Conventions for the Use of SI Units

CONVENTIONS FOR THE USE OF SI UNITS.

Q. What are the RULES for the use of SI units?

Ans. 1) Units of every physical quantity should be represented by its symbol.

         2) Full name of a unit always starts with a smaller letter even if the name is after a person, e.g., 1 newton, 1 joule, etc. But symbol for a unit named after a person should be in capital letter, e.g., N after scientist Newton, J after scientist Joule, etc.

         3) Symbols for units do not take plural for example, force of 20 N and not 20 newtons or not 20 Ns.

         4) Symbols for units do not contain any full stop at the end of recommended letter, e.g., 25 kg and not 25 kg..

         5) The units of physical quantities in numerator and denominator should be written as one ratio for example the SI unit of acceleration is m/s² or m s^-2 but not m/s/s.

         6)  Use of combination of units is and symbols for units is avoided when physical quantity is expressed by combination of two. e.g., The unit J/kg K is correct while joule/kg K is not correct.

         7) A prefix symbol is used before the symbol of the unit. Thus prefix symbol and unit symbol constitute a new symbol for the unit which can be raised to a positive or negative power of 10.

         8) Space or hyphen must be introduced while indicating multiplication of two unit e.g., m/s should be written as m s or m-s² and not as ms² (because ms² will be read as millisecond). Discussed earlier is length. To measure the length or discussed the SI unit used is metre (m). In 1960, a standard for the metre based on the wavelength of orange-red light emitted by atoms of krypton was adopted. By 1983 a more precise measurement was development. It says that a metre is length of the path travelled by light in vacuum during a time interval of 1/299792458 second. This was possible as by that time the speed of light in vacuum could be measured precisely as c=299792458 m/s.