Electric Current ( I )
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 )
The conventional direction of electric current is the direction of motion of positive charge .
The current is the same for all cross-sections 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 larger in cross-section , so that charge rate remains unchanged .
If a charge q revolves in a circle with frequency f , the equivalent current , i = qf
( In a metallic conductor current flows due to motion of free electrons while in electrolytes and ionized gases current flows due to electrons and positive ions .)
Types of Electric Current
According to its magnitude and direction electric current is of two types
( i ) Direct Current ( DC ) Its magnitude and direction do not change with time . A ceil , battery or DC dynamo are the sources of direct current . ( ii ) Alternating Current ( AC ) An electric current whose magnitude changes continuously and changes its direction periodically is called alternating current . AC dynamo is source of alternating current .
Current Density
The electric current flowing per unit area of cross-section of conductor is called current density . Current density ( J ) = I / A
Its S1 unit is ampere metre -2 and dimensional formula is [ AT -2 ]. It is a vector quantity and its direction is in the direction of motion positive charge or in the direction of flow of current .
Thermal Velocity of Free Electrons
Free electrons in a metal move randomly with a very high speed of the order of 10 5 ms-1 . This speed is called thermal velocity of free electron .
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