Why can't transformers operate with direct current?

I. Clarify the core working logic of The Transformer: Electromagnetic induction requires "a changing magnetic field".
The core components of the transformer are the primary coil, the secondary coil, and the iron core. Its voltage transformation function is based on two major electromagnetic induction laws:
Mutual induction phenomenon: After current is passed through the primary coil, a magnetic field will be generated in the iron core; when the magnetic field changes, an induced electromotive force (voltage) will be generated in the secondary coil, thereby achieving voltage increase (such as a step-up transformer for long-distance power transmission, and a step-Down Transformer for household electricity).
Key prerequisite: The magnetic field must "change" - only when the magnitude or direction of the current changes, will the magnetic flux in the iron core change, and the secondary coil can induce voltage.
II. Characteristics of direct current (DC): Leads to "static magnetic field, unable to induce"
The core characteristic of DC (Direct Current) is that the current direction remains unchanged and the magnitude is stable (such as batteries, DC power supplies output). When it is passed through the primary coil of the transformer, the following problems occur:
Iron core magnetic field is constant: Stable DC current will form a "static constant magnetic field" in the iron core, and the magnetic flux does not change.
No induced voltage in the secondary coil: According to the electromagnetic induction law, the magnitude of the induced electromotive force is proportional to the rate of change of the magnetic flux. Since the magnetic flux is constant, no induced voltage can be generated in the secondary coil, and the transformer is equivalent to a "short circuit", unable to achieve the voltage transformation function.
The primary coil is severely overheated and burned: The resistance of DC current is extremely small (the coil is copper wire, resistance R is very small), according to Ohm's law I = U/R, when passing through the rated voltage of DC current, the primary coil will generate an extremely large "direct current large current", causing the coil to overheat (P = I²R), and it will burn the insulation layer of the coil within a short time, even causing a fire.
III. Why alternating current (AC) can be adapted to transformers?
AC (Alternating Current) has current directions and magnitudes that change periodically (such as the 50Hz power grid in China, changing 50 times per second). When passed through the primary coil:
The changing current generates "alternating magnetic field", and the magnetic flux in the iron core changes rapidly over time;
The secondary coil cuts the alternating magnetic field's magnetic lines, continuously generating induced voltage, and the magnitude of the induced voltage is proportional to the匝数比 of the primary and secondary coils (U₁/U₂ = N₁/N₂)， thereby achieving precise voltage transformation. Summary
The essence of transformer design is "to transfer electrical energy through changing magnetic fields". However, the "constant nature" of direct current prevents the magnetic field from changing, which means it cannot achieve voltage transformation. Moreover, it can also cause equipment to burn out due to excessive current. This is why transformers are widely used in AC power grids, while direct current circuits (such as power supply for electronic devices) need to be converted through rectification and inversion equipment before they can be compatible.