Electric Current and Theory of Electricity

Definition of Electric Current

While a potential difference is applied across a conductor, electrical charge flows through it and electrical current is the measure of the quantity of the electrical charge flowing through the conductor per unit time.

Theory of Electricity

All matter in this universe is made up of atoms, and there are a number of models developed to explain the physical behaviors of matters. Bohr model of atomic structure is one of the oldest and simplest of models of atom. According to this model an atom consists of a central nucleus containing minute particles called protons and neutrons.

Surrounding the nucleus there are a number of electrons in various orbits. This model is illustrated in figure below.

bohr model of atomic structure

Figure-1


It should be noted that we have over-simplified this atomic model for better understanding. This is done for the beginners so that they can understand easily what electric current is and how many electrical devices operate.

The model shown in figure 1 is not drawn to scale since a proton is 1,836 times more massive than an electron. Due to this relatively large mass the proton does not play an active part in electrical current flow. It is due to the movement of electrons; hence behavior of electrons is more important. But there is one thing common for both protons and electrons and that is their electric charge. This charge is considered as the quantity of electricity it is given the symbol Q. Electron and proton both possess exactly same amount of charge. Although, the charge of protons and electrons are same but they are opposite in nature. The electron has a negative charge, whereas the proton has a positive charge. All atoms in their normal state are electrically neutral means the atoms have no net charge in their normal state. Hence, the number of electrons and protons are same in a normal atom. In other words, in this state the atom possesses as many orbiting electrons as there are protons in its nucleus. The protons are strongly bonded to nucleus and they cannot be separated from nucleus normally. But there may be some electrons in the outer orbits of the atoms which are loosely associated with atom. These electrons can be separated from atom if adequate condition is imposed. If one or more of these types of loosely associated orbiting electrons leave the parent atom, the charge balance in atom is disturbed. Because of that the atom acquires a net positive charge, and it is then called positive ion. On the other hand, if extra electrons can be made to orbit the nucleus then the atom acquires a net negative charge. It then becomes a negative ion.

There are some materials whose valances electrons are so loosely associated with the parent atoms that numbers of these electrons can be disassociated form their orbit even at room temperature. These disassociated electrons are called “free electrons”.

All the materials in this universe fall under three major groups. According to the presence of free electrons the materials are classified as conductors, semiconductors and insulators. In general we can say that conductors have many free electrons which will be drifting in a random manner within the material. Insulators have very few free electrons (ideally none), and semiconductors fall somewhere between these two extremes.

electric current
Whenever a conductor comes under influence of an electric field, which means when potential difference is applied across the conductor, the free electrons experience a force towards the higher potential side. This force makes the free electrons to drift through the conductor towards higher potential end of the conductor. This flow of electrons is the cause of electric current.

Flow of electrons means migration of negative electric charge from one point to another inside the conductor. Electric current is the rate at which free electrons can be made to drift through a material in a particular direction. In other words, it is the rate at which charge moves through a conductor.

Since charge is measured in coulombs and time in seconds then logically the unit for electric current would be the coulomb/second. In fact, the amount of current flowing through a conductor or circuit may be calculated by dividing the amount of charge passing a given point by the time taken. The unit however is given a special name, the ampere. Ampere is often abbreviated to amp. It is also represented as ‘A’. This is fairly common practice with SI units, whereby the names chosen are those of famous scientists whose pioneering work is thus commemorated. The relationship between current, charge and time can be expressed as a mathematical equation as follows:

Where I is the current and Q is the charge passing through a particular cross section of a conductor during time t.

