Transformer work on A.C

Transformer work on principle that a changing magnetic field is required to set up an induced emf or current.As we know that d.c has a constant magnitude so it does not set up a varying magnetic field in the coil,therefore there is no induced emf or current  so a transformer does not work on D.C.Only at the starting time when we supply direct current there is a change in the magnitude of current so at a small interval of time we get varying magnetic field but this field cannot be mantain for a whole time so we cannot use D.C current.

   But if we use A.C then there is a changing magnetic field because A.C does not  have a constant magnitude as its current varies with time.So a induced emf or current is produced in the second coil that is why alernating current is used for working of transformer.