Question

explain theory of ohm's law

Answer 1

Ohm's law states that the current through a conductor between two points is directly proportional to the voltage across the two points. Introducing the constant of proportionality, the resistance,[1] one arrives at the usual mathematical equation that describes this relationship:[2]

{\displaystyle I={\frac {V}{R}},}

where I is the current through the conductor in units of amperes, V is the voltage measured across the conductor in units of volts, and R is the resistance of the conductor in units of ohms. More specifically, Ohm's law states that the R in this relation is constant, independent of the current.[3]

The law was named after the German physicist Georg Ohm, who, in a treatise published in 1827, described measurements of applied voltage and current through simple electrical circuits containing various lengths of wire. 

Answer 2

the ratio of potential difference V across of a conductor to the current I flowing through it remain constant provided at temperature and other physical condition of the conductor remains the same.
V=IR
R=V/R