The SI system

The SI system (from the French Systeme International) is a worldwide measurement system based on the older metric system that most of us learned in school. There are minor differences between the SI and metric systems, but, for purposes of this book, they re interchangeable. 

Many years ago, there was a movement in the United States to convert to the metric system. But, alas, Americans are still buying their potatoes by the pound and their gasoline by the gallon. Don t worry about it. Most professional chemists 1 know use both the U.S. and SI systems without any trouble. It s necessary to make conversions when using two systems, but 1 show you how to do that right here. 

The basic unit of length in the SI system is the meter (m). A meter is a little longer than a yard there are 1.094 yards in a meter, to be exact. But that s not a really useful conversion. The most useful Sl/English conversion for length is 

The basic unit of mass in the SI system for chemists is the gram (g). And the most useful conversion for mass is 

The basic unit for volume in the SI system is the liter (L). The most useful conversion is 

In 1960, the General Conference adopted an extensive revision and simplification of the system. The name Le Systeme International d Unites (International System of 

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In addition to fundamental units from the SI system, i.e., m, kg, s, mol, K, A, and cd, multiples and sub-multiples of these units as well as derived or combined units are also used and indicated in parentheses. 

One complication is the matter of units while the Systeme International d Unites (SI) requires additional and sometimes awkward constants, its broad use requires attention [1]. Hence, while we present the derivation in the cgs/esu system, we show alternative forms appropriate to the SI system in Tables V-1 and V-2. 

Show what Eq. V-7 (for k ) becomes when written in the SI system. Calculate k for 0.05M, 2 2 electrolyte at 25°C using SI units repeat the calculation in the cgs/esu system and show that the result is equivalent. 

Show what form Eq. V-36 takes if written in the SI system. Refening to the equation of Problem 12, show what number replaces 12.9 if f is in volts, v in meters per second, and F in volts per meter. 

Make a calculation to confirm the numerical illustration following Eq. V-42. Show what form the equation takes in the SI system, and repeat the calculation in SI units. Show that the result corresponds to that obtained using cgs/esu units. 

Referring to Eq. V-69, calculate the value of C for a 150-A film of ethanol of dielectric constant 26. Optional Repeat the calculation in the SI system. 

The SI Systeme International d Unites) unit of energy is the joule (J) An older unit is the calorie (cal) Most or game chemists still express energy changes in units of kilocalories per mole (1 kcal/mol = 4 184 kJ/mol)... 

The surface tension of a liquid, -y, is the force per unit length on the surface that opposes the expansion of the surface area. In the literature the surface tensions are expressed in dyn cm 1 dyn cm = 1 mN in the SI system. For the large majority of compounds the dependence of the surface tension on the temperature can be given as... 

A final interpretation of the regrouped expression given in item (2) is that 2D equals the diffusion velocity for a particle undergoing unit displacement, X = 1 m in the SI system. 

The situation is not so simple when these various parameters are time dependent. In the latter case, the moduli, designated by E(t)and G(t), are evaluated by examining the (time dependent) value of o needed to maintain a constant strain 7o- By constrast, the time-dependent compliances D(t) and J(t)are determined by measuring the time-dependent strain associated with a constant stress Oq. Thus whether the deformation mode is tension or shear, the modulus is a measure of the stress required to produce a unit strain. Likewise, the compliance is a measure of the strain associated with a unit stress. As required by these definitions, the units of compliance are the reciprocals of the units of the moduli m in the SI system. 

In this expression, called Pick s first law, the proportionality constant D is the diffusion coefficient of the solute. Since J = (l/A)(dQ/dt) and c = Q/V, where Q signifies the quantity of solute in unspecified units, it follows that D has the units length time", or m sec in the SI system. The minus sign in Eq. (9.69)... 

Units. The SI system of units and conversion factors (qv) has been formally adopted worldwide, with the exception of Bmnei, Burma, Yemen, and the United States. The participation of the United States in the metrication movement is evident by the passage of the Metric Acts of 1866 and 1975 and the subsequent estabUshment of the American National Metric Council (private) and the U.S. Metric Board (pubHc) to plan, coordinate, monitor, and encourage the conversion process. 

