Questions on transformers at the 12^{th} class level are simple. But unusually simple questions may make you unusually careless in answering. See the following question which appeared in H.P.P.M.T. 2005 question paper:

An ideal transformer has *N*_{P} turns in the primary and *N*_{S} turns in the secondary. If the voltage per turn is *V*_{P} for primary and *V*_{S} for secondary *V*_{S}/*V*_{P} is equal to

(a) 1

(b) *N*_{S}/*N*_{P}

(c) *N*_{P}/*N*_{S}

(d) (*N*_{P}/*N*_{S})^{2}

In an ideal transformer the magnetic flux linked per turn of the primary and the secondary windings are the *same*. Therefore, the voltage per turn of the primary is the *same* as the voltage per turn of the secondary. The required voltage ratio is 1 [Option (a)].

Here is another simple question:

A small transformer with 80% efficiency has turns ratio 10:1. If a dry cell of emf 1.5 volt is connected across the primary, the voltage across the secondary will be

(a) 15 V

(b) 0.15 V

(c) 12 V

(d) 0.12 V

(e) zero** **

Don’t waste your time thinking of the efficiency and the turns ratio. A transformer requires a varying voltage across its primary to produce a voltage across its secondary. Since the dry cell supplies a steady voltage and therefore a steady current, there cannot be a flux change. The voltage induced across the secondary is zero.