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 the other end of the conductor in one second.

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. Its SI unit is ohm.

If physical conditions like temperature, intensity of light etc remains unchanged then electric current flowing through a conductor is directly proportional to the potential difference across its ends. If Vis the potential difference across the ends of a conductor and I is the current through it, then according to Ohm's law -

**V ∝ I
V = IR**

where R is a constant called resistance of conductor.

The resistances of such conductors which obey ohm's law are called ohmic resistance.

**example :** resistance of manganin wire.

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.

Reciprocal of resistance of a conductor is called its conductance

**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 *l* and *A* are respectively length and cross sectional area of a conductor and *R* is its resistance.

**R ∝ l / A
R = ρ l / A**

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

The reciprocal of a conductor is called resistivity of a conductor is called its conductivity (s). Its SI unit is mho m^{-1} or siemen/ meter (sm^{-1}).

Various resistances can be combined to form a network mainly in two ways :

1. Series combination

2. Parallel combination

» In series combination, the equivalent resistance is equal to the sum of the resistances of individual conductors.

(R = R_{l} + R_{2} + ...... R_{n})

» Parallel combination, the reciprocal of equivalent resistance is equal to the sum of the reciprocal of individual resistances.