Conductors vs insulators

Whether a material carries current comes down to one thing: do its electrons roam free, or are they locked down? A conductor has a sea of free electrons and lets current flow easily — copper, gold, aluminium and silver. An insulator grips its electrons hard and blocks current — plastic, rubber, glass, dry wood and air.

This is why every cable is built from both at once: a conductor (copper) to carry the current where you want it, sheathed in an insulator (plastic) to stop it going where you don’t. The two materials do opposite jobs in the same wire.

Between the extremes sit semiconductors like silicon — insulators we can switch on to conduct on command. That controllability is the entire foundation of modern electronics: every chip, transistor and LED is built from semiconductors.