A hydroelectric power plant situated at a distance of 20 km from a town. Whose energy demand is 100 MW at 240 V. At hydro electric power plant, the water pressure head is at a height of 400 m and the water flow available is \( 100\,\mathrm{m^3/s} \). Electrical energy generated in power plant is transmitted to the town with the use of transformers voltage output of the AC generator at power plant is stepped up and is there transmitted over long distances to a sub-station near the consumers in town. Sub-stations and utility poles steps down the voltage finally the 240 V which finally reaches the houses.
In ideal transformer back emf \( (e_p) \) in primary coil is equal to the applied voltage \( (V_p) \) across primary coil. If this were not so then.
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Primary current would be infinite
In an ideal transformer the primary winding has negligible resistance. The current in it is limited only by the back emf induced by the changing flux, which opposes the applied voltage.
The net voltage that drives current through the primary resistance \( R_p \) is the difference between the two:
\( \displaystyle i_p = \frac{V_p - e_p}{R_p} \)
With \( R_p \rightarrow 0 \), the current stays finite only if \( e_p = V_p \). If \( e_p \) were not equal to \( V_p \), a non-zero voltage would act across a coil of zero resistance, and the primary current would become infinite.
NCERT (2026-27):
Transformers – “… We consider an ideal transformer in which the primary has negligible resistance and all the flux in the core links both primary and secondary windings. … The alternating flux φ also induces an emf, called back emf in the primary. … But εp = vp. If this were not so, the primary current would be infinite since the primary has zero resistance (as assumed).”