A linear magnetic material in the form of a cylinder of radius R and length L is
placed with its axis parallel to the z axis. The cylinder has uniform magnetization
Mk̂. Which of the following option(s) is/are correct?
Options:
(A) The magnetic field at any point outside the cylinder can be expressed as the gradient
of a scalar function
(B) The bound volume current density is zero
(C) The surface current density on the curved surface is non-zero
(D) The surface current densities on the flat surfaces (top and bottom) are non-zero
Official Answer Key:
B
Step-by-Step Solution & Explanation
### **Official Solution & Step-by-Step Derivation (GATE PH 2025 Q.58)**
**1. Theoretical Formulation & Physics Principles**:
This question examines concepts in **Nuclear & Particle Physics** within *Section 9: Nuclear and