Fundamental (or base) units and derived units

The metre, kilogram, second and ampere are called fundamental units or base units. See: Base units

From these units we can derive some more units that are called derived units.
These units are built up step by step from the base units and usually given a distinct name.

Examples of derived units are those for velocity, acceleration, force (Newton), work (Joule), energy (Joule), charge (Coulomb), pressure (Pascal) and density

As an example we will look at how the unit for potential difference (volt) is derived from the four base units

The volt is defined as the work done per unit charge

1. Combining two base units, metre and second, we have the derived unit for velocity (ms^-1)
2. Combining two base units, amp and second, we have the derived unit for charge (As) (simply called the Coulomb)(C)
3. Combining the base unit second with the derived unit for velocity we have another derived unit for acceleration (ms^-2)
4. Combining the base unit kilogram with the derived unit for acceleration we have another derived unit, this time for force (ms^-2) (simply called the Newton)(N)
5. Combining the base unit metre with the derived unit for force we have another derived unit, this time for work (or energy) (kgm²s^-2) (simply called the Joule)(J)
6. Combining two derived units, that for work (kgm²s^-2) with that for charge (As) we have the derived unit for potential difference (kgm²s^-3A^-1) (simply called the volt)(V)

You can also show how to build up the derived unit for the volt using the following table.