CGS system

While the Systeme International d Unites (SI) system of units is not particularly relevant to physical chemistry and requires additional and sometimes awkward constants, its broad use deserves attention. The majority of the derivations are made in the cgs/esu (centimeter-gram-second/electrostatic unit) system of units however, both the SI and cgs systems are explained and tables for their interconversion are given in Chapters V and VI. 

The authors of this research selected units which could be written with maximum simplification to report their results. We must replace the factors which have canceled out. Assuming that the cgs system of units was used throughout, we note that possible units for H are grams per square centimeter and possible unts for C2 are grams per cubic centimeter, which yield the required units for n/c2 Note that these units of H must be multiplied by the gravitational constant to give H in dynes per square centimeter ... 

Coulomb s law. This relationship poses no particular difficulties as a qualitative statement the problem arises when we attempt to calculate something with it, since the proportionality constant depends on the choice of units. In the cgs system of units, the electrostatic unit of charge is defined to produce a force of 1 dyne when two such charges are separated by a distance of 1 cm. In the cgs system the proportionality factor in Coulomb s law is unity and is dimensionless. For charges under vacuum we write... 

Units employed in diffusivity correlations commonly followed the cgs system. Similarly, correlations for mass transfer correlations used the cgs or Enghsh system. In both cases, only the most recent correlations employ SI units. Since most correlations involve other properties and physical parameters, often with mixed units, they are repeated here as originally stated. Common conversion factors are listed in Table 1-4. 

The first step is to determine the number of turns needed for the primary winding. For this, the parameters from the core data sheet of the particular core and core material are used. Also, the minimum level of flux density already should have been determined (refer to Appendix D). The equation for determining the number of turns for the primary winding in the CGS System (U.S.) is... 

An air-gap is required for all unipolar flux drive applieations sueh as this. One method of aehieving this is shown in Equation 3.28a (CGS system (U.S.)). 

In old literature the cgs system is found, in which the dynamic viscosity is measured in centipoise = 0.1 poise = 0,001 dyne s cm - ... 

The assumptions made in tlie development of Eq. 12.6.1 are (1) tlie plume spretid lias a Gaussian distribution in both tlie horizontal and vertical planes witli standard deviations of plume concentration distribution in the horizontal and vertical of Oy and respectively (2) tlie emission rate of pollutants Q is uniform (3) total reflection of tlie plume takes place at tlie eartli s surface and (4) tlie plume moves downwind with mean wind speed u. Altliough any consistent set of units may be used, tlie cgs system is preferred. 

The most commonly used system apart from SI is the cgs system based on the =ntiinetre, gram and second as the only base units. The unit of force is the dyne, ind the unit of energy is the erg. In electromagnetism, SI is associated with an independent base quantity of current, whereas cgs is associated with current as a derived quantity. 

A quick conversion table is given in Table 0.1. It isn t comprehensive, but you should find it useful. Unfortunately, many texts dealing with molecular modelling still use the cgs system. 

These systems are in essence modifications of the cgs system but employ larger units. The basic dimensions are again of L, M, and T. 

It should be noted that a force in the cgs system is sometimes expressed as a gram force and in the mks system as kilogram force, although this is not good practice. It should also be noted that the gram force = 980.665 dyne and the kilogram force = 9.80665 N)... 

The three fundamental units of the SI and of the cgs systems are length, mass, and time. It has been shown that force can be regarded as having the dimensions of MLT-2, and the dimensions of many other parameters may be worked out in terms of the basic MLT system. For example ... 

Prior to the now almost universal adoption of the SI system of units, the unit of heat was defined as the quantity of heat required to raise the temperature of unit mass of water by one degree. This heat quantity is designated the calorie in the cgs system and the kilocalorie in the mks system, and in both cases temperature is expressed in degrees Celsius (Centigrade). As the specific heat capacity is a function of temperature, it has been necessary to set a datum temperature which is chosen as 298 K or 25°C. 

