si units
1.2.1 base units
the international system of units (si) is a standard system used in physics. it is built from a small set of fundamental units known as base units.
all other units can be derived from these base units.
| s.no | base quantity | si base unit | symbol |
|---|---|---|---|
1 |
mass | kilogram | kg |
2 |
length | metre | m |
3 |
time | second | s |
4 |
electric current | ampere | a |
5 |
temperature | kelvin | k |
6 |
amount of substance | mole | mol |
7 |
luminous intensity | candela | cd |
knowledge about the mole and candela is not required at this level.
1.2.2 derived units
many physical quantities are formed by combining base quantities. the units of these quantities are known as derived units.
- derived quantities are expressed using base quantities
- derived units are written in terms of base units
for example, speed is derived from distance and time.
speed = distance / time
the unit of speed is therefore m s−1.
1.2.3 principle of homogeneity
the principle of homogeneity states that all terms in a physical equation must have the same units.
- each term added or subtracted must have identical units
- equations with mismatched units are physically incorrect
this principle can be used to check whether an equation is valid.
1.2.4 multiples and sub-multiples
very large and very small quantities are expressed using prefixes. these prefixes represent powers of ten.
| prefix | symbol | multiplying factor |
|---|---|---|
| tera | T |
1012 |
| giga | G |
109 |
| mega | M |
106 |
| kilo | k |
103 |
| deci | d |
10−1 |
| centi | c |
10−2 |
| milli | m |
10−3 |
| micro | μ |
10−6 |
| nano | n |
10−9 |
| pico | p |
10−12 |