The SI units

There are seven basic SI (Systeme International) emits from which all other units can be derived. These seven are assumed to be independent of each other and have various specific definitions that you should know for the examination. The acronym is SMMACKK. The base SI units  

From these seven base SI units, many others are derived. For example, speed can be denoted as distance per unit time (m.s-1) and acceleration as speed change per unit time (m.s-2). Some common derived units are given below. 

These derived units may have special symbols of their own to simplify them. For  

Some everyday units are recognized by the system although they themselves are not true SI units. Examples include the litre (10-3 m3), the minute (60 s), and the bar (105 Pa). One litre is the volume occupied by 1 kg of water but was redefined in the 1960s as being equal to 1000 cm3. 

Interestingly, lO100 is known as a googol, which was the basis for the name of the internet search engine Google after a misspelling occurred. 

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FIGURE 2 12 Boiling points of unbranched alkanes and their 2 methyl branched iso mers (Temperatures in this text are expressed in de grees Celsius C The SI unit of temperature is the kelvin K To convert degrees Celsius to kelvins add 273 15 )... 

The SI units of frequency are reciprocal seconds (s ) given the name hertz and the symbol Hz m honor of the nineteenth century physicist Heinrich R Hertz The constant of proportionality h is called Planck s constant and has the value... 

The SI unit for magnetic field strength is the tesla (T) named after Nikola Tesla a contemporary of Thomas Edison and who like Edison was an inventor of electrical devices... 

Frequency (Section 13 1) Number of waves per unit time Although often expressed in hertz (Hz) or cycles per sec ond the SI unit for frequency is s ... 

The SI unit of viscosity is pascal-second (Pa s) or newton-second per meter squared (N s m ). Values tabulated are mN s (= centipoise, cP). The temperature in degrees Celsius at which the viscosity of a... 

The dynamic viscosity, or coefficient of viscosity, 77 of a Newtonian fluid is defined as the force per unit area necessary to maintain a unit velocity gradient at right angles to the direction of flow between two parallel planes a unit distance apart. The SI unit is pascal-second or newton-second per meter squared [N s m ]. The c.g.s. unit of viscosity is the poise [P] 1 cP = 1 mN s m . The dynamic viscosity decreases with the temperature approximately according to the equation log rj = A + BIT. Values of A and B for a large number of liquids are given by Barrer, Trans. Faraday Soc. 39 48 (1943). 

Kinematic viscosity v is the ratio of the dynamic viscosity to the density of a fluid. The SI unit is meter squared per second [m s ]. The c.g.s. units are called stokes [cm -s l] poises = stokes X density. 

Spandex size is usually expressed in denier which is weight in g/9000 m length. However, the SI unit is tex, the weight in g/1000 m. Rubber size is expressed as gauge, which is the reciprocal of diameter or size in inches. 

Eneigy. The SI unit of energy is the joule which is the work done when the point of appHcation of a force of one newton is displaced a distance... 

Power, P, defiaed as the rate at which work is performed, is expressed ia terms of energy divided by time and is most commonly given in units of horsepower, as for the power suppHed by mechanical devices such as diesel engines, or in the SI units of watts, especially when measuring electrical power. One horsepower is equivalent to the amount of power needed to lift 33,000 pounds (14,982 kg) one foot (30.5 cm) in one minute. One watt is equivalent to the power required to perform one joule of work per second. In a simple direct-current circuit where potential is represented by E ... 

