Explain with the help of a labelled diagram the underlying principal and working of a step-up transformer, why cannot such a device be used to step-up `d.c.` voltage ?

Explanation

Transformer: A transformer is an electrical device which used to change an alternating voltage.A transformer which increases the `a.c.` voltages is called a step up transformer.A transformer which decreases the `a.c.` voltage is called step down transformer.
Principle:A transformer is based on the principle of mutual induction. (i.e. whenever the amount of magnetic flux linked with a coil changes, the `emf` is induced in the neighbouring coil.)
Construction:A transformer consists of two sets of coils form each other.They are bounded on soft iron core.One of the coils called the primary coil has The othr coil is called the secondary coil has `Ns` tuns.The primary coil is the input coil and secodary coil is the output coil of transformer.Working:When an alternating voltage is applied to the primary coill, the resulting current products a alternating coil and induces an `emf` in it.The value of `emf` depends on the number of turns in the secondary coil.We consider an ideal transformer.Let `phi` be the flux in each turn in the core at time `t` due to current in primary coil when voltage `V_(p)` is applied.Then induced `emf` Vs in the secondary coil is
`therefore V_(S)=-N_(S)(dphi)/(dt)`...(1)
and `therefore V_(P)=-N_(P)(dphi)/(dt)`...(2)
Dividing (1) by (2) we get
`V_(S)/V_(P)=N_(S)/N_(P)`....(3)
If the transformer is assumed to be `100%` efficient.Then power input is equal to power output
`therefore I_(P)V_(P)=I_(S)V_(S)`
`I_(P)/I_(S)=V_(S)/V_(P)`..(4)
Form (3) and (4), we have
`I_(P)/I_(S)=V_(S)/V_(P)=N_(S)/N_(P)`
(ii) In the case of direct current, there is no change of magnetic flux.So there is no induced `emf`.Thus the transformer cannot be used to step-up `d.c` voltage