Dimensions, Quantities and Units

Abstract

The dimensions of all physical quantities can be expressed in terms of the four basic dimensions: mass, length, time and temperature. Thus velocity has the dimensions of length per unit time and density has the dimensions of mass per unit length cubed. A system of units is required so that the magnitudes of physical quantities may be determined and compared one with another. The internationally agreed system which is used for science and engineering is the Systeme International d’Unites, usually abbreviated to SI. Table 2.1 lists the SI units for the four basic dimensions together with those for electrical current and plane angle which, although strictly are derived quantities, are usually treated as basic quantities. Also included is the unit of molar mass which somewhat illogically is the gram molecular weight or gram mole and which is usually referred to simply as a ‘mole’. However, it is often more convenient to use the kilogram molecular weight or kmol.