Viscosity is equal to the slope of the flow curve, Tf = dr/dj. The quantity r/y is the viscosity Tj for a Newtonian Hquid and the apparent viscosity Tj for a non-Newtonian Hquid. The kinematic viscosity is the viscosity coefficient divided by the density, ly = tj/p. The fluidity is the reciprocal of the viscosity, (j) = 1/rj. The common units for viscosity, dyne seconds per square centimeter ((dyn-s)/cm ) or grams per centimeter second ((g/(cm-s)), called poise, which is usually expressed as centipoise (cP), have been replaced by the SI units of pascal seconds, ie, Pa-s and mPa-s, where 1 mPa-s = 1 cP. In the same manner the shear stress units of dynes per square centimeter, dyn/cmhave been replaced by Pascals, where 10 dyn/cm = 1 Pa, and newtons per square meter, where 1 N/m = 1 Pa. Shear rate is AH/AX, or length /time/length, so that values are given as per second (s ) in both systems. The SI units for kinematic viscosity are square centimeters per second, cm /s, ie, Stokes (St), and square millimeters per second, mm /s, ie, centistokes (cSt). Information is available for the official Society of Rheology nomenclature and units for a wide range of rheological parameters (11). 

The gray is also used for the ionising radiation quantities, specific energy imparted, kerma, and absorbed dose index, which have the SI unit joule per kilogram. 

Time. Although the SI unit of time is the second, the minute, hour, day, and other calendar units may be necessary where time relates to calendar cycles. Automobile velocity is, for example, expressed in kilometers per hour. 

Area. The SI unit of area is the square meter (m ). The hectare (ha) is a special name for the square hectometer (hm ). Large land or water areas are generally expressed in hectares or in square kilometers (km ). 

Temperature. The kelvin is the SI unit of thermodynamic temperature, and is generally used in scientific calculations. Wide use is made of the degree Celsius (°C) for both temperature and temperature interval. The temperature interval 1°C equals 1 K exacdy. Celsius temperature, t, is related to thermodynamic temperature, T, by the following equation ... 

Dimensionless Quantities. Certain quantities, eg, refractive index and relative density (formerly specific gravity), are expressed by pure numbers. In these cases, the corresponding SI unit is the ratio of the same two SI units, which cancel each other, leaving a dimensionless unit. The SI unit of dimensionless quantities may be expressed as 1. Units for dimensionless quantities such as percent and parts per million (ppm) may also be used with SI in the latter case, it is important to indicate whether the parts per million are by volume or by mass. 

Viscosity. Although traditionally of Httle importance in the evaluation of vegetable and insect waxes, viscosity is an important test for mineral and synthetic waxes. One of the most frequently used tests, ASTM D88, is used to measure the time in seconds required for a specified quantity of wax at a specified temperature to flow by gravity through an orifice of specified dimensions. This viscosity is expressed in Saybolt Universal Seconds (SUS) at the temperature of the test. The SI unit for kinematic viscosity is mm /s (=cSt). 

The Iduematic viscosity (v) is defined as the ratio of the absolute viscosity to density at the same temperature and pressure. The most common unit corresponding to the poise is the stoke (1 cmVsec). The SI unit would be mVsec. 

More closely related to health effects and material damage is the energy deposited in a mass of material. The Rad was an early unit the SI unit is the Gray. 

Mole The SI unit of quantity the amount of a pure element or chemical compound that contains the same number of atoms or molecules. It is often simpler to use moles rather than volume or mass when working with gases. Moles are given by... 

In this section there are a number of AG —T diagrams in which AG is expressed in the original unit of kilocalories, rather than in the SI units of joules. This has been done in order to avoid the difficult redrawing and recalculating of the diagrams involved. 

As you can see from this discussion, a wide number of different units can be used to express measured quantities in the metric system. Ibis proliferation of units has long been of concern to scientists. In 1960 a self-consistent set of metric units was proposed. This so-called International System of Units (SI) is discussed in Appendix 1. The SI units for the four quantities discussed are... 

Throughout this text, we will use the SI unit joule (J)> defined in Appendix 1, to express energy. A joule is a rather small quantity. One joule of electrical energy would keep a 10-W lightbulb burning for only a tenth of a second. For that reason, we will often express energies in kilojoules (1 kj = 103 J). The quantity h appearing in Planck s equation is referred to as Planck s constant... 

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