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Concept of Electric Charge

The most basic quantity in an electric circuit is the electric charge q. Electric charge is a fundamental conserved property of some subatomic particles, which determines their electromagnetic interaction. Electrically charged matter is influenced by, and produces, electromagnetic fields.
The SI unit of charge is Coulomb (C). The charge of an electron is 1.602 × 10–19C. Thus, one Coulomb charge is defined as the charge possessed by Description: 3106.pngelectrons.
∴ 1 Coulomb charge = charge of 6.24 × 1018 electrons

Conductors, Insulators and Semiconductors

In some materials, there is a large number of free electrons or loosely bound valence-band electrons present. These electrons are easily knocked out of their orbit and easily constitute a large current. Such materials are known as conductors. Almost all metals and some liquids are good conductors.
In some materials, no free electrons are available; the valence-band electrons are tightly bound to the nucleus. Such materials are known as insulators. Examples of some insulators include glass, mica, plastics, etc.
In between the limits of these two major categories is a third general class of materials called semiconductors; where there are no such free electrons present, but free electrons can easily be created by adding some impurities. Examples of some insulators include germanium, silicon, etc. For example, germanium, a semiconductor, has approximately one trillion times (1 × 1012) the conductivity of glass, an insulator, but has only about one thirty-millionth (3 × 10–8) part of the conductivity of copper, a conductor.

Concept of Electric Current

The phenomenon of transferring electric charge from one point in a circuit to another is described by the term electric current. Electric current is defined as the rate of flow of electric charges or electrons through a cross-sectional area. By convention, the electric current flows in the opposite direction to the electrons.

Current Density

Current density at any point is a vector whose magnitude is the electric current per unit cross-sectional area and whose direction is normal to the cross-sectional are, i.e., Description: 3133.png. Its unit is Ampere per square meter (A/m2).

Concept of Electric Potential and Potential Difference

To move an electron in a conductor in a particular direction, or to create a current, requires some work or energy. This work is done by the potential or the potential difference. This is also known as voltage difference or voltage (with reference to a selected point such as earth). The unit of potential is volt.
The potential of a point is 1Volt if 1Joule work is done in bringing 1Coulomb charge from infinity to that point.
The voltage Vab between two points a and b is the energy (or work) w required to move a unit positive charge from a to b. [Unit of voltage is volt (V).]
Description: 3141.png
The potential difference between two points is 1Volt if 1Joule work is done to displace 1 Coulomb charge from one point to the other.

Drift Velocity

Electric current is the number of coulombs of charge which pass a point in the circuit per unit time. Because of its definition, it is often confused with the quantity drift velocity. Drift velocity refers to the average distance traveled by a charge carrier per unit time. Like the velocity of any object, the drift velocity of an electron is the distance to time ratio.
(a) Typical path of an electron
(b) Current is constituted by flow of many charge carriers through a cross section
Mathematically, if n number of charge carriers (electrons) with charge Q each passes through an area A with drift velocity v, then the current is given by, Description: 3147.png.

Concept of Electromotive Force (EMF)

The phenomenon of electric current depends on the presence of free electrons. If a material is having a large number of free electrons, these electrons will always move in random directions as shown in the Figure (a). If an external effort is applied to the material, it is possible to drift all the electrons in a definite direction as shown in the Figure (b). Such an external factor is known as electromotive force (emf). In other words, the voltage or potential of an electrical energy source is known as emf.

Electric Circuits and Networks

Any combination and interconnection of network elements like resistor or inductor or capacitor or electrical energy sources are known as ‘networks’. However, a closed energised network is known as ‘circuit’. A network need not contain an energy source; but a circuit must contain an energy source. Therefore, it can be stated that all circuits are networks, but all networks are not circuits.

Loop and Mesh

A loop or mesh denote a closed path obtained by starting at a node and returning back to the same node through a set of connected circuit elements without passing through any intermediate node more than once. However, the difference between mesh and loop is that a mesh does not contain any other loop within it, i.e., mesh is the smallest loop.

Node and Branch

A node is a point in a circuit where two or more circuit elements join. A node is said to be an essential node if it joins three or more elements.
A branch is a path that connects two nodes. Those paths that connect essential nodes without passing through an essential node are known as essential branches.

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