Metric System Base units

The name SI is derived from Systbme International d Unites and has evolved from an original basis of a given length (meter) and mass (kilogram) established by members of the Paris Academy of Science in the late eighteenth century. The original system was known as the metric system, but there are differences in the modem SI system and the old metric system based primarily on new names being added for derived terms. 

SI units International System of units based on the metric system and units derived from the metric system. (A2.1)... 

Because of its convenience and consistency, scientists have used the metric system of units for more than 200 years. Originally, the metric system was based on only three fundamental units the meter for length, the kilogram for mass, and the second for time. Today, there are more than 50 officially recognized SI units for various scientific quantities. 

Recall from Chapter 2 that the universal unit system used hy scientists is called Le Systeme Internationale d Unit6s or SI. It is a metric system based on seven base units—meter, second, kilogram, kelvin, mole, ampere, and candela—from which all other units are derived. The size of a unit in a metric system is indicated by a prefix related to the difference between that unit and the base unit. For example, the base unit for length in the metric system is the meter. One tenth of a meter is a decimeter where the prefix deci- means one tenth. And, one thousand meters is a kilometer. The prefix kilo- means one thousand. 

The SI and metric systems of units are based on powers of ten. This means that many unit conversions within these systems can be carried out just by shifting the decimal point. Eor instance, the conversion from milligrams to grams in Example 1-6 just... 

As measuring techniques became more precise and the demand for accuracy increased, the standards on which people based their units were improved. In the 18 century, the French invented the metric system, based on a more consistent, systematic, and carefully defined set of standards than had ever been used before. For example, the meter (or metre, from the Greek metron, a measure ) became the standard for length. The first definition for the standard meter was one ten-millionth of the distance from the North Pole to the Equator. This became outdated as the precision of scientist s measuring instruments improved. Today, a meter is defined as the distance light travels in a vacuum in 1/299,792,458 second. Technical instruments for measuring length are calibrated in accordance with this very accurate definition. 

The metric system has long been preferred for most scientific work. In 1960 an infernafional agreement set up a comprehensive system of units called the International System (le Systeme Internationale in French), or SI. The SI units are based on the metric system and units derived from the metric system. The most important fundamental SI units are listed in Table 5.1. Later in this chapter we will discuss how to manipulate some of these units. 

SI units International System of units based on the metric system and units derived from the metric system. (A2.1) Side chain (of amino acid) the hydrocarbon group on an amino acid represented by H, CH3, or a more complex substituent. (21.6)... 

SI units (SI Systeme fnternational d Unites) The modern coherent rationalized internationally adopted metric system of units. It has seven BASE UNITS and two dimensionless units, formerly called supplementary units. DERIVED UNITS are formed by multiplication and/or division of base units. Standard prefixes are used for decimal multiples and submultiples of SI units, along with standard symbols for both units and prefixes. 

Base unit Derived unit Metric system SI units Section 3.2 Base... 

Metric system, SI units, derived unit, base unit... 

Scientific measurements are expressed in the metric system. As you know, this is a decimal-based system in which all of the units of a particular quantity are related to one another by factors of 10. The more common prefixes used to express these factors are listed in Table 1.2 (page 7). 

The Systeme Internationa] (SI) is the internationally accepted form and elaboration of the metric system. It defines seven base units in terms of which all physical quantities can be expressed. At this stage all we need are ... 

Still other units encountered in the literature and workplace come from various other systems (absolute and otherwise). These include metric systems (c.g.s. and MKS), some of whose units overlap with SI units, and those (FPS) based on English units. The Fahrenheit and Rankine temperature scales correspond to the Celsius and Kelvin, respectively. We do not use these other units, but some conversion factors are given in Appendix A. Regardless of the units specified initially, our approach is to convert the input to SI units where necessary, to do the calculations in SI units, and to convert the output to whatever units are desired. 

The measurement system that you will most likely encounter is the SI (Metric) system. Each quantity (such as mass and volume) has a base unit and a prefix that modifies the base unit. The prefixes are the same for all quantities and are based on a decimal system. Below are some basic SI units we will introduce others in later chapters ... 

The "Systeme International" (S ) units, based on the metric system, were designed to achieve maximum internal consistency. The SI system is based on the following set of defined units ... 

The system of units used in chemistry is the SI system (Systeme International), which is related to the metric system. There are base units for length, mass, etc. and decimal prefixes that modify the base unit. Since most of us do not tend to think in these units, it is important to be able to convert back and forth from the English system to the SI system. These three conversions are useful ones, although knowing the others might allow you to simplify your calculations ... 

Familiarizing loursetf ulith Base Units and Metric System Prefixes... 

Chemists routinely measure quantities that run the gamut from very small (the size of an atom, for example) to extremely large (such as the number of particles in one mole). Nobody, not even chemists, likes dealing with scientific notation (which we cover in Chapter 1) if they don t have to. For these reasons, chemists often use a metric system prefix (a word part that goes in front of the base unit to indicate a numerical value) in lieu of scientific notation. For example, the size of the nucleus of an atom is roughly 1 nanometer across, which is a nicer way of saying 1x10- meters across. The most useful of these prefixes are in Table 2-2. 

---