The electron in the outermost orbit of an atom is referred to as valence electrons. When sufficient energy or external force is imparted to these electrons, it moves from a negatively charged atom to a positively charged atom. This flow of electrons is called current. The lower case or upper-case alphabet of I is used to represent current. 

An electric current may be defined as the rate of motion of electric charge across a cross-sectional boundary. The unit of current is the ampere (A) named after the French physicist A.M. Ampere. One Ampere corresponds to the charge moving at the rate of one-coulomb per second (C/s). 

The current is expressed as 

       \tiny i=\frac{dq}{dt}\;C/s\;(A)

The term ‘dq’ is the amount of charge that passes by in a time interval ‘dt’


If 0.5 Coulombs of charge flow in a conductor across a cross section in 2 sec, find the current through it. 

    \bg_white \tiny I=\frac qt=\frac{0.5}2\;=\;0.25\;A


How many electrons pass a fixed point on the conductor in 1 minute and 1 second if it carries a current of five amperes. 


As one coulomb is equal to 6.24x1018 electrons, the number of electrons passing a fixed point on the conductor in one second and one minute is calculated as 3.12x1019 electrons per second and 1.872x1021 electrons per minute respectively. 


Find the time taken by the one coulomb to cross a given point in the circuit which constitutes four amperes of current in the circuit. 

       I\;=\;\frac qt\;A

       t=\frac QI\;=\;\frac14\;=\;0.25\;s