The SI Base Units

In fact, the number of base quantities chosen and the selection of the quantities which are considered as base quantities are a matter of expediency in different fields and applications of physics, it might weU be expedient to use different numbers of base quantities and different selections of base quantities. It should be kept in mind, however, that the number and selection of base quantities are only a matter of different representations of physics physics itself is not affected by the choice that is made. 

many scientists therefore prefer to use the conventional system recommended by ISO. This system uses seven base quantities, which are selected according to the seven base units of the SI system. Table 1.2-1 shows the recommended names, s)rmhols, and measuring devices for the conventional seven hase quantities. 

All other physical quantities can then be defined as derived quantities this means they can be defined by equations in terms of the seven base quantities. Within this conventional system , the set of all defining equations for the derived physical quantities also defines the units for the derived quantities in terms of the units of the hase quantities. This is the great advantage of the conventional system . 

In this way, velocity is traced back to the two hase quantities length / and time t. On the right-hand side of this equation, we have a differential of length dl divided hy a differential of time dt. The algebraic combination of the hase quantities in the defining equation for a derived quantity is called the dimensions of the derived quantity. So velocity has the dimensions length/time, acceleration has the dimensions length/time squared, and so on. 

Data for a physical quantity are always given as a product of a number (the numerical value of the physical quantity) and a unit in which the quantity has been measured. 

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We are asked to make a unit conversion. The SI base unit of length is the meter, and the SI base unit of time is the second. It is necessary to convert from miles to meters and from hours to seconds. The appropriate unit equivalences are... 

The prefixes should be attached directly to the SI base unit e.g., kilogram, millisecond, gigameter, etc. Similarly, the abbreviations attach directly to the abbreviation for the SI units e.g., cm, Mg, mK, etc. Do not use two or more of the SI units. Although kilogram is the normal base unit for mass, the prefixes are added to gram (g), not kilogram (kg). 

The problem with the SI base unit being a kilogram is the kilo part. The philosophical idea behind the SI system says any parameter (physical, chemical, mechanical, etc.) can be derived from a suitable combination of the others. For example, the SI unit of velocity is metres per second (m s-1), which is made up of the two SI fundamental units of length (the metre) and time (the second). A few of these combinations are cited in Table 1.3. 

The SI base unit for electric current is the ampere (A). In addition to being an SI base unit, an ampere is a coulomb (C) per second, and a faraday is 96485 C/mol of electrons. Therefore ... 

As shown above, the SI unit for volume is the cubic meter (m3), but most chemists use the liter (L, which is equal to 1 cubic decimeter (dm3)) or milliliter (mL). Appendix A lists the SI base units and prefixes, as well as some English-SI equivalents. 

The first step in mastering the SI system is to figure out the base units. Much like the atom, the SI base units are building blocks for more-complicated units. In later sections of this chapter, you find out how more-complicated units are built from the SI base units. The five SI base units that you need to do chemistry problems (as well as their familiar, non-SI counterparts) are in Table 2-1. 

At the top of the traceability chain is the stated metrological reference, which for our purposes is the definition of a unit. It might not be obvious how a piece of paper from Paris giving the interesting, but rather esoteric, definitions of the SI base units can be used as a reference. The metrological reference allows the creation of a primary calibrator that embodies the value of the unit (or some multiple or submultiple of it). The international prototype... 

If necessary, as seen earlier, we can manipulate the various prefixes for the SI base units as required. Further practice is given in the following problem. 

Traceability for measurements in a particular area can take some time to develop. It is not always possible or even desirable to have traceability back to the SI base units. 

By weighing the receptor cylinder, which has been selected and treated, before and after each introduction of component gas and by means of Eq. 2, we are able to get the mole fraction of each component. This method is known as gravimetric preparation. Since mass is one of the seven base quantities and the atomic or molecular weight M can be determined very precisely, the application of gravimetric preparation makes the value of quantity traceable to the mole, the SI base unit in chemistry. 

Amperes and candelas are rarely used in chemistry. The name "kilogram" occurs for the SI base unit of mass for historical reasons. Derived units are formed from the kilogram, but appropriate decimal prefixes are attached to the word "gram."... 

K) the SI base unit of temperature, defined by assigning 273.16 K to the temperature at which steam, ice, and water are at equilibrium (called the triple point of water). The freezing point of water is 273.15 K. 

The names and symbols of decimal multiples and submultiples of the SI base unit of mass, the kg, which already contains a prefix, are constructed by adding the appropriate prefix to the word gram and symbol g. 

