Why the output of the transformer is zero if DC voltage is applied on its primary coil?

The output of a transformer is zero when DC voltage is applied to its primary coil because a transformer operates on the principle of electromagnetic induction, which requires a changing magnetic field to induce a voltage in the secondary coil.

In a DC circuit, the voltage applied to the primary coil is constant, and therefore, the magnetic field around the coil remains constant as well. Since there is no changing magnetic field, there is no voltage induced in the secondary coil, and the output of the transformer is zero.