Verified Solution • 2025–26 Session moderate

QPages 230-234: Q26: HC Verma: Concepts of Physics - Volume 2 Magnetic Field Pages 230-234: Q26 Solution: Suppose that the radius of cross-section of the wire used in…

⚠️ Common Exam Traps & Conceptual Pitfalls

Students often apply Hooke's law as Δa = F/(Y A) and omit the necessary factor of the loop radius $a$, forgetting that strain is defined by $\Delta a/a = F/(Y A)$. Remember to multiply the stress $F/A$ by the original radius $a$ to obtain the absolute change, and use $A=\pi r^2$ for the wire’s cross‑section.

Suppose that the radius of cross-section of the wire used in the previous problem is

r

. Find the increase in the radius of the loop if the magnetic field is switched off. Young's modulus of the material of the wire is

Y

.

Step-by-Step Verified Solution

**Answer:** Increase in radius

\Delta a = \frac{i B a^2}{Y A}

. **Step-by-Step Explanation:** The compression force causes a strain

\Delta a/a = F/(YA)

. With

F = i B a

,

\Delta a = \frac{i B a^2}{Y A}

.

All Questions in this Chapter

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