When a sinusoidal AC voltage U1 is applied to both ends of the primary coil, there is an alternating current I1 in the wire and an alternating magnetic flux ф1 is generated, which forms a closed magnetic circuit through the primary coil and the secondary coil along the iron core. A mutual inductance potential U2 is induced in the secondary coil, and ф1 also induces a self-inductance potential E1 on the primary coil. The direction of E1 is opposite to the applied voltage U1 and the amplitude is similar, thus limiting the size of I1. In order to maintain the existence of the magnetic flux ф1, a certain amount of power consumption is required, and the transformer itself also has a certain amount of loss. Although the secondary is not connected to the load at this time, there is still a certain current in the primary coil. This current is called "no-load current". ".