square inches, the pressure unit is pounds per square inch (psi). The SI unit of pressure is the pascal (Pa), which is one newton per square meter. (The newton is the SI unit of force.) Although chemists generally follow SI guidelines, one pascal is a very small pressure the kilopascal (kPa) is a more practical unit. Many other pressure units are commonly used. One is the bar, which is 1 X 10 Pa or 100 kPa. The millimeter of mercury, or its equivalent, the torr, and the atmosphere are other common units of pressure. The millimeter of mercury is usually abbreviated mm Hg. (Hg is the elemental symbol for mercury.)... 

Unit pascal Symbol Pa Definition/relationship SI pressure unit m Application scientific (kPa)... 

Although the pascal is the SI unit of pressure, there are other units of pressure that are more commonly used. Table 11.2 lists the units in which pressure is most commonly expressed in chemistry and their definitions in terms of pascals. Which of these units you encounter most often will depend on your specific field of study. Use of atmospheres (atm) is common in chemistry, although use of the bar is becoming increasingly common. Use of millimeters mercury (mmHg) is common in medicine and meteorology. We will use all these units in this text... 

This is the basic expression in kinetic theory for the pressure of an ideal gas. In equation 19.8, pressure has standard SI units of kg/(m-s ) or N/m (which is consistent with the original definition of pressure as a force per unit area). We define the unit pascal (abbreviation Pa) as 1 N/m. The pascal is the basic SI unit of pressure, although the bar (100,000 Pa) and atmosphere (101,325 Pa or 1.01325 bar) are commonly used. 

The point is that Mathcad allows an effective (quick) problem definition and solution. There was such enthusiasm when high-level programming languages (Fortran, Pascal, BASIC, etc.) substituted assembly (low-level) computer codes. Exchanging higher-level computer languages for Mathcad now has removed the additional link between a computer and a specialist known as a programmer. 

A draft formal specification for the revised Version of SMD Format has recently been published for comment in the Journal of Chemical Information and Computer Sciences. The format is defined by means of a set of syntax diagrams, similar to those used in the definition of programming languages such as Pascal. 

Pascal (N/m ) is the unit of pressure. Pressure is usually expressed with referenee to either absolute zero pressure (a complete vacuum) or local atmospheric pressure. As shown in Figure 1.4, the following definitions apply ... 

Here, m is the mass of the substance, c the specific heat and AT the temperature difference. The boiling point of a liquid is the temperature at which the vapor pressure equals the environmental pressure surrounding the liquid. The SI unit of pressure is N/m which is called Pascal, Pa. The standard pressure is the pressure at 1 bar (100 kPa, this is the current lUPAC definition). The standard reference conditions are temperature 0°C, pressure 100 kPa. 

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