Though it seems inconsistent, the SI base unit is the gram-mole. As Mario Iona reminds me, SI is not an MKS system. Some textbooks still prefer to use use the kilogram-mole, or worse, use it... 

Seconds, (s), the SI base unit, is inadequate for long periods of time, and so a minute, (min) of 60 s, an hour (h) of 3600 s, and a day (d) of 86,400 s are accepted. Year and month are problems in that they each have varying magnitude, however AlChE recognizes the 365-day year (yr) as an accepted time unit. 

The SI units for derived physical quantities are those coherently derived from the SI base units by multiplication and division. Some of the Sl-derived units that have special names and symbols are presented in Table 16.12. 

A star s temperature and size determine its brightness, or luminous intensity. The SI base unit for luminous intensity is the candela. The more massive a star and the hotter its temperature, the brighter the star will be. How bright a star appears from Earth can be misleading because stars are at different distances from Earth. Light spreads out as it travels from its source. Thus, distant stars will appear less bright than stars of equal intensity that are closer to Earth. 

Time The SI base unit for time is the second (s). The frequency of microwave radiation given off by a cesium-133 atom is the physical standard used to establish the length of a second. Cesium clocks are more reliable than the clocks and stopwatches that you use to measure time. For ordinary tasks, a second is a short amount of time. Many chemical reactions take place in less than a second. To better describe the range of possible measurements, scientists add prefixes to the base units. This task is made easier because the metric system is a decimal system. The prefixes in Table 2-2 are based on multiples, or factors, of ten. These prefixes can be used with all SI units. In Section 2.2, you will learn to express quantities such as 0.000 000 015 s in scientific notation, which also is based on multiples of ten. 

Mass Recall that mass is a measure of the amount of matter. The SI base unit for mass is the kilogram (kg). A kilogram is about 2.2 pounds. The kilogram is defined by the platinum-iridium metal cylinder shown in Figure 2-2. The... 

The Kelvin scale was devised by a Scottish physicist and mathematician, William Thomson, who was known as Lord Kelvin. A kelvin (K) is the SI base unit of temperature. On the Kelvin scale, water freezes at about 273 K and boils at about 373 K. Figure 2-5 compares the two scales. You will use the Celsius scale for your experiments. In Chapter 14, you will learn why scientists use the Kelvin scale to describe the behavior of gases. 

What is a mole The mole, commonly abbreviated mol, is the SI base unit used to measure the amount of a substance. It is the number of representative particles, carbon atoms, in exactly 12 g of pure carbon-12. Through years of experimentation, it has been established that a mole of anything contains 6.022 136 7 X 10 representative particles. A representative particle is any kind of particle such as atoms, molecules, formula units, electrons, or ions. The number 6.022 136 7 X 10 is called Avogadro s number in honor of the Italian physicist and lawyer Amedeo Avogadro who, in 1811, determined the volume of one mole of a gas. In this book, Avogadro s number will be rounded to three significant figures—6.02 X 10. ... 

As a simple example, suppose the mass of a sample is measured to be 64.3 g. If this is to be used in a formula involving SI units, it should be converted to kilograms (the SI base unit of mass). To do this, we use the fact that 1 kg = 1000 g and write... 

The SI base unit for time is the second (sec). Often it is convenient to use units considerably longer than the second, such as hours (hr), days, and... 

Some properties, such as time or length, can be expressed in terms of SI base units. Other properties, such as volume or density, are expressed in SI derived units, which are really made by combining the SI base units. Following are some examples of physical properties and the Si-derived units, which can be used to measure them. 

Current ampere. A, the SI base unit of electric current -> Charge coulomb, C = As, the SI derived unit of charge Energy joule, J = kg-m s , the SI derived unit of energy Potential volt, V = JC" , SI derived unit of the cell potential... 

The symbols used to denote units are printed in roman font those denoting physical quantities or mathematical variables are printed in italics and should generally be single letters that may be further specified by subscripts and superscripts, if required. The unit of any physical quantity can be expressed as a product of the SI base units, the exponents of which are integer numbers, e.g. [ ] = m2 kg s 2. Dimensionless physical quantities, more properly called quantities of dimension one, are purely numerical physical quantities such as the refractive index n of a solvent. A physical quantity being the product of a number and a unit, the unit of a dimensionless quantity is also one, because the neutral element of multiplication is one, not zero. 

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