The rate of flow of charge through any cross –section of a wire is called electric current flowing through it . Electric current (i) = q / t . Its SI unit is ampere (A). Current is a rate quantity. There are several rate quantities in physics. For instance, velocity is a rate quantity - the rate at which an object changes its position. Mathematically, velocity is the position change per time ratio. Acceleration is a rate quantity - the rate at which an object changes its velocity. Mathematically, acceleration is the velocity change per time ratio. And power is a rate quantity - the rate at which work is done on an object. Mathematically, power is the work per time ratio. In every case of a rate quantity, the mathematical equation involves some quantity over time .The current is the same for all cross-section of a conductor of non – uniform cross – section similar to the water flow ,charge flows faster ,where the conductor is smaller in cross section and slower where the conductor is large in cross-section , so that charge rate remains unchanged .
If a charge q revolved in a circle with frequency f ,the equivalent currentI = qf ,
' 1 ampere = 1 coulomb / 1 second
Types of Electric Current
The electric current flowing per unit area of cross-section of conductor is called current density .
The current density (j) = i / A
Its SI unit is ampere m-2 and dimension formula at [AT-2]
It is a vector quantity and its direction of motion positive charge or in the direction of flow of current .