Introductory concepts

Electric current

            Electric current is the rate of flow of electric charges. Electrons are negatively charged particles and their flow in conductors results in electric current.

            By definition, current, 

      If Q is the amount of charge flowing through a conductor in T seconds, then current I is given by the expression,

                                               

      The unit of charge is Coulomb. 1 Coulomb = 6.24×10¹⁸ electrons. The unit of current is Ampere(A). 1 Ampere= 1 Coulomb/sec.

Electromotive force(e.m.f)

            Energy is required for the movement of charge from one point to another. Inorder to move electrons along a conductor, some amount of work is required. The work required is supplied by an electromotive force(e.m.f) provided by a battery or a similar device.

Potential difference

Potential difference is the difference between the voltages at two ends of a conductor.

A current carrying element is shown in the fig above. The voltage across the element, V_ab is given by,    

                                

      Where work done (W) is measured in Joules and the charge (Q) in Coulombs.

            The positive (+) and negative (-) signs shown in fig. define the polarity of the voltage V_ab. With this definition, V_ab represents the voltage at point ‘a’ relative to point ‘b’.