Resistance, resistivity and factors

Resistance

Resistance is the opposition a material offers to the flow of electric current. R = V/I, measured in ohm (Ω).

Resistivity

Resistivity (ρ), or specific resistance, is the resistance between opposite faces of a unit cube of the material. It is a property of the material only, measured in ohm-metre (Ω·m).

Factors affecting the resistance of a conductor

R = ρ l/A

1. Length (l): R is directly proportional to length.
2. Cross-sectional area (A): R is inversely proportional to area.
3. Nature of material: through resistivity ρ (silver and copper low, nichrome high).
4. Temperature: R_t = R₀ (1 + α t). For metals, R rises with temperature.

Example: a copper wire (ρ = 1.7 × 10⁻⁸ Ω·m), l = 100 m, A = 1 mm² gives R = 1.7 × 10⁻⁸ × 100/10⁻⁶ = 1.7 Ω.