Draw a labelled diagram of a step-up transformer. Obtain the ratio of secondary to primary voltage in terms of number of turns and current in the two coils.

Explanation

When an alternating potential `V_p` is applied to the primary coil, an emf (`e_p`) is induced in it.
`thereforee_p=-N_p(dphi)/(dt)`
It resistance of primary coil is low, `V_p=e_p`
`thereforeV_p=-N_p(dphi)/(dt)`
As same flux is liked with the secondary coil with the help of soft iron core due to mutual induction, emf `(e_s)` is induced in it.
`thereforee_s=-N_s(dphi)/(dt)`
If the output circuit is open then, `V_s=e_s`.
`thereforeV_s=-N_s(dphi)/(dt)`
Thus, `V_s/V_p=N_s/N_p`
For an ideal transformer , `P_(out)=p_(i n)`
or,`I_sV_s=I_pV_p`
`thereforeV_s/V_p=I_p/I_x=N_s/N_p`
For step-up transformer `N_s/N_pgt1`.
In case of dc voltage, flux does not change. Thus no emf is induced in the circuit.