Now to look at the very fundamental concept of how wireless power transfer works. Remember the basic idea learned in school – if AC current is passed through a wire (i.e. “primary coil”), it generates a magnetic field. Apply that field to another piece of wire (secondary coil), and it induces a current in the secondary coil. This is true for any inductively coupled system – and in the case of wireless power transfer, the key to the system working is that the vertical distance between primary and secondary coils is relatively small compared to the coil diameter. The key to maintaining efficiency of the power transfer is in the geometry of the design. Here, TI shows two coils with diameters “D” and “D2”. The displacement between the coils is defined as “z”.