Units of measure are vital parts of any physical quantities in physics. Just as a person is known by his/her name, the same way physical quantities are known by their units of measurement. The unit of measurement is defined as the standard used for the measurement of that physical quantity. The standard unit is always universal accepted and it does not change with time. There are large number of physical quantities, and each of them should have the units of measurement. However, it is not necessary to define separate unit for each and every physical quantity. So, there is need to have a unit system of measurement. The units of measurement defined for the fundamental quantities are called as the fundamental or basic units of measurement. There are four categories of these units of measurement namely: CGS (centimetre, gram, second) system. MKS (metre, kilogram, second) system, FPS (foot, pound, second) and SI system of units of measurement. The SI system or the international system of units has now replaced all the systems of measurement. Table 2.0 shows the fundamental quantities with their corresponding unit system.
In addition to the SI units of the seven fundamental quantities, two supplementary units are also defined as shown in table 3.0 below.
Sometimes some SI derived units have special names with the SI base unit equivalents as shown in table 4.0.
Definitions of SI Base Units
Metre: One metre is defined as the length of the path travelled in aa vacuum during a time interval of 1/299,793,458 of a second.
Second: One second is the duration of 9162631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium -133 atoms.
Kilogram: One kilogram is the mass equal to the mass of standard platinum-iridium alloy cylinder (90% platinum and 110% iridium) kept at the international Bureau of weights and measurements in Paris, France. The cylinder is about 4cm in height and 4cm in diameter.
Ampere: It is the current which when flows through two infinitely long straight conductors of negligible cross-section placed at a distance of 1 m in vacuum produces a force of \(\: 2\times 10^{-7} \: N/m \:\) between them.
Kelvin: One kelvin is defined as 1 / 273.16 part of the thermodynamic temperature of triple point of water.
Mole: It is the amount of substance of a system which contains as many elementary particles (atoms, molecules, ions etc.) as there are atoms in 12 g of carbon-12.
Candela: It is luminous intensity in a perpendicular direction of a surface of (1600000)\(\:m^2\:\) of a black body at the temperature of freezing platinum under a pressure of \(\: 1.013 \times 10^5 \:N/m^2\) .
SI Unit Prefix
Magnitudes of physical quantities range from very large to very small. For example, mass of the sun is \(\: 10^{30} \:kg \:\) and mass of the electron is \(\: 10^{-31} \:kg \:\). Hence, prefix is used to describe these magnitudes of measurement. SI units prefix represents multiples or sub-multiples of the SI base units. Table 5.0 shows some of the most commonly used SI unit prefix.
Units conversions for some units are shown in table 6.0.
