Resistiviteettia

Resistivity, or resistiviteetti in Finnish, is a material property that quantifies how strongly a substance resists electric current. It is intrinsic to the material and independent of shape, and is denoted by ρ. It is defined as ρ = E/J, where E is the electric field and J is the current density, with the standard unit of ohm-meters (Ω·m). The reciprocal quantity, conductivity σ = 1/ρ, has units of siemens per meter (S/m).

Resistance depends on geometry via R = ρL/A, so resistivity describes bulk behavior. Volume resistivity refers to the interior; surface resistivity is used for coatings and is often given in ohms per square.

Temperature alters resistivity: metals typically increase resistivity with temperature, while many semiconductors decrease. A common linear approximation is ρ(T) ≈ ρ0[1 + α(T − T0)], with α as the temperature coefficient (material dependent).

Typical values span many orders of magnitude. Copper has about 1.68×10^-8 Ω·m at 20°C; insulators exceed 10^12 Ω·m. Semiconductors show wide ranges depending on doping. Resistivity is measured by methods such as the four-point probe or van der Pauw technique for arbitrary-shaped samples.

Resistivity guides material choice in electrical design, geophysics, and coatings, and it is central to understanding heating, contact resistance, and device reliability.