Resistance
The ratio R = V/I for a conductor obeying Ohm's law. Measured in ohms (Ω). Determined by the conductor's geometry (R = ρℓ/A) and material resistivity ρ.
Definition
Resistance R is the proportionality constant between voltage and current in a conductor, R = V/I, measured in ohms (1 Ω = 1 V/A). For a uniform wire of length ℓ and cross-section A made of a material with resistivity ρ, the resistance is R = ρℓ/A — longer wires resist more, thicker wires resist less, and the material sets the prefactor.
Resistance also controls how much electrical energy a conductor dissipates as heat. The power dissipated is P = VI = I²R = V²/R, measured in watts. This is called *Joule heating*, and it is the mechanism behind every resistive heater, light bulb filament, kitchen toaster, and space heater. In most circuits it is a loss to be minimised (through low-R conductors); in heating elements it is the design goal (through high-ρ materials like nichrome, chosen to reach red-hot temperatures without oxidising).
Typical values span thirteen orders of magnitude: a 30 cm length of heavy copper wire, about 10⁻³ Ω; a room-temperature 1 kΩ resistor, 10³ Ω; a dry human body between hand and foot, about 10⁵ Ω; a glass insulator, 10¹⁰ Ω or higher. Resistance depends weakly on temperature for most metals (about +0.4% per °C for copper), strongly on temperature for semiconductors (often decreasing by decades as the material warms), and drops to exactly zero in superconductors below their critical temperature.