Basic Concepts

Introduction

Electric Circuit Theory & Electromagnetic Theory are the two fundamental theories upon which all branches of electrical engineering are built.

Our objective in this article is not the study of various uses and applications of circuits. Rather our major concern is the analysis of the circuits. By analysis of a circuit we mean study of the behavior of the circuit: How does it respond to a given input? How do the interconnected elements and devices in the circuit interact?

We will be using the International System of Units (SI), adopted by the General Conference on Weights and Measures in 1960.

Electric Charge and Current

Electric Charge is the property of matter by virtue of which it responds to an electromagnetic field. Charge is measured in Coulomb.

Charge is Quantized: The only charges that occur in nature are integral multiples of the electronic charge 

The law of conservation of charge states that charge can neither be created nor destroyed, only transferred. Thus the algebraic sum of the electric charges in a system does not change.

Electric Current is the time rate of change of charge, measured in Ampere (A).

1 Ampere=1 Coulomb/second

A direct current (dc) is a current that remains constant with time.

Alternating Current is a current that varies with time.

Voltage or Potential Difference

The voltage  between two points in an electric circuit is the energy (or work) needed to move a unit charge from a to b; mathematically,

Where w is energy in Joules (J) and q in coulombs (C).

Polarity of Voltage        

The (+) and (–) signs are used to define reference direction or voltage polarity.

    Polarity of Voltage

The common term signal is used for an electric quantity such as a current or voltage (or electromagnetic wave) when it is used for conveying information.

Power and Energy

Power is the time rate of expending or absorbing energy. Power is measured in Watt (W).

If current leaves from the +ve terminal of an element then the element is delivering power and if the current enters the +ve terminal of an element then the element is absorbing power.

Associated Variable Convention      

1) Active Sign Convention: If the current leaves from the +ve terminal of an element  and if the current enters the +ve terminal of an element p = -vi.

2) Passive Sign Convention: If the current leaves from the +ve terminal of an element  and if the current enters the +ve terminal of an element p = +vi.

We use Active Sign Convention: If the element delivers power (or energy) then the power is taken +ve

                                                                                                                       else the power is taken –ve.

The law of conservation of energy must be obeyed in any electric circuit. 

Consequently, the algebraic sum of power in a circuit at any instant of time is zero.

Energy is the capacity to do work, measured in Joule (J).

Circuit Elements

There are two types of elements: Active Elements and Passive Elements.

An active element is one that can generate energy and a passive element is one that can’t generate energy. So Passive elements include resistor, capacitor & inductor & Active elements include op-amps, BJT, current source, voltage source etc.

There are two kinds of sources: Independent Sources and Dependent Sources.

An ideal independent source is an active element that provides a specified voltage or current that is independent of other circuit elements.

Following pictures show the symbols used:

An ideal dependent source is an active element in which the source quantity is controlled by another voltage or current.

1)      VCVS – Voltage Controlled Voltage Source

2)      VCCS –  Voltage Controlled Current Source

3)      CCVS- Current Controlled Voltage Source

4)      CCCS- Current Controlled Current Source

Following pictures show the symbols used:

A note to the reader:  In the next article I will be covering the Basic Laws including Ohms Law, Kirchhoff’s Laws, Combination of Cells and resistors, Voltage Division & Current Division.

For any questions or queries feel free to contact: abhisekroutwrites@gmail.com