ELECTRIC CIRCUIT THEORY CONCEPTS

Introduction

Electric Circuit Theory and Electromagnetic field theory are two foundational subject on which Electrical Engineering is supported. To understand core subject like Electrical machines, Power Electronics and Power system one have to good understanding of both Electric circuit Theory and Field theory. So we are going to start our journey in Electrical Engineering by learning Concept related to Electric Circuit Theory.

Starting with circuit theory We have clear understanding of terminology used in Circuit theory. So below Some terms are described which we encountered in our path to learning circuit theory.

Circuit: The interconnection of Elements which forms closed path for flow of current is called circuit.

Network: The interconnection of Elements are called Network.

Now by reading this definition we got confused Circuit and Network supposed to be the same thing but there is just small difference between them. I'll explain it through images shown one of them is circuit and other one is not circuit but we call it network. We can clearly see in second one the terminals are open circuit so there is not a closed path but interconnection of element is there so called it network not circuit so closed path for flow of current is essential for the network called as electric circuit. On the other hand the first image is  circuit because it consist of a closed path for flow of current.

So we can conclude that all circuits are network but vice versa not possible.

So far we understand circuit and network now we move on to terms which have greatest significance in Electrical Engineering.

We know that all matter made up of atoms and atoms too divisible in protons, electrons and neutrons.

Atom is Electrically neutral i.e. overall charge on atom is zero. Now new term introduced in our path is charge. So we discuss it.

Charge: Charge is the  electrical property of atom due to which it attract or repel another atom or particle consisting charge.

Now the question arise "why atom is electrically neutral or why its overall charge is zero?".

Here's the answer  We all know that atom consists of equal number of protons and electrons both electrons and protons carry same magnitude of charge shown as 'e'  called elementary charge and proton have positive in charge and electron charge is negative so while considering whole atom total protons positive charge is equal to total electron negative charge so their sum comes out to be zero.

Now we discuss more about charge.

• Charge is Quantized that means charge on any atom or body is integral multiple of elementary charge e.

                                 Q=n x e

          where n is any integer and e is elementary charge of magnitude 1.602 X 10^-19

• Charge is Conserved i.e. overall charge of a system is always same.
• SI unit of charge is Coulomb (C) where 1C is consists of 6.25 X 10^18 electrons or protons. 1C charge is very large so smaller charge values generally used such as μC, nC and pC.

Current: The rate of flow of charge is called current.

Another important term in our path of learning is current it describe as the rate of flow of charge but in very much detail it describes as the rate of flow of positive charge is called current. Although we know that charge in metals generally due to electron still due to universally accepted idea of things goes from positive to negative we consider current as flow of positively charge particles from high potential to low potential terminal.

The SI unit of current is Ampere and expression for current is given below:-

If we had a graph of charge with respect to time the slope of graph gives us the current or if we had function represent charge its first derivative with respect to time gives us the instantaneous current. I would like my viewers to remember above point so they can find the current easily.

Now what if we have current(i) vs time (t) graph and we wanted to find total charge of the system ,then what we do? The Answer is simple the Area under the i vs t graph gives us the total charge. If we had function of i then its integration gives us the charge(Q).

Direct Current(DC): The current which flows in one direction is called dc current although it may be constant or time varying.

Alternating Current(AC): The current which changes direction with respect to time is called Ac current.

Voltage: Since we know that the charge is flow through a conductor from one end to other. To move the electron in a conductor in a particular direction some work and energy transfer. This work is done by electromotive force or Voltage difference. Voltage v_ab between two points a nd b in the circuit is the energy required to move unit positive charge from b to a.  

Its SI unit is volt(V).

Power: Power is defined as much energy a system consume or expend with respect to time. Technically it defined as time rate of consumption or absorption of energy in a system.

Mathematically it shown as

In our circuit theory we consider passive sign convention means if current enter the positive terminal of element and power is positive means p=+vi that means power is absorbed by element. If power comes out negative in passive sign convention p=-vi that means power is delivered by an element.

Energy supplied or absorbed by element in an interval is given by

Now we get the basic idea about the terms we encountered in our journey to learn Electrical circuits further we get the idea about elements and limitations of circuit theory. 

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