Sunday, 23 September 2018

CURRENT ELECTRICITY NOTE

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) 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.Thermal Velocity of Free Electrons: Free electrons in a metal move randomly with a very high speed of the order of 105 ms-1. This speed is called thermal velocity of free electron. Average thermal velocity of free electrons in any direction remains zero. Drift Velocity of Free Electrons:When a potential difference is applied across the ends of a conductor, the free electrons in it move with an average velocity opposite to direction of electric field. which is called drift velocity of free electrons. Drift velocity vd = eEτ / m = eVτ / ml

where, τ = relaxation time, e = charge on electron,
E = electric field intensity, 1 = length of the conductor,
V = potential difference across the ends of the conductor
m = mass of electron. Relation between electric current and drift velocity is given by vd = I / An e
Mobility:The drift velocity of electron per unit electric field applied is mobility of electron.

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