On what factors does the resistance of a conductor depend? Write the relation between them and define resistivity.

1. 1 · Open with the definition. Resistance is the property of a conductor that opposes the flow of charge.
2. 2 · List the factors.
   • Length: R ∝ l
   • Area of cross-section: R ∝ 1/A
   • Nature of the material
   • Temperature
3. 3 · Combine them. R ∝ l/A, so R = ρl/A, where ρ is the resistivity of the material.
4. 4 · Define resistivity. Resistivity is the resistance of a conductor of unit length and unit area of cross-section. SI unit: Ω m.
5. 5 · Real life. Heating coils use nichrome (high ρ); house wiring uses copper (low ρ).
6. 6 · Close in one line. A long, thin wire of a high-resistivity material has the most resistance.

Scoring tip: Write both proportions, R ∝ l and R ∝ 1/A, before the final formula. Always give the unit of ρ as Ω m.