(a) OHM'S LAW:

STATEMENT: The electric current flowing through a metallic conductor is DIRECTLY PROPORTIONAL to the potential difference applied across its ends, provided the temperature and other physical conditions of the conductor remain constant.

$$V \propto I \qquad \Rightarrow \qquad V = IR$$

where the constant of proportionality R is the RESISTANCE of the conductor. Its SI unit is the ohm ($\Omega$).

(b) V-I GRAPH (what to draw):

Take the electric current I (in ampere) along the X-axis and the potential difference V (in volt) along the Y-axis.

The graph obtained is a STRAIGHT LINE PASSING THROUGH THE ORIGIN.

This shows that V is directly proportional to I, which verifies Ohm's law.

The SLOPE of this straight line gives the resistance of the conductor:

$$\text{Slope} = \frac{V}{I} = R$$