The International System Of Units: In earlier times scientists of different countries were using different systems of units for measurement. Three such systems, the CGS, the FPS (or British) system, and the MKS system were in use extensively till recently. The base units for length, mass, and time in these systems were as follows:
The system of units that is at present internationally accepted for measurement is the international system of units, abbreviated as SI. Because SI units used decimal system, conversions within the system are quite simple and convenient. We shall follow the SI units.
Table2.1: SI Base Quantities and Units
| Base quantity | Name | Symbol | Definition |
| Length | meter | m | The meter, symbol \(\mathrm{m}\), 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 \(\mathrm{m} \mathrm{s}^{-1}\), where the second is defined in terms of the caesium frequency \(\Delta V c s\). |
| 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 \(6.62607015^{-10} \times 10^{-34}\) when expressed in the unit \(\mathrm{J} \mathrm{s}\), which is equal to \(\mathrm{kg} \mathrm{m}^{2} \mathrm{~s}^{-1}\), where the metre and the second are defined in terms of \(c\) and \(\Delta v c s\). |
| 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 \(\Delta v c s\), the unperturbed groundstate hyperfine transition frequency of the caesium-133 atom, to be 9192631770 when expressed in the unit \(\mathrm{Hz}\), which is equal to \(\mathrm{s}^{-1}\). |
| Electric | ampere | A | The ampere, symbol A, is the SI unit of the electric current. It is defined by taking the fixed numerical value of the elementary charge \(e\) to be \(1.602176634 \times 10^{-19}\) when expressed in the unit \(C\), which is equal to \(A\) s, where the second is defined in terms of \(\Delta v c s\). |
| Thermodynamic temperature | Kevin | k | The kelvin, symbol K, is the SI unit of thermodynamic temperature. It is defined by taking the fixed numerical value of the Boltzmann constant \(\mathrm{k}\) to be \(1.380649 \times 10^{-23}\) when expressed in the unit \(\mathrm{J} \mathrm{K}^{-1}\), which is equal to \(\mathrm{kg} \mathrm{m}^{2} \mathrm{~s}^{-2} \mathrm{k}^{-1}\), where the kilogram, metre and second are defined in terms of \(h, c\) and \(\Delta v e s\). |
| Amount of substance | mole | mol | The mole, symbol mol, is the SI unit of the amount of substance. One mole contains exactly \(6.02214076 \times 10^{23}\) elementary entities. This number is the fixed numerical value of the Avogadro constant, \(N_{A}\), when expressed in the unit \(\mathrm{mol}^{-1}\) and is called the Avogadro number. The amount of substance, symbol \(n\), of a system is a measure of the number of specified elementary entities. An elementary entity may be an atom, a molecule, an ion, an electron, any other particle or specified group of particles. |
| Luminous Intensity | Candela | cd | The candela, symbol cd, is the SI unit of luminous intensity in given direction. It is defined by taking the fixed numerical value of the luminous efficacy of monochromatic radiation of frequency \(540 \times 10^{12} \mathrm{~Hz},{K_{cd}}\), to be 683 when expressed in the unit \(1 \mathrm{~m} \mathrm{~W}^{-1}\), which is equal to \(\mathrm{cd} \mathrm{sr} \mathrm{} \mathrm{W}^{-1}\), or \(\mathrm{cd} \mathrm{sr} \mathrm{} \mathrm{kg}^{-1} \mathrm{~m}^{-2} \mathrm{~s}^{3}\), where the kilogram, metre and second are defined in terms of \(h, c\) and \(\Delta v c s\). |
Table2.2: SI Prefixes
| Power of 10 | Prefix | Symbol |
| 18 | exa | E |
| 15 | peta | p |
| 12 | tera | T |
| 9 | giga | G |
| 6 | mega | M |
| 3 | kilo | k |
| 2 | hecto | h |
| 1 | deca | da |
| -1 | deci | d |
| -2 | centi | c |
| -3 | milli | m |
| -6 | micro | μ |
| -9 | nano | n |
| -12 | pico | p |
| -15 | femto | f |
| -18 | atto | a |
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