resistivity

Resistivity is a material property that quantifies how strongly a substance opposes the passage of electric current. Unlike resistance, which depends on an object's shape and size, resistivity is intrinsic to the material itself. It is defined as the resistance of a cube of that material with sides of 1 metre and current flowing perpendicular through opposite faces, measured in ohm-metres (Ωm). Copper at 20°C has a resistivity of approximately 1.68 × 10−8 Ωm; aluminium about 2.65 × 10−8 Ωm. These low values explain why both are standard conductors in electrical wiring and components.

The relationship between resistivity and practical resistance is direct: R equals resistivity multiplied by length, divided by cross-sectional area (R = ρL/A). This formula shows why a thin, long wire has higher resistance than a thick, short one made from the same material. Engineers use this relationship constantly when selecting wire gauges for circuits, designing busbars, or calculating current-carrying capacity. A designer choosing between copper and aluminium for a cable run must account for both their different resistivities and their different densities, which affect cost and weight.

Temperature dependence and material selection

Resistivity changes with temperature. For most metals, it increases linearly with increasing temperature; the temperature coefficient of resistance for copper is about 0.0039 per degree Celsius. This matters in high-power applications where joule heating can alter conductor properties. Nichrome (an alloy of nickel and chromium) has high resistivity (about 1.0 × 10−6 Ωm) and a lower temperature coefficient, making it suitable for heating elements. Semiconductors like silicon have resistivity values millions of times higher than metals, but their resistivity can be controlled through doping to create functional electronic devices.

Resistivity values also reflect electron mobility within the material lattice. Impurities, defects, and crystal structure all influence how freely electrons move. Annealed (heated and cooled) copper conducts better than cold-worked copper because the crystal structure is more ordered. This is why electrical-grade copper must meet purity standards: even small amounts of impurities significantly increase resistivity and reduce efficiency in long-distance power transmission.

The term appears across the trade in specifications, data sheets, and design calculations. You will encounter it when selecting materials for resistors, evaluating conductor losses, designing heating systems, or assessing the quality of recycled metals. Standards such as those from IEC and ASTM define resistivity values at reference temperatures (typically 20°C) to ensure reproducibility and comparability across suppliers and applications.