Quantities and Units and Prefixes

Quantities and Units

In science research, scientists have devised and stick to an International System of Units (also know as SI units). This is a coherent system of units comprising 5 base units on which all other units are based. (The units are the same measure across the board and not subjective to)

The first table (Table 1) shows 7 units (exclude the mole and the candela since they are not required for your syllabus)

When one or more basic quantities are multiplied or divided, a derived quantity is produced. The units are therefore a combination of basic units. In some cases, the combination is very complex and shortened by using names of famous scientists e.g.. Joule and Newton. Table 2 lists a few of the derived quantities.

Quantities without units are dimensionless quantities. e.g. magnification, relative density and refractive index.

Prefixes

Prefixes are used in front base or derived SI units when their values are too large or too small to deal with easily. For instance a length of 400 000 m can be re written as 400 km or 0.4 Mm. Table 3 below shows a list of common prefixes and their symbols. You should be able to comfortably change values between different notations.