How does wireless charging work, and why is it less efficient?
By inductive coupling — a coil in the pad creates a changing magnetic field, which induces a current in a coil in your phone. It is a transformer split into two halves, with air and a phone case in the gap, and the losses come from exactly that gap.
The mechanism. Alternating current through the transmitter coil produces an alternating magnetic field. The receiver coil, placed within that field, has a current induced in it, which is rectified and used to charge the battery. The two coils communicate as well, negotiating power level and confirming a compatible device is present rather than a set of keys.
Why efficiency is lower. A cable delivers energy directly; induction loses it at several points:
Coupling losses, since only part of the field passes through the receiver coil. Alignment matters enormously — a few millimetres off centre costs real efficiency, which is why magnetic alignment systems exist and improve things measurably.
Distance, with thick cases and metal or card-carrying cases degrading or blocking it entirely.
Conversion losses at both ends, in the inverter and rectifier.
Heat, which is both a loss and a problem in itself: the phone throttles charging when hot, so a warm phone charges slower, and charging heat is one of the things that ages a lithium battery fastest. Typical end-to-end efficiency is meaningfully below a cable's, with figures varying widely by alignment and design.
The standard. Qi is the near-universal interoperable standard; its newer revision adopts magnetic alignment and higher power. Manufacturer-specific fast wireless charging frequently requires that brand's own pad.
Why metal objects are a hazard. A conductive object in the field has currents induced in it too, and heats up. Pads use foreign object detection to shut down when they sense this — which is also why a card with a metal-backed sticker, or a coin, interrupts charging.
Worth it? For convenience and for reducing port wear, yes. For speed, battery longevity and energy use, a cable remains better.