Introduction to Circuits and Networks

INTRODUCTION TO CIRCUITS AND NETWORKS

Basic Phenomena

The energy associated with flow of electrons is called electrical energy. The flow of electrons is called current. The current can flow from one point to another point of an element only if there is a potential difference between these two points. The potential difference is called voltage. When electric current is passed through a device or element, three phenomena have been observed. The three phenomena are,

(i) Opposition to flow of current,

(ii) Opposition to change in current or flux, and

(iii) Opposition to change in voltage or charge.

The various effects of current like heating, arcing, induction, charging, etc., are due to the above phenomena. Therefore, three fundamental elements have been proposed which exhibit only one of the above phenomena when considered as an ideal element (of course, there is no ideal element in nature). These elements are resistor, inductor and capacitor.

Ideal Elements

The ideal resistor offers opposition only to the flow of current. The property of opposition to the flow of current is called resistance and it is denoted by R.

The ideal inductor offers opposition only to change in current (or flux). The property of opposition to change in current is called inductance and it is denoted by L.

The ideal capacitor offers opposition only to change in voltage (or charge). The property of opposition to change in voltage is called capacitance and it is denoted by C.

Electric Circuits

The behaviour of a device to electric current can be best understood if it is modelled using the fundamental elements R, L and C. For example, an incandescent lamp and a water heater can be modelled as ideal resistance. Transformers and motors can be modelled using resistance and inductance.

Practically, an electric circuit is a model of a device operated by electrical energy. The various concepts and methods used for analysing a circuit is called circuit theory. A typical circuit consists of sources of electrical energy and ideal elements R, L and C. The practical energy sources are batteries, generators (or alternators), rectifiers, transistors, op-amps, etc. The various elements of electric circuits are shown in Figs 1.1 and 1.2.

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Elements which generate or amplify energy are called active elements. Therefore, energy sources are active elements. Elements which dissipate or store energy are called passive elements. Resistance dissipates energy in the form of heat, inductance stores energy in a magnetic field, and capacitance stores energy in an electric field. Therefore, resistance, inductance and capacitance are passive elements. If there is no active element in a circuit then the circuit is called a passive circuit or network.

Sources can be classified into independent and dependent sources. Batteries, generators and rectifiers are independent sources, which can directly generate electrical energy. Transistors and op-amps are dependent sources whose output energy depends on another independent source. Practically, the sources of electrical energy used to supply electrical energy to various devices like lamps, fans, motors, etc., are called loads. The rate at which electrical energy is supplied is called power. Power, in turn, is the product of voltage and current.

Circuit analysis relies on the concept of law of conservation of energy, which states that energy can neither be created nor destroyed but can be converted from one form to other. Therefore, the total energy/power in a circuit is zero.

Units

SI units are followed in this book. The SI units and their symbols for various quantities encountered in circuit theory are presented in Table 1.1. In engineering applications, large values are expressed with decimal multiples and small values are expressed with submultiples. The commonly used multiples and submultiples are listed in Table 1.2.

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