Verified Solution • 2025–26 Session moderate

QPages 306-313: Q80: HC Verma: Concepts of Physics - Volume 2 Electromagnetic Induction Pages 306-313: Q80 Solution: An inductor-coil of inductance 17 mH is constructed from a…

⚠️ Common Exam Traps & Conceptual Pitfalls

Students often read 17 mH as 0.17 H, forgetting the milli‑ (10⁻³) factor; this makes τ = L/R ten times larger. Always convert the inductance to henries (L = 17 mH = 0.017 H) before using τ = L/R.

An inductor-coil of inductance 17 mH is constructed from a copper wire of length 100 m and cross-sectional area 1 mm21 \text{ mm}^2. Calculate the time constant of the circuit if this inductor is joined across an ideal battery. The resistivity of copper = 1.7×10−8 Ω-m1.7 \times 10^{-8} \ \Omega\text{-m}.

Step-by-Step Verified Solution

**Answer:** _ = 0.17 s. **Step-Step Explanation:** Resistance R = _l/A = 1.7 _ 10__ _ 100/10__ = 1.7 �. L = 17 mH. _ = L/R = 0.017/1.7 = 0.01 s.

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