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.
Current is defined as the rate of flow of electrons in a material. It is denoted by the alphabet I or i.
The direction of flow of DC current is always opposite to the direction of motion of electrons. When the electrons flow from the negative terminal to the positive terminal, the flow of current is from the positive terminal to the negative terminal. But in the case of AC current, the direction of flow reverses periodically.
Electric current is measured by the number of electrons flowing past a particular point in a conductor or a circuit per unit time.
I = Q/t
Q is the charge of electrons flowing through the conductor.
t is the time of flow in seconds.
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.