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Question 3: What conventions are used in SI to indicate units?



In science, convention is a generally accepted rule which has no scientific proof but gives accurate results whenever used. For example, by convention, the charge on an electron is taken to be negative. The results would be the same if scientists, initially, decided the charge on an electron to be positive and that on a proton to be negative.

Conventions in indicating units

  • Names of units are treated as ordinary nouns. They begin with small letter. For example, newton, hertz, meter, second. However, at the beginning of the sentence, the first letter is capitalized. Similarly, the first letter of unit name after a scientist is capital and in case of letters, the second one is small. For example, N and Pa.

    Take an example. City is the name of place and common or ordaniry noun. On the other hand Peshawar is a particular city and proper noun. We start common nouns with small letter and proper nouns with capital one. Thus we write, “Peshawar is a city.” When we mean unit of force, it is considered as common noun and if we mean the scientist (Sir Isaac Newton), it is considered as proper noun and started with capital letter like Peshawar. Therefore, we say newton is the unit of force named after famous physicist Newton.

  • When it is necessary to write full name of the unit instead of its symbol, it is necessary to write it in full. For example, 2.1 meters per second. (2 m/second is considered against the convention, for example).
  • The combination of a prefix name (e-g, micro, kilo etc) and unit name (e-g, meter, watt) is considered a single word and no space or hyphen is used between them. For example, millimeter, kilowatt is considered correct. Milli meter or kilo-watt is against the convention.
  • Use of compound prefixes is not allowed. For example, pm (= 10-12m) but never μμm. (µm = 10-6m)
  • Multiple or sub-multiple prefix is part of the unit. Write it before the unit symbol without any separators. For example, μC is correct. μ C is against the convention.
  • In forming product or quotient of units (in case of derived units), the normal rules of algebra apply. Multiplication is indicated by putting a space or a half-high dot. For example, N m. Division is indicated by an oblique stroke (/). For example m/s.
  • It is not permissible to use abbreviations. For example cc is not allowed to be used for cubic centimeter or cm3.
  • When multiple of a unit is raised to a power, the power applies to the whole multiple. For example, km2 = (km)2 = (103m)2 = 106m2.


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