Verified Solution • 2025–26 Session moderate

QPages 328-329: Q11: HC Verma: Concepts of Physics - Volume 2 Alternating Current Pages 328-329: Q11 Solution: A transformer is designed to convert an AC voltage of 220 V to…

⚠️ Common Exam Traps & Conceptual Pitfalls

Students often treat a transformer as a pure inductor and conclude that because $X_L=\omega L=0$ for DC the circuit has no opposition, overlooking core saturation and the fact that only the small winding resistance limits the current, leading to a huge $I=V/R$ and rapid overheating. Remember that a transformer requires a time‑varying flux; applying DC removes the inductive reactance and forces the core into saturation, so a transformer must never be powered by DC.

A transformer is designed to convert an AC voltage of 220 V to an AC voltage of 12 V. If the input terminals are connected to a DC voltage of 220 V, the transformer usually burns. Explain.

Step-by-Step Verified Solution

**Answer:** Because the inductive reactance is zero for DC, causing a large current. **Step-by-Step Explanation:** A transformer is essentially an inductor. For DC, ω=0\omega = 0 so XL=ωL=0X_L = \omega L = 0. The only opposition to current is the small resistance of the coil. The current I=VRI = \frac{V}{R} becomes very large, heating the coil and burning the transformer [citation:1][citation:14][citation:16].

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