• Components that are used to store and /or pass electrical energy from one terminal to another terminal are referred to as passive elements. Resistors, inductors, capacitors. and a pair of  mutually coupled inductors are considered as passive elements. The electronic components such as diode, transistor and FET will be represented by their equivalent circuits (in the form of resistor, capacitor and inductor) in an electric circuit.

Resistor:

• The resistor is a passive component. The resistance is the property of a resistor by which it opposes the flow of current (irrespective of dc or ac) and is measured in ohms (Ω). The resistance of the material is expressed by the following equation.  

• Where R is the resistance measured in ohms. The rho (⍴) is the resistivity or specific resistance of the material, measured in ohm-metre (Ω-m). Length (l) and area (A) are the length and area of the material used.

Series and Parallel Connections of Resistors:

• Two or more resistors can be connected in series or parallel as shown in figure 1.4. 
• If R₁,R₂ and Rn resistors are connected in series as shown in figue 1.4 (a), the equivalent resistance R is expressed by

                   (7)

• If R1, R2 and Rn resistors are connected in parallel as shown in figue 1.4 (b), the equivalent resistance R is expressed by

                 (8)

• If two resistors R1 and R2 are connected in parallel, then the equivalent resistance is expressed by         

                                   (9)

• Behaviour of resistor for ac and dc sources: The resistor opposes the flow of current irrespective of the ac or dc applied voltages. The current flowing through the resistor is expressed by

                                  (10)

• The voltage across the resistor is expressed by

V=IR                                               (11)