Video narration
Your phone charger changes voltage-but how can a transformer do that when its two coils have no wire between them? Inside, two insulated coils are wound around a laminated iron core. The input coil, called the primary coil, receives AC: current that changes direction. The separate output coil is the secondary coil. Here's the link: AC in the primary creates a changing magnetic field in the core, inducing voltage in the secondary. The field transfers energy between the coils; no wire joins them. Compare the turns, the loops of wire. If the secondary has more turns than the primary, its voltage rises; fewer turns lower it. Ideally, output-to-input voltage equals secondary-to-primary turns: V-s over V-p equals N-s over N-p. Real transformers have losses, and voltage trades off with current; they don't create power. That changing field is the connection: it induces voltage in the separate coil, though the coils have no wire between them.