Transformers and AC power distribution

A transformer has no moving parts and no electrical connection between its two coils — everything happens through the magnetic flux threading both of them. Drive an alternating current through the primary coil and its magnetic field constantly changes, which by Faraday's law induces an EMF in the secondary coil wound on the same core. Because both coils share the same changing flux, the induced voltages scale directly with how many turns each coil has: V_s = V_p(N_s/N_p). More turns on the secondary than the primary steps the voltage up; fewer steps it down.

What a transformer cannot do is create energy. To an excellent approximation, the power flowing into the primary equals the power flowing out of the secondary, so whatever factor the voltage is multiplied by, the current is divided by. This is precisely why long-distance power lines run at hundreds of kilovolts rather than the few hundred volts used in a home: resistive loss in a wire scales with the square of the current, so stepping voltage up (and current down) by a step-up transformer before transmission, then stepping back down near the destination, cuts transmission losses dramatically for the same delivered power.

That AC current, incidentally, doesn't spread evenly through a thick conductor's cross-section — it crowds toward the surface, an effect called the skin effect, and the higher the frequency the more pronounced the crowding. It's a secondary reason (alongside easy voltage transformation) that power engineers think carefully about frequency, conductor thickness and stranding when designing transmission lines, on top of the basic step-up/step-down arithmetic above. None of this works with steady DC: a transformer needs a changing flux, and a DC current, once established, produces a magnetic field that isn't changing at all.