Density is defined as the mass of a substance contained in a unit volume. In the SI system of units, the ratio of the density of a substance to the density of water at I5°C is known as its relative density, while the older term specific gravity is the ratio relative to water at 60°F. Various units of density, such as kg/m, Ib-mass/fF, and g/cm, are commonly used. In addition, molar densities, or the density divided by the molecular weight, is often specified. This section briefly discusses methods of correlation of density as a function of temperature and presents the most common accurate methods for prediction of vapor, liquid, and solid density. 

Vacuum flow is usually described with flow variables different from those used for normal pressures, which often leads to confusion. Pumping speed S is the actual volumetric flow rate of gas through a flow cross section. Throughput Q is the product of pumping speed and absolute pressure. In the SI system, Q has units of Pa m vs. 

The primai y distinguishing charac teristic of gas dispersoids is particle size. The generally accepted unit of particle size is the micrometer, [Lm. (Prior to the adoption of the SI system, the same unit was known as the micron and was designated by 1.) The particle size of a gas dis-persoid is usually taken as the diameter of a sphere having the same... 

This example can also he carried out in the SI system hy using Fig. 21-41... 

F1G. 21-60a Biilkhopp er truck, Tractor trailer used for plastics, (Butler Mfg. Co. ). To convert data to the SI system, change the dimensions shown to inches and multiply by 25,4, To convert volume to cubic meters, multiply cubic feet by 0,02832,... 

Two systems of units are in common usage in mechanics. The first, the SI system, is an absolute system based on the fundamental quantities of space, time, and mass. All other quantities, including force, are derived. In the SI system the basic unit of mass is the kilogram (kg), the basic unit of length (space) is the meter (m), and the basic unit of time is tbe second (s). The derived unit of force is the Newton (N), which is defined as the force required to accelerate a mass of 1 kg at a rate of 1 m/s-. 

Since these groups in the SI system are non-dimensional, they can be used to present the results of tests of pumps in a family of pumps that are geometrically and dynamically similar. This may be done as shown in... 

The volume V is the space occupied by the system. It is usually expressed in cubic meters (m3) or cubic decimeters (dm3). A dm3 is the same volume as a liter (L), but dm3 is preferred to the liter because it is a part of the SI (Systeme International d Unites) system of units. 

For many purposes, the chosen unit in the SI system will be either too large or too small for practical purposes, and the following prefixes are adopted as standard. Multiples or sub-multiples in powers of 103 are preferred and thus, for example, millimetre should always be used in preference to centimetre. 

The most important practical difference between the mks and the SI systems lies in the units used for thermal energy (heat), and this topic is discussed in Secton 1.2.7. 

A detailed account of the structure and implementation of the SI system is given in a publications of the British Standards Institution11, and of Her Majesty s Stationery... 

The units, dimensions, and normal form of expression for these quantities in the SI system are ... 

In the SI system, the unit of heat is taken as the same as that of mechanical energy and is therefore the Joule. For water at 298 K (the datum used for many definitions), the specific heat capacity Cp is 4186.8 J/kg K. 

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. 

In this expression consistent units must be used. In the SI system each of the terms in equation 2.1 is expressed in Joules per kilogram (J/kg). In other systems either heat units (e g. cal/g) or mechanical energy units (e.g. erg/g) may be used, dU is a small change in the internal energy which is a property of the system it is therefore a perfect differential. On the other hand, Sq and SW are small quantities of heat and work they are not properties of the system and their values depend on the manner in which the change is effected they are, therefore, not perfect differentials. For a reversible process, however, both Sq and SW can be expressed in terms of properties of the system. For convenience, reference will be made to systems of unit mass and the effects on the surroundings will be disregarded. 

It may be noted that the pressure measuring devices (a) to (e) all measure a pressure difference AP(— Pj — P ). In the case of the Bourdon gauge (0, the pressure indicated is the difference between that communicated by the system to the tube and the external (ambient) pressure, and this is usually referred to as the gauge pressure. It is then necessary to add on the ambient pressure in order to obtain the (absolute) pressure. Even the mercury barometer measures, not atmospheric pressure, but the difference between atmospheric pressure and the vapour pressure of mercury which, of course, is negligible. Gauge pressures are not. however, used in the SI System of units. 

Units in this book conform to the SI system (Systeme International d Unites). They are listed in the following tables with the relevant conversion factors. 

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