Verified Solution • 2025–26 Session moderate

QPages 249-253: Q47: HC Verma: Concepts of Physics - Volume 2 Magnetic Field Due to a Current Pages 249-253: Q47 Solution: A thin but long, hollow, cylindrical tube of radius carries…

⚠️ Common Exam Traps & Conceptual Pitfalls

Students frequently mistake the given distance $r/2$ as the radial distance from the central axis, leading to an incorrect application of Ampere's law. Always verify whether the distance is measured from the axis or the surface; here, the radial distance is $R = r + r/2 = 3r/2$.

A thin but long, hollow, cylindrical tube of radius

r

carries

i

along its length. Find the magnitude of the magnetic field at a distance

r/2

from the surface a) inside the tube b) outside the tube.

Step-by-Step Verified Solution

**Answer:** (a) Inside: 0 (b) Outside:

\frac{\mu_0 i}{3\pi r}

. **Step-by-Step Explanation:** (a) Inside a hollow tube,

B = 0

. (b) Outside at

r/2

from surface: distance from axis =

3r/2

.

B = \frac{\mu_0 i}{2\pi (3r/2)} = \frac{\mu_0 i}{3\pi r}

.

All Questions in this Chapter

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