المقاومة النوعية

Electrical resistivity is an intrinsic material property describing how strongly a material opposes electrical current flow. It is the reciprocal of electrical conductivity, meaning materials with high conductivity have low resistivity and vice versa. Resistivity is defined as the ratio of electrical field to current density and is measured in ohm-meters in SI units.

Good conductors like copper and silver have high electrical conductivity and very low resistivity, typically around 10^-8 ohm-meters. Good insulators like glass and amber have low conductivity and extremely high resistivity, ranging from 10^9 to 10^14 ohm-meters. The greater the resistivity, the larger the electrical field needed to produce a given current density.

Temperature influences resistivity through a material-dependent relationship that is often approximately linear. In metals like copper, resistivity increases with temperature due to increased atomic vibrations. In semiconductors and carbon, resistivity decreases as temperature increases because the negative temperature coefficient enhances charge carrier availability.

Resistivity at any temperature can be modeled using a linear equation: ρ = ρ₀(1 + αΔT), where ρ is resistivity at temperature, ρ₀ is resistivity at room temperature, and α is the temperature coefficient. The sign of α determines whether resistivity increases or decreases with temperature, with positive coefficients indicating increased resistivity at higher temperatures.

Resistivity is defined as the ratio of the electrical field to current density. When voltage is applied to a conductor, an electrical field is generated, and the resulting current density depends on both the field strength and the material's resistivity. Lower resistivity produces larger current density from a given electrical field.

Silver has the lowest resistivity at 1.59 × 10^-8 ohm-meters, followed by copper at 1.68 × 10^-8 ohm-meters. Carbon shows 3.50 × 10^-5 ohm-meters, while semiconductors like silicon range from 1 to 2300 ohm-meters. Insulators like glass and amber have resistivity values exceeding 10^9 ohm-meters, making them extremely resistant to current flow.

In metals at a given temperature, current density is approximately proportional to the electrical field, a relationship governed by the theory of metallic conduction. This proportionality allows resistivity to be treated as a constant material property. Understanding how electrons interact with the lattice structure through the theory of metallic conduction explains why different metals have different resistivity values.