There are two types of wireless power transferring. The first is inductive power transfer. The Power is transferred, wireless through an inductive coupling at a short range. Typically a few millimeters max. The transmitter and receiver coils are inductively coupled which means that the AC current in the transmitter coil generates a magnetic field which induces a voltage in the receiver coil. The receiver voltage may be used to power a device. The magnetic field is highly concentrated in between the transmitter/receiver, this causes users to be only exposed to a small leakage field which is further minimized by an EMI shield on the back of the coil. The coupling effect is influenced by the following factors: matching coil size, a flat surface between transmitter and receiver as well as the shielding and the alignment of the coils. The second is Resonant Power Transfer. When two resonant circuits resonating at the same frequency are brought into close proximity (near the field area), evanescent wave coupling occurs and the energy is transferred from transmitter to receiver. Resonant power transfer offers more freedom in positioning the transmitter and receiver and allows multiple receivers charging on one transmitter. Efficiency, power level and EMC/EMI are the challenges.