Transformers work only with AC (alternating current) and not with DC (direct current) because of how they operate:

 Transformers work only with AC (alternating current) and not with DC (direct current) because of how they operate:

How Transformers Work 
A transformer uses electromagnetic induction.

An AC current in the primary coil creates a changing magnetic field.

This changing magnetic field induces a voltage in the secondary coil.

Why DC Doesn't Work:

DC is constant (it doesn't change direction or magnitude).

A constant magnetic field from DC doesn't induce a changing magnetic field.

Without a changing field, no voltage is induced in the secondary coil.

So, no energy transfer happens.

What Happens if You Use DC:

If you apply pure DC to a transformer:

It will only induce a voltage for a brief moment (when switching it on or off).

Then, nothing happens.

Worse, the transformer can overheat or burn out because DC causes a constant current, which creates heat in the coil but no useful induction.

So in short:
Transformers need changing magnetic fields → AC provides that → DC doesn't