Q.Explain the underlying principle of a transformer, describe briefly its construction, and derive the turns-ratio relation $V_s/V_p=N_s/N_p$ for an ideal transformer.

Principle. A transformer changes an AC voltage using mutual induction: a changing current in the primary coil sets up a changing magnetic flux, which (guided almost entirely through the shared iron core) links the secondary coil as well, inducing an emf in it purely through electromagnetic induction -- with no direct wire connection between the two coils.

Construction. A closed, laminated iron core carries two separate windings: the primary, of $N_p$ turns, connected to the input supply $V_p$; and the secondary, of $N_s$ turns, connected to the load, delivering output voltage $V_s$. Both windings link essentially the same flux $\Phi$ per turn, since they share the same closed core.

Deriving the turns ratio. By Faraday's law, applied to each winding separately, …

State the mutual-induction principle and construction, write Faraday's law for each winding separately, a …

• Writing the turns-ratio relation upside down (e.g. $V_p/V_s=N_s/N_p$).
• Omitting that the two windings share essentially the SAME flux $\Phi$ -- this …