The author proposed mutual energy theorem in 1987. Similar formula has been proposed as reciprocity theorem by Welch in 1960. Considering the different positioning of the same formula, the author has studied this theorem in recent years and found that it is not only an energy theorem, but also an energy conservation law in electromagnetics. The author also puts forward the principle of mutual energy and the theorem of mutual energy flow. The mutual energy flow theorem further makes the that formula a localized law of conservation of energy. Considering that the mutual energy flow has the properties of photons, the author has used the mutual energy flow to interpret photon and quantum and solve the problem of wave particle duality. This paper verifies that this theorem is indeed the law of energy conservation through a transformer environment. Then it is further extended from transformer to antenna system. The author believes that when the secondary coil of the transformer is moved to a place far away from the primary coil, the primary coil becomes the transmitting antenna and the secondary coil becomes the receiving antenna. Such antenna systems and transformer systems meet the same law of energy conservation.

reciprocity theorem, conservation of energy, Poynting theorem, energy flow, transformer, primary coil, secondary coil, transmitting antenna, receiving antenna, retarded wave, retarded potential, advanced wave, advanced potential, absorber, emitter, photons, quantum, electromagnetic wave, electromagnetic field, electromagnetics