commercial work. Because SI units used decimal system, conversions within the system are quite simple and convenient. We shall follow the SI units in this book. In SI, there are seven base units as given in Table .
. Besides the seven base units, there are two more units that are defined for (a) plane angle d θ as the ratio of length of arc d s to the radius r and (b) solid angle d Ω as the ratio of the intercepted area d A of the spherical surface, described about the apex O as the centre, to the square of its radius r , as shown in Fig. . (a) and (b) respectively.
The unit for plane angle is radian with the symbol rad and the unit for the solid angle is steradian with the symbol sr. Both these are dimensionless quantities. (a) (b) Fig. .
Description of (a) plane angle d θ and (b) solid angle d Ω . Base SI Units quantity Name Symbol Definition Length metre m The metre, symbol m, is the SI unit of length. It is defined by taking the fixed numerical value of the speed of light in vacuum c to be 299792458 when expressed in the unit m s – , where the second is defined in terms of the caesium frequency ∆ ν cs . Mass kilogram kg The kilogram, symbol kg, is the SI unit of mass.
It is defined by taking the fixed numerical value of the Planck constant h to be .62607015× – when expressed in the unit J s, which is equal to kg m s – , where the metre and the second are defined in terms of c and ∆ ν cs . Time second s The second, symbol s, is the SI unit of time. It is defined by taking the fixed numerical value of the caesium frequency ∆ ν cs , the unperturbed ground- state hyperfine transition frequency of the caesium- atom, to be 9192631770 when expressed in the unit Hz, which is equal to s – . Electric ampere A The