The SI system of units

Before proceeding further with dimensional considerations we first need to discuss units of measurement. In this course we will adopt the SI system of units ⁽¹⁾, which is described in some detail in the Physics Quick Reference Guide [4] (which I will abbreviate as PQRG from now on), pp. 4-10. In the SI system the base, or defined, units, are the meter (m), the kilogram (kg), the second (s), the kelvin (K), and the ampere (A).⁽²⁾ The definitions of these units in terms of fundamental physical processes are given in the PQRG. All other units are derived. For instance, the SI unit of energy, the joule (J), is equal to 1 kg m²/s². The derived units are also listed in the PQRG. The SI system is referred to as a LMT-class, since the defined units are length L, mass M, and time T (if we add thermal and electrical phenomena, then we have a - class in the SI system).

Choice of units

In this course, where several fields of pure and applied physics are introduced, it makes sense to use SI units throughout and save time on unit conversions. However, in the real world, the specialists in different fields often use their own peculiar units. Sometimes they do this for the sake of tradition, like US engineers who persist in using British units; but often enough they have good reasons for departing from SI. Here are a few illustrative examples.
These examples and the summary should be read after the rest of the chapter:

• Atomic units
• Astronomical units
• Units of pressure
• Summary

Footnotes:

⁽¹⁾ In some older texts this is referred to as the MKS system.

⁽²⁾ We should also add the mole (mol) and the candela (cd), but these will seldom enter into our models.