Factors Affecting Resistance

Why do different components resist current by different amounts? Electrons moving through a conductor are restrained by the atoms around them — that restraint is resistance. A component that offers very low resistance is a good conductor; higher resistance makes it a resistor, poor conductor, or (at the extreme) an insulator.

Experiments show resistance depends on three factors. It's directly proportional to length l — double the length, double the resistance (R ∝ l). It's inversely proportional to the area of cross-section A — a thicker wire has *less* resistance (R ∝ 1/A). And it depends on the material the conductor is made of.

Combining these: R = ρl/A, where ρ (rho) is the resistivity of the material — a fixed property of that material, independent of its length or thickness, measured in ohm-metre (Ω m). Metals and alloys have very low resistivity (10⁻⁸ to 10⁻⁶ Ω m) and are good conductors; insulators like rubber and glass have resistivity many orders of magnitude higher (10¹² to 10¹⁷ Ω m).

Alloys generally have higher resistivity than their constituent metals, and don't oxidise easily at high temperatures — which is exactly why heating devices like electric irons and toasters use alloys such as nichrome, not pure metals. Tungsten (very high melting point) is used for bulb filaments, while copper and aluminium (low resistivity) are used for power transmission lines.