Question

Can a transformer be used to step down dc? Why?

Answer 1

$No$ it can't be used because $dc$ current don't change with time so it can't produce induced current in the secondary coil so power can't be transferred thus $no$ output at secondary coil.

$Principle-$

When primary coil get supply of current it generate magnetic field and thus secondary coil get a flux due to being near to primary coil, as the current supplied to primary coil is always $alternating$ current so its varying with time so the flux of secondary coil changes with time and thus $induced$ current get generated in the secondary coil due to changing flux according to Len's Law.

This whole process is called Mutual Magnetic Induction and thus electrical energy transferred to secondary coil in form of $induced$ current