Q: State Ohm’s law.

**Answer:** Ohm’s law states that the current (I) flowing through a conductor is directly proportional to the potential difference (V) across the two ends of the conductor, provided temperature, strain etc. are kept constant. Mathematically, V = RI, where R is resistance of the conductor.

Q: What is resistivity (or specific resistance)?

**Answer:** Resistivity is a property of a material which determines the obstruction offered by the material to the flow of electric current through it. Its magnitude is equal to the resistance of a wire of the material of unit length and unit area of cross-section.

Its unit is ohm-metre (Ω m).

Point to Remember: Resistivity of conductors increases with the increase in temperature; but *resistivity of semi-conductors and insulators decreases with the increase in temperature*.

Q: Define current density.

Ans: Current density (j) is defined as current per unit area normal to the current. Its unit is Am^{-2}.

Q: Define electrical conductivity.

Ans: Electrical conductivity (σ) is defined as the reciprocal of resistivity (ρ).

[** Note**: An alternate form of Ohm’s law is: j = σ E, where E is uniform electric field in the conductor.