Every time I hear the description of a wireless charging system it goes like this: "remember a transformer? It's the same here – one coil is in the base and another is in the device and when you put the device onto the base the transformer gets assembled and starts transmitting power".
One problem with this description is there's no magnetic core in the system. AFAIK each transformer has a magnetic core in its design. A magnetic core is a huge set of steel plates that goes though both coils and this set of plates in fact conducts energy – the alternating current in the primary coil induces alternating electromagnetic field in the core and that field induces current in the secondary coil.
So looks like the core is a key component and a transformer can't possibly work without a core. Yet I don't see a magnetic core in a wireless charging system.
How does it work without a magnetic core?
Best Answer
A metal core is advisable because it concentrates the magnetic field. This makes magnetic coupling between coils more effective, but you can do it without the core too, especially if both coils are tightly coupled (mechanically close to each other). If you don't have a core and the two coils are some distance apart, part of the generated field will pass outside of the other coil.
(In a transformer the outside metal "shortcuts" the field, increasing it. Without this outside metal the field would fill a large volume around the core.)