ABSTRACT

This chapter has been designed to provide you with the background knowledge required to help you understand the concepts introduced in the later chapters. If you have studied electrical science, electrical principles, or electronics beyond school level then you will already be familiar with many of these concepts. If, on the other hand, you are returning to study or are a newcomer to electronics or electrical technology this chapter will help you get up to speed. Fundamental units

You will already know that the units that we now use to describe such things as length, mass and time are standardized within the International System of Units. This SI system is based upon the seven fundamental units (see Table 1.1). Derived units

All other units are derived from these seven fundamental units. These derived units generally have their own names and those commonly encountered in electrical circuits are summarized in Table 1.2 together with the corresponding physical quantities. Table 1.1 SI units

(Note that 0 K is equal to 273°C and an interval of 1 K is the same as an interval of 1°C.)

Quantity Unit Abbreviation

Current ampere A

Length metre m

Luminous intensity candela cd

Mass kilogram kg

Temperature Kelvin K

Time second s

Matter mol mol

If you find the exponent notation shown in Table 1.2 a little confusing, just remember that V1 is simply 1/V, s1 is 1/s, m2 is 1/m2, and so on. Example 1.1

The unit of flux density (the Tesla) is defined as the magnetic flux per unit area. Express this in terms of the fundamental units. Solution

The SI unit of flux is the Weber (Wb). Area is directly proportional to length squared and, expressed in terms of the fundamental SI units, this is square metres (m2). Dividing the flux (Wb) by the area (m2) gives Wb/m2 or Wb m2. Hence, in terms of the fundamental SI units, the Tesla is expressed in Wb m2.