0195136047.pdf

4 CIRCUIT CONCEPTS

for analyzing and designing simple but illustrative practical circuits. Later, a brief introduc-

tion is given to meters and measurements. Finally, the analogy between electrical and other

nonelectric physical systems is pointed out. The chapter ends with a case study of practical

application.

1.1 ELECTRICAL QUANTITIES

In describing the operation of electric circuits, one should be familiar with such electrical quantities

as charge, current, and voltage. The material of this section will serve as a review, since it will

not be entirely new to most readers.

Charge and Electric Force

The proton has a charge of+1.602 10−^19 coulombs (C), while the electron has a charge of

− 1. 602 × 10 −^19 C. The neutron has zero charge. Electric charge and, more so, its movement

are the most basic items of interest in electrical engineering. When many charged particles are

collected together, larger charges and charge distributions occur. There may be point charges (C),

line charges (C/m), surface charge distributions (C/m^2 ), and volume charge distributions (C/m^3 ).

A charge is responsible for anelectric fieldand charges exertforceson each other. Like

charges repel, whereas unlike charges attract. Such an electric force can be controlled and utilized

for some useful purpose.Coulomb’s lawgives an expression to evaluate the electric force in

newtons (N) exerted on one point charge by the other:

Force onQ 1 due toQ 2 =F ̄ 21 =

Q 1 Q 2

4 πε 0 R^2

a ̄ 21 (1.1.1a)

Force onQ 2 due toQ 1 =F ̄ 12 =

Q 2 Q 1

4 πε 0 R^2

a ̄ 12 (1.1.1b)

whereQ 1 andQ 2 are the point charges (C);Ris the separation in meters (m) between them;ε 0

is the permittivity of the free-space medium with units of C^2 /N·m or, more commonly, farads

per meter (F/m); anda ̄ 21 anda ̄ 12 are unit vectors along the line joiningQ 1 andQ 2 , as shown in

Figure 1.1.1.

Equation (1.1.1) shows the following:

1. ForcesF ̄ 21 andF ̄ 12 are experienced byQ 1 andQ 2 , due to the presence ofQ 2 andQ 1 ,
   respectively. They are equal in magnitude and opposite of each other in direction.


2. The magnitude of the force is proportional to the product of the charge magnitudes.


3. The magnitude of the force is inversely proportional to the square of the distance between
   the charges.


4. The magnitude of the force depends on the medium.


5. The direction of the force is along the line joining the charges.
   Note that the SI system of units will be used throughout this text, and the student should be
   conversant with the conversion factors for the SI system.
   The force per unit charge experienced by a small test charge placed in an electric field is
   known as the electric field intensityE ̄, whose units are given by N/C or, more commonly, volts
   per meter (V/m),

E ̄=lim

Q→ 0

F ̄

Q

(1.1.2)