The SI Derived Units

SI derived units — The following Table gives the SI derived units ... 

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 international unit itself may eventually be replaced by the SI unit termed the katal, the SI derived unit for catalytic activity (see Chapter 1). It is defined as moles per second. The name katal had been used for this unit for decades, but did not become an official SI derived unit until 1999 with Resolution 12 of the 21st CGPM, on the recommendation of the International Federation of Cfinical chemistry and Laboratory Medicine. Both the International Union of Pure and Applied Chemistry and the lUB now recommend that enzyme activity be expressed in moles per second and that the enzyme concentration be expressed in terms of katals per liter (kat/L). Thus, lU = lO mol/fiOs = 16.7 X 10 mol/s, or l.Onkat/L - 0.06U/L. The formal adoption of the katal is hoped to discourage the use of a non-SI unit called unit, symbol U, defined as micromoles per minute. Units are more commonly used than the katal in practice at present, but their definition lacks coherence with the SI system. 

SI Derived Units You can see that quantities such as area and volume are missing from the table. The quantities are omitted because they are derived—that is, computed—from one or more of the SI base imits. For example, the unit of area is computed from the product of two perpendicular length units. Because the SI base imit of length is the meter, the SI derived unit of area is the square meter, m. Similarly, the imit of volume is derived from three mutually perpendicular length imits, each represented by the meter. Therefore, the SI derived unit of volume is the cubic meter, m. The SI derived units used in this text are listed in Table A.2. 

The SI derived units with special names may he used in com-hinations to provide a convenient way to express more complex physical quantities. Examples are given in the next table ... 

Pressure is a derived unit that represents the force applied to an area perpendicular to the force. The SI derived unit is the Pascal with the symbol Pa ... 

Other quantities, called derived quantities, are defined in terms of the seven base quantities via a system of quantity equations. The SI derived units for these derived quantities are obtained from these equations and the seven SI base units. Examples of such SI derived units are given in the Table 2, where it should be noted that the symbol 1 for quantities of dimension 1 such as mass fraction is generally omitted. 

The SI derived unit by which pressure is measured is the pascal (Pa). 

The SI derived units are the SI units for derived physical quantities. In accordance with the defining equations for derived physical quantities in terms of the base physical quantities, the units for derived quantities can be expressed as products or ratios of the units for the base quantities. Table 1.2-4 shows some examples of SI derived units in terms of SI base units. 

The International System of Units (SI), Physical Quantities, and Their Dimensions I 2.4 The SI Derived Units 17... 

The derived unit known as the plane angle refers to the angular separation of two lines from a common point in a two-dimensional plane. The SI derived unit for plane angles is the radian. A complete rotation about a point origin is an angular displacement of 2ti radians. The extension of this into three dimensions is known as the solid angle and is measured by the SI derived unit called the steradian. 

The basic SI units are the metre (m), kilogramme (kg), second (s), kelvin (K not K), mole (mol), ampere (A) and candela (cd). The mole is the unit of amount of substance and is defined as that amount of substance which contains as many elementary entities as there are atoms in 0.012 kg of carbon-12. It is thus precisely the same amount of substance as, in the c.g.s. system, had been called the gramme-molecule. Some of the SI derived units which are important in the present volume, together with their symbols, are as follows ... 

To convert Jm to the SI derived unit of pressure, the pascal (Pa), we can use the following relations from Table 1.2 ... 

---