I am simply trying to understand the difference between wireless power transmission( in which a source device could wirelessly power another device) and something like RFID or WIFI or Bluetooth where signals at various frequencies are being sent and demodulated as useful information?
And also, what frequency is wireless power transmission being used on?, depends or application, range, power?
Wireless power transmission and RFID use the magnetic “near-field”. This means they operate rather like a transformer, coupling two circuits via a changing magnetic field shared between them. For wireless power you need efficient coupling, which means really good proximity - basically its an air-cored transformer. Typical frequencies are 10’s of kHz to 100’s of kHz (induction hobs work just the same, but generate heat in the secondary, which is the metal pan base)
RFID only has to transmit information, efficiency is less important, but you still need enough coupling to detect the signal against background noise, which means the range is only a few times the size of the transmit coil usually. Typically the RFID tag modulates the magnetic field seen by the transmitter a tiny tiny fraction, and the transmitter filters out only the expected frequency of modulation. Frequencies are higher to make the RFIDs smaller. 150kHz, 10MHz, that sort of range, there are various RFID standards.
(Though some RFIDs are actually far-field, and work by back-scattering a signal I think).
WiFi and BT are both radio transmission, ie electromagnetic radiation, propagating in the “far-field”, electromagnetically. The antennas used are tuned to the appropriate wavelength and a lot of amplification is
needed to boost the tiny signals (microvolt levels are common) till they can be detected/demodulated.
Far field propagation involves both an electric field and a magnetic field component, filtering either one out will
swallow the signal. Radio antennas always generate both electric and magnetic fields. Radio waves can propagate long distances as the power falls of with distance squared (near fields fall off with distance cubed).
Near field can be done just using magnetic field, or just using electric field. Usually magnetic field is used and the electric field is screened out to reduce radio transmission. Electric field coupling is seldom used as for any useful power levels the voltages are dangerous. High voltage electricity pylon lines couple to fluorescent light bulbs rather well. See this link (but note their explanation is completely wrong!) Fluorescent tubes under power lines - Larkinweb.co.uk