Two Questions on Transformers

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.

You will find a few multiple choice questions (with solution) on electromagnetic induction here