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Conductors and insulators
Why materials differ in how they carry current.
Why materials differ in how they carry current.
Current is the movement of charged particles, so a material carries current only if it has charges free to move. In metals, each atom lets go of an outer electron, and these electrons wander freely among the atoms. A little voltage moves them all, so metals are conductors. In an insulator such as glass, plastic or ceramic, the electrons are held tightly to their atoms and nothing moves. Semiconductors, such as silicon, sit between: very few free charges when pure, but their conduction can be controlled, which makes transistors possible (see Semiconductor basics).
The scale is logarithmic and enormous. Copper is a conductor and glass an insulator, but the gap between them is at least seventeen orders of magnitude. Conduction is not only for metals: salt water carries current through dissolved ions, which is why a wet antenna insulator, a salty coastal deposit or the human body can all conduct. Even "pure" water is a poor conductor until impurities dissolve in it.
Resistance depends on the material's resistivity (ρ), the wire's length, and its cross-section: R = ρ × L ÷ A. Longer wire resists more; thicker wire resists less, and doubling the diameter cuts resistance to a quarter.
Take 10 m of 14 AWG copper wire (area about 2.08 mm², resistivity about 1.68 × 10⁻⁸ Ω·m). Its resistance is 1.68 × 10⁻⁸ × 10 ÷ 2.08 × 10⁻⁶ = 0.081 Ω. A radio drawing 10 A over a 10 m run needs 20 m of wire for the round trip, so the drop is 10 × 0.161 = 1.6 V. That is 12% of a 13.8 V supply, lost in the wiring: this is why DC power leads are short and thick. See Wire, cable and gauge for sizes and ratings.
Insulators hold conductors apart: the plastic on a wire, the dielectric inside coax, the ceramic or glass insulator at the end of a dipole. Insulators are not perfect. Dirt and moisture make a surface conductive, and a high enough voltage forces a spark through even dry air, at roughly 3 kV per millimetre (less near sharp points). Equipment designers allow for this by spacing high-voltage parts well apart. Treat anything as live until you know otherwise (Electrical safety).