Resistivity
An intrinsic material property measuring how strongly a material opposes current flow, measured in ohm-metres (Ω·m). Resistivity (ρ) is independent of the sample's shape or size and depends only on the material and temperature. The resistance of a conductor is R = ρL/A, where L is length and A is cross-sectional area.
Formula
R = ρL/A
Real World
Copper wiring in a house uses copper (ρ ≈ 1.7×10⁻⁸ Ω·m) rather than iron because its lower resistivity means thinner, cheaper cables can carry the same current without overheating.
Exam Focus
Always substitute consistent SI units (metres, m²) into R = ρL/A before rearranging — unit errors are the most common mark loss.
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