Why is the iron core in the transformer not magnetic?
Is it related to the shape of the iron core?
It looks like an iron coil is energized. Although there is a magnetic field on the iron ring, because the iron ring is closed and there is no magnetic pole, the magnetism cannot be displayed.
The transformer core is a soft magnetic material, it is magnetic when it is energized, and there is no magnetism after power off (in fact, there is a small amount of residual magnetism, which can be ignored)
The toroidal coil and the magnetic circuit of the iron core of the transformer are closed, and the magnetic lines of force are confined in the iron core, which is magnetic to the outside (actually a small amount of magnetism).
The iron core in the transformer is superimposed with a very thin iron sheet to reduce eddy current losses.