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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

Quantity

Unit

Symbol

Plane angle

radian

rad

Socid angle

steradian

Sr

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.

 



 


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