Engineering Fundamentals: An Introduction to Engineering, 4th ed.c

346 Chapter 12 Electric Current and Related Parameters

amount of substance, andluminous intensity. Moreover, with the help of these base

dimensions we can derive all other necessary physical quantities that describe how nature

works.

In the previous chapters, we discussed the role of the fundamental dimension length

and length-related parameters, the fundamental dimension mass and mass-related

parameters, the fundamental dimension time and time-related parameters, and the fun-

damental dimension temperature and temperature-related parameters in engineering

analysis and design. We now turn our attention to the fundamental dimensioncurrent.

Table 6.7 is repeated again to remind you of the role of fundamental dimensions and

how they are combined to define engineering quantities used in analysis and design.

12.1 Electric Current as a Fundamental Dimension

As explained in Chapter 6, it was not until 1946 that the proposal forampereas a base unit for

electric current was approved by the General Conference on Weights and Mea-

sures (CGPM). In 1954, CGPM includedampereamong the base units. Theampereis

defined formally as that constant current which, if maintained in two straight parallel conduc-

tors of infinite length, of negligible circular cross section, and placed 1 meter apart in a

vacuum, would produce between these conductors a force equal to 2 10

 7

newton per meter

of length.

To better understand what the ampere represents, we need to take a closer look at the

behavior of material at the subatomic level. In Chapter 9 we explained what is meant by atoms

and molecules. An atom has three major subatomic particles, namely, electrons, protons, and

neutrons. Neutrons and protons form the nucleus of an atom. How a material conducts elec-

tricity is influenced by the number and the arrangement of electrons. Electrons have negative

charge, whereas protons have a positive charge, and neutrons have no charge.

Simply stated, the basic law of electric charges states thatunlike charges attract each other

while like charges repel. In SI units, the unit of charge is the coulomb (C). One coulomb is

defined as the amount of charge that passes a point in a wire in 1 second when a current of

1 ampere is flowing through the wire. In Chapter 10, we explained the universal law of gravi-

tational attraction between two masses. Similarly, there exists a law that describes the attractive

electric force between two opposite-charge particles. The electric force exerted by one point

charge on another is proportional to the magnitude of each charge and is inversely propor-

tional to the square of the distance between the point charges. Moreover, the electric force is

attractive if the charges have opposite signs, and it is repulsive if the charges have the same sign.

The electric force between two point charges is given by Coulomb’s law:

(12.1)

wherek8.99 10

9

Nm

2

/C

2

,q 1 andq 2 (C) are the point charges, andris the distance ( m)

between them. Another important fact that one must keep in mind is that the electric charge

F 12 

kq 1 q 2

r

2

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