BASIC

Custom Search

ELECTRICAL THEORY

APPENDIX A METRIC SYSTEM AND POWERS OF TEN

Metric Svstem

Units of the international metric system, commonly called SI (system internationale), are used in electricity. The seven basic units are shown on Table A-1. Two supplementary units of SI are shown on Table A-2.

TABLE A-1 BASE UNITS of the INTERNATIONAL METRIC SYSTEM

 Quantity Base Unit Symbol Length meter m Mass kilogram Kg Time second s Electric current ampere A Thermodynamic temperature Kelvin K Light intensity Candela cd Amount of substance Mole mol

TABLE A-2 SUPPLEMENTARY SI UNITS

These base and supplemental units are used to derive other units. Most of the units of measure used in electricity are derived units. These units are shown on Table A-3. Some of these are derived from the base units, and some are derived from other derived units, or a combination of both. For example, the unit of current is the ampere, which is derived from the base units of second and coulomb. The derivation of these units is beyond the scope of this text. However, units commonly used in the study and use of electricity and their relationship to other units of measure are discussed in Chapter 3, Units of Electrical Measurement.

TABLE A-3 DERIVED SI UNITS

 Quantity Derived Unit Symbol Energy joule J Force newton N Power watt W Electric charge coulomb C Electric potential volt V Electric resistance ohm Electric conductance mho Electric capacitance farad F Electric inductance henry H Frequency hertz Hz Magnetic flux weber Magnetic flux density tesla T

Metric Prefixes

When working with basic electrical measurement units, sometimes the values are too large or too small to express conveniently. For convenience, add metric prefixes (Table A-4) to the basic unit. For example, if we have a resistor that is 1,000,000 ohms (), we can add the prefix kilo, or its designated symbol K, and express the value as 1,000 K, or the metric prefix mega (symbol M) and express the value as 1 M. In the case of capacitance or current, we may have a very small value such as 0.002 amperes (symbol A) or 0.000005 farads (symbol F). These values can be expressed by adding the prefix milli (symbol m) and expressing the current as 2 milli amperes or 2 mA, or adding the prefix micro (symbol ) and expressing the capacitance as 5 micro farads, or 5 F.