conventional current flow

Here in the figure it is shown that the electric current is coming out from the positive terminal of the battery and passing through the circuit and enters in the negative terminal of the battery. This is called conventional current flow. However, since electrons are negatively charged, these must move from negative terminal to the positive terminal of battery. That means it is in the opposite direction of the conventional electric current. So it can be concluded that the current is always in opposite direction of negative charge flow. The definition of electric current is the rate at which electric charge flowing through a circuit. So it is very much confusing, why is direction of conventional current always in opposite of charge flow. Actually, from very beginning, it was assumed that positive potential is higher level potential than negative potential. When concept of electrons was not introduced, people used to think that electric current always flow from higher potential to lower potential. This compares with water which can naturally only flow from a high level to a lower level. Thus the concept of conventional current flow was adopted. All the subsequent ‘rules’ and conventions were based on this direction of conventional current flow. On introduction of concept of electrons, it was decided to retain the concept of conventional current flow. Had this not been the case then all the other rules and conventions would have needed to be changed! Hence, true electron flow is used only when it is necessary to explain certain effects (as in semiconductor devices such as diodes and transistors). Whenever we are considering basic electrical circuits and devices we shall use conventional current flow i.e. current flowing around the circuit from the positive terminal to the negative terminal.

Unit of Electric Current

As in the definition of electric current we have already told, that this is nothing but transferring charge per unit time in other view it can be seen as amount of charge crossing the perpendicular cross section of a conductor per unit time. So due to application of electrical potential difference, if ΔQ Coulomb charge crosses a particular cross section of any conductor at Δt second then,

Hence, unit of electric current is Coulomb per Second and it is named as Ampere, after the name of famous French mathematician Andre-Marie Ampere.

SI unit of electric current is Ampere and it is abbreviated as A or Amp. In CGS system its unit is Biot and abbreviated as Bi. 1 Bi = 10 A.

Explanation of Electric Current as a Phenomenon

Current is associated with charge carried by charged particles. Electrical current means the charge flows to one end from other by means of charged particles. The phenomenon of transferring charge from one place to another is referred as electric current.

electric current

Electric Current

It can be assumed that a beam of positively charged holes moving from one side to another. If that beam of holes moving from left to right, the current would be assumed, directed from left to right. As the holes are associated with atoms generally they cannot move. Then what we mean by movement of positive holes? Actually negatively charged free electrons move from right to left, which is assumed as if positive holes are moving in opposite direction of electrons movement that is from left to right. According to the general agreement, the direction of current is chosen to coincide with the direction in which positive charge carriers or holes move even the actual movable carriers of charge are electrons and they move in opposite direction. So direction of conventional current flow is in opposite of electrons movement.

So it can be concluded like this, if electrical potential difference is applied across a conductor, then due to electrical field, free electrons in the conductor start moving toward positive or higher potential end of the conductor. The direction of the electric current is considered to be flowing from higher potential end to lower, as the relative motion of static positive charge is assumed to be in that direction.

André-Marie Ampère

André-Marie Ampère

Therefore the unit of electrical current is Coulomb/second which is known as ′Ampere′ after the name of André-Marie Ampère (20 January 1775 – 10 June 1836), a French physicist and mathematician. This was all about basic theory of electricity.

Types of Current

There are only two types of electrical current, direct current and alternating current. We abbreviate them as DC and AC respectively. Concept of DC was developed before AC. But AC becomes most popular means of generating, transmitting and distributing of electric power. The direction of the flow of direct current is unidirectional,means this electric current does not alter its direction during flowing. Most common example of DC in our daily life is the current we get from all kind of battery system. All the house hold products, operated by battery, utilize DC as their source. One of the most important uses of DC is in relay and control system of electrical power system. Actually in all the circuit breakers, DC is only means of electrical ON / OFF and TRIP / CLOSE operation. Another important use of DC is, excitation of field magnetic system of all kinds of DC rotating machine, synchronous rotating machines.

Most popular form of electrical current is alternating current or AC. As the current produced by battery system, is always DC, the current produced form rotating generator, is AC. Even in DC generators, the fundamental current produced is alternating current but by means of commutator system, it is converted to DC. In case of AC generators, the produced alternating current at armature is directly taken. Alternating current periodically alters its direction of flow and forms a particular alternating waveform. The most common among them is sinusoidal wave form but in some special case these wave form can also be triangular type or square type. AC does have some advantages over DC for generating, transmitting and distributing and that is why the current we get from our electric supply companies, is normally alternating current.


Video on Conventional Current Flow

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