Charging a phone without a wire

Set a phone on a pad and it charges. Nothing is beamed at it. The pad and the phone each hold a flat coil of wire, and together the two coils make a transformer with an air gap where the iron should be.

What is happening on the pad

A coil in the pad carries an alternating current. That current makes a magnetic field that swings back and forth with it, thousands of times a second. A second coil, inside the phone, sits a few millimetres above. Wherever the swinging field passes through that second coil, it drives a current in it. The phone rectifies that current into its battery. No radio wave travels from pad to phone; the field simply reaches across the gap.

Why the distinction matters

Wireless power comes in two kinds, and they behave nothing alike. Far-field transfer radiates a wave — microwaves or a laser beam — from a transmitter to a distant receiver, and its efficiency is low and sensitive to weather and to how the transmitter is built. Near-field transfer does not radiate at all. The energy lives in the magnetic field close around the coil and is handed to a second coil that overlaps it. Over short distances it reaches 68 to 95 percent efficiency, the best of any wireless method, and it is what every charging pad uses. Its price is the word short.

How it works

A transformer is two coils sharing one changing magnetic field. Faraday's law says the voltage induced in a coil is its number of turns times how fast the field through it changes. In a grid transformer an iron ring guides essentially all of the field from one coil through the other. A charging pad has no ring. Its field spreads out in loops through the air, and only the loops that happen to thread the phone's coil do any work.

The fraction of the field that does thread both coils is called the coupling coefficient, written k. Coils sharing an iron core couple far better. Coils in air manage far less, and k is what the transfer lives or dies on. Measurements collected in the review show that moving the coils from 20 mm to 100 mm apart drops k from 0.6 to 0.1, and the efficiency from 80 percent to about 40. Sliding one coil sideways by roughly 200 mm halves k. Tilting it a few degrees barely matters. Typical inductive chargers therefore keep the gap under 40 mm and run in the kilohertz range, which the review counts as a safety advantage.

This is why a pad cares so much about placement. The Qi standard, introduced in 2008, includes a "guided positioning" design with a magnet in the centre of the pad coil that pulls a steel piece in the phone directly over it. The magnet moves no power; it exists to keep k high.

Tuning both coils to resonate at one frequency stretches the range to metres, at a cost in efficiency — still near-field, still nothing radiated.

In short

A charging pad is half a transformer, and the phone is the other half. The pad's coil makes a changing magnetic field; the loops of that field which pass through the phone's coil induce a current there. There is no iron to guide the field, so only a fraction couples, and that fraction collapses as the coils separate or slide out of line. Centimetres of gap, not metres, is the honest range — and nothing is being broadcast.