Every particle in the nature other than the insulators* has a large number of free electrons in them. These electrons move randomly in all directions within the material under normal conditions. If a certain amount of voltage is applied across these materials, all these electrons start moving from the region of higher potential towards the region of lower potential. This movement of electrons from the region of higher potential to the region of lower potential under electric field constitutes the electric current.
*there will be no free electrons in insulator provided that it is maintained at a normal temperature or room temperature.
Definition of current
Electric current is normally referred to as the flow of charges through a conductor. It can be defined as the amount of charge that flows past a cross-section area in a conductor. In other words, the term “current” can be defined as the rate of flow of charges through a conductor. Read more about Electic Current
The mathematical expression of electric current
Electric current is measured by the number of electrons flowing past a particular point in a conductor or a circuit per unit time.
Where Q is the charge of electrons flowing through the conductor. t is the time of flow in seconds.
The direction of flow of current
The direction of current flow is a little difficult to understand to those who have been taught that current flows from positive to negative. There are two theories behind this phenomenon. One is the theory of conventional current and the other is the theory of actual current flow. When Benjamin Franklin was studying charges, the structure of an atom and atomic particles were unknown. Hence he assumed the point of charge accumulation as positive and the point which is deficient of charges as negative. Therefore, charge is said to flow from positive to negative. This is called conventional current.
But in reality, an electric current is nothing but the flow of electrons. Electrons are negatively charged particles and are attracted towards the positive charge. Also, many experiments have revealed that it is free electrons in a conductor that flows. Negatively charged electrons move from the negative terminal to the positive terminal. This is the direction of the actual current flow.
In terms of circuit analysis
In terms of circuit analysis, we normally consider the flow of current from positive to negative. Mathematically, negative charge flowing in one direction is equivalent to positive charges flowing in the opposite direction. Hence it does not make a difference. One can either consider the flow of current from positive to negative or vice versa during circuit analysis. In fact, positively charged ions can be attracted by negatively charged electrons.
Unit of current
Unit of current is ampere or A. one ampere is equal to the one coulomb per second whereas one coulomb is equal to 6.25 x 1018 electrons. By saying that one ampere of current is flowing through a circuit, it is meant that 6.25 x 1018 electrons are crossing a point in the circuit per second.