Pressure Force per unit area

I he mixture clement shown in Fig. 14.15 contains the flowing gas and solid particles. The partial densities of these two elements are pg and p. respectively. The void fraction is and this can be interpreted as the partial cross-sectional area for gas flow (see Eq. (14.13)). This means that if the pressure of the gas is p, then the pressure force per unit area of the total mixture affecting the flow of gas is (pp and the pressure force affecting the flow of solids is 1 -

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Reciprocating compressors compress gases by a piston moving backwards and forwards in a cylinder. Valves control the flow of low-pressure gas into the cylinder and high-pressure gas out of the cylinder. The mechanical work to compress a gas is the product of the external force acting on the gas and the distance through which the force moves. Consider a cylinder with cross-sectional area A containing a gas to be compressed by a piston. The force exerted on the gas is the product of the pressure (force per unit area) and the area A of the piston. The distance the piston travels is the volume V of the cylinder divided by the area A. Thus ... 

Pressure force per unit area Primary Amine an amine in which the nitrogen atom is bonded to a single carbon atom, when only one hydrogen in ammonia has been replaced by an organic R group Primary Cells batteries that cannot be recharged... 

Find the pressure (force per unit area) exerted by a 1 m high column of mercury (density = 13.6 kg L-1) on the base of the column, in Pascals (the answer is independent of the width of the column). Compare this to atmospheric pressure. 

In reality, atoms and molecules (and container walls, made of atoms and molecules) can be somewhat compressed, and collisions last for a time on the order of picoseconds. The force is concentrated on a spot which is about the same size as the molecule (typically on the order of 1 nm2) which still makes the pressure (force per unit area) at the point of impact quite high (Problem 7-2). So the wall deforms briefly, but in general the impact is not large enough to break chemical bonds, so it quickly recovers its shape. 

The problem of recovery leads to the question of hardness. Hard substances have a high number of strongly directed, covalent chemical bonds per unit volume. Soft substances generally have fewer bonds per unit volume or bonds that are weak or weakly directed, such as ionic or dipole attractive forces. Bond energy per unit volume has the same dimensions as pressure (force per unit area), and a plot of hardness measured by the Knoop indenter versus the bond energy per molar volume for various substances is essentially linear, provided that one chooses substances for which the bonding is predominantly of one type (i.e., not mixed, as in graphite or talc). 

The hydrostatic (turgor) pressure (force per unit area in a liquid), P, acts uniformly in all directions in a cell. This internal pressure pushes against the... 

Shepherd satellite—planetary satellite whose gravitational perturbations on a particle tend to keep it in a stable orbit around the planet. The Pascal (Pa) is the metric unit of pressure (force per unit area). One Pascal is defined as a pressure of one Newton per square meter. The standard sea level atmospheric pressure on Earth is 101,200 Pascals. 

A barometer measures the pressure (force per unit area) of the atmosphere. 

Just as the properties of molecules in a liquid differ from the properties of molecules in a gas, so do the properties of molecules at a surface between a liquid and a gas. That boundary is called the interphase. To increase the surface area of a liquid sample, or in other words, to bring molecules from the interior of a liquid to the surface, requires work to overcome the cohesive force operating between the molecules in the surface. This force is called the surface tension. Surface tension opposes the expansion of the surface of the liquid. The shape of any object which minimizes the amount of surface for a given volume is a sphere. Surface tension forces a droplet of any liquid small enough to escape the influence of gravity to assume the shape of a sphere. Table 5-4 lists the surface tension, given the symbol y, of several liquids at 20 °C. The units of y are force per unit length compared with the units of pressure, force per unit area. 

As formula (4.1) indicates, E has units of pressure — force per unit area. This is why we express it in Pascals. Let s remind that the unit Pa is defined as one Newton per m, and that normal atmospheric pressure is very close to 10 Pa. Since we are going to understand the molecular world, it is also useful to realize that one MPa (megaPascal) can be thought of as 1, where pico Newton is 1 pN = 10 N and 1 nm = 10 m. 

Rather than perform the derivation, however, let s consider in qualitative terms how the ideal-gas equation might follow from these statements. The total force of the molecular collisions on the walls and hence the pressure (force per unit area. Section 10.2) produced by these collisions depend both on how strongly the molecules strike the walls (impulse imparted per collision) and on the rate at which the collisions occur ... 

Pressure Force per unit area Miseibility Ability to mix in all proportions... 

Precipitation Formation of a solid product during a chemical reaction between reactants in solution Prescription drugs Drugs that can be purchased only at the direction of a physician Pressure Force per unit area... 

Having obtained the pressure (force per unit area), we may now work backward using the first of Eqs. [Ill] to find the potential per unit area via quadrature ... 

Work. Work is energy transferred through some physical process, such as mechanical agitation. Work is given the symbol W and is defined as positive when work is done on the system. The fundamental definition of work (from solid mechanics) being done on an object is the force on the object multiplied by the distance over which the force acts. In thermodynamics, we are interested in pressure (force per unit area) and volume (distance times area) so the work done by the system becomes... 

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