The author has put forward the viewpoint and theory that electromagnetic energy flow is transmitted by mutual energy flow rather than self energy flow corresponding to the Poynting vector for 7 years, but it has been still in theory, and there is no specific example to verify this theory. This paper studies the examples of electromagnetic field and electromagnetic wave of plane-sheet current. Through this example, the author explains why the theory of mutual energy flow transmitting electromagnetic wave energy should be used to replace the theory of self energy flow transmitting energy. The author studies the magnetic quasi-static electromagnetic field and the radiated electromagnetic field. Corresponding to the magnetic quasi-static electromagnetic field, the power transmitted by self energy flow is reactive power. Energy flow can only be transferred by mutual energy flow. There is no dispute. Since there is no theory of mutual energy flow in the previous classical electromagnetic theory, so far the classical electromagnetic theory can not explain the energy flow from the primary coil to the secondary coil of the transformer. This paper achieves this goal for the first time. For radiated electromagnetic fields, both self energy flow and mutual energy flow are active power, but the author believes that self energy flow does not transfer energy, so Maxwell electromagnetic theory should add a time reversal wave to offset self energy flow. Or modify the electromagnetic theory to make the radiated electromagnetic field consistent with the magnetic quasi-static electromagnetic field, and the self energy flow transmits the electromagnetic wave of reactive power. In short, the classical electromagnetic theory is not a complete theory and must be modified. This example also tells us that the mutual energy flow is generated at the source of the electromagnetic wave and annihilated at the sink of the electromagnetic wave, which is very similar to the photon described by Cramer's transactional interpretation of quantum mechanics. Therefore, the mutual energy theory proposed by the author can be regarded as the concrete realization of the transactional interpretation. The interpretation of mutual energy flow proposed by the author is also a further development of transactional interpretation.