CBSE Class 12 · 2007 · Alternating CurrentShort Answer

Principle: A transformer works on mutual induction — a changing current in the primary sets up a changing flux in the soft-iron core that links the secondary and induces an emf in it.
Working: The primary ($N_p$ turns) and secondary ($N_s$ turns) are wound on a common laminated core. An alternating primary voltage drives an alternating flux $\varphi$ shared by both coils, so

For a step-up transformer $N_s>N_p$, hence $V_s>V_p$. For an ideal transformer $V_p{I}_p=V_s{I}_s$, so the stepped-up voltage comes with a reduced current.
A d.c. voltage gives a steady current and therefore a constant flux; with $\frac{d\varphi }{dt}=0$ no emf is induced in the secondary, so a transformer cannot step up d.c.