Before closing this section, it is worth mentioning that the hyperpolarizability tensors are complex quantities usually given in the old cgs system of units of esu (electrostatic units). The transformation into the International System is readily obtained with the relationship ... 

In addition to SI units ("Systeme International ), the still occasionally used cgs system and the list of symbols at the beginning of the book, the following standard abbreviations are used ... 

As the derivation in the Giorgi system of units (ref. 8 Moore, 5th ed.) is more complicated (e.g., a factor of 1000 1 in b should not be overlooked), we followed the older version (Moore, 4th ed., pp. 352-356) in the electrostatic cgs system because the concepts of molecularity and ionic strength are more easily fitting in. 

The abbreviation au stands for atomic units , which is a system of units meant to simplify the equations of molecular and atomic quantum mechanics. The units of the au system are combinations of the fundamental units of mass (mass of the electron), charge (charge of the electron), and Planck s constant. By setting these three quantities equal to unity one gets simpler equations. The au system has a simple relation to the SI and Gaussian (cgs) systems of units. For example, 1 au of length = aQ (Bohr radius) = 5.29 x 10"9 cm =... 

Physical quantities relevant to analytical measurements and the units and symbols used to express them are given in Table 1.3. Both SI and CGS units have been included because of current widespread use of the latter and for ease of comparison with older literature. However, only the SI nomenclature is now officially recognized and the use of the CGS system should be progressively discouraged. 

Viscosity is the force in dynes required to move a plane 1 cm in area at a distance of 1 cm from another plane 1 cm in area through a distance of 1 cm in 1 s. In the centimeter-gram-second (cgs) system, the unit of viscosity is the poise (P) or centipoise (1 cP = 0.01 P). Two other terms in common use are kinematic viscosity and fluidity. The kinematic viscosity is the viscosity in centipoise divided by the specific gravity, and the unit is the stoke (cm /s), although the centistoke (0.01 st = 1 cSt) is in more common use fluidity is simply the reciprocal of viscosity. 

Unfortunately, little direct information is available on the physicochemical properties of the interface, since real interfacial properties (dielectric constant, viscosity, density, charge distribution) are difficult to measure, and the interpretation of the limited results so far available on systems relevant to solvent extraction are open to discussion. Interfacial tension measurements are, in this respect, an exception and can be easily performed by several standard physicochemical techniques. Specialized treatises on surface chemistry provide an exhaustive description of the interfacial phenomena [10,11]. The interfacial tension, y, is defined as that force per unit length that is required to increase the contact surface of two immiscible liquids by 1 cm. Its units, in the CGS system, are dyne per centimeter (dyne cm" ). Adsorption of extractant molecules at the interface lowers the interfacial tension and makes it easier to disperse one phase into the other. 

A short digression on units is perhaps appropriate here. We shall use either Gaussian rrrrits in this book or, much more frequently, Hartree s atomic units. Gaussian units, as far as we are concerned, are identical with the old cgs system of units with the added proviso that charges are measured in unnamed electrostatic units, esu. The value of e is thus 4.803206808 X 10 esu. Keeping this number at hand is all that will be required to use Gaussian rmits in this book. 

Pdt which is the area under the P-t curve of the pulse. For triangular pressure pulses of peak amplitude Pm, I = Pmt/2, and for an exponentially decaying pressure pulses I=Pm/B(l—e" 1) where B is the decay constant. In the cgs system I has the dimensions of dyne sec/cm2... 

Equation (1) defines the units of surface tension to be those of force per length or Nm 1 in SI or dynes cm"1 in the cgs system. We see presently that these are not the only units used for 7. 

This supplies a second definition of surface tension It equals the work per unit area required to produce a new surface. In terms of this definition, the units of 7 are energy per area — J m 2 in SI or erg cm"2 in the cgs system. 

In recent engineering research papers, units with the International System of Units (SI) are generally used. The SI system is different from the CGS system often used by scientists or from the conventional metric system used by engineers [4]. In the SI system, kilogram is used for mass only, and newton (N), which is the... 

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