Question

state ohm's law.express it mathematically and apply it to define the unit of resistance

Answer 1

The difference of potential V across the ends of metallic wire in an electric circuit is directly proportional to the flow of current through this. whereas temperature will remain constant. 

so, 
V = IR
I = flow of current
R = constant

Answer 2

The law states that the potential difference V across the ends of a given metallic wire in an electric circuit is directly proportional to the current flowing through it provided the temperature remains the same.

V = IR

Derivation:

V is directly proportional to I

V/I = constant

V/I = R where R is the resistance

∴ V = IR