Current Electricity

Electric Current: Electric current is defined as the rate of flow of charge or charge flowing per unit time interval. Its direction is the direction of flow of positive charge. Its SI unit is ampere (A). It is a scalar quantity.

A current of one ampere flowing through a conductor means 6.25 x 10 ¹⁸ electrons are entering at one end or leaving or leaving the other end of the conductor in one second.

Resistance: The opposition offered by a conductor to the flow of current through it is called resistance. It arises due to collisions of drifting electrons with the core ions. It SI unit is ohm.

Ohm’s law: if physical conditions like temperature, intensity of light etc. remains unchanged then electric current flowing thorough a conductor is directly proportional to the potential difference across its ends. If V is the potential difference across the ends of a conductor and I is the current through it, then according to ohm’s law V & I   or,   v = RI

Where r is a constant called resistance of conductor.

Ohmic resistance: The resistances of such conductors which obey ohm’s law are called ohmic resistance. For example resistances of maintaining wire.

Non Ohmic resistance: The resistances of such materials which do not obey ohm’s law are called non ohmic resistance.

Example: resistance of diode valve, resistance of triode valve. Conductance: Reciprocal of resistance of a conductor is called its conductance i.e. conductance =     1 / Resistance

It is denoted by G and (G = 1/R)

Its SI unit is ohm⁻¹ (also called mho or siemen.)

The resistance of a conductor is directly proportional to its length and inversely proportional to its cross sectional area. I.e. if I and A are respectively length and cross sectional area of a conductor and R is its resistance then R & I/A or, R= ρ I/A

Where ρ is a constant of material of conductor called specific resistance or resistivity. Its SI unit is ohm meter.

Specific conductance or conductivity: The reciprocal of resistivity of a conductor is called its conductivity (s). Its SI unit is mho m⁻¹ or siemen / meter (sm⁻¹)