Work-energy principle (WEP) governing wireless power transfer (WPT) process is derived. Driving power as the source to sustain a steady WPT is obtained. Transferring coefficient (TC) used to quantify power transfer efficiency is introduced.

WEP gives a clear physical picture to WPT process. The physical picture reveals the essential difference between transferring problem and scattering problem. The essential difference exposes the fact that the conventional characteristic mode theory (CMT) for scattering systems cannot be directly applied to transferring systems.

Under WEP framework, this paper establishes a CMT for transferring systems. By orthogonalizing driving power operator (DPO), the CMT can construct a set of energy-decoupled characteristic modes (CMs) for any pre-selected objective transferring system. It is proved that the obtained CM set contains the optimally transferring mode, which can maximize TC.

Employing the WEP-based CMT for transferring systems, this paper does the modal analysis for some typical two-coil transferring systems, and introduces the concepts of co-resonance and ci-resonance, and reveals some important differences and connections “between transferring problem and scattering problem”, “between co-resonance phenomenon of transferring systems and external resonance phenomenon of scattering systems”, and “between so-called magnetic resonance and classical electric-magnetic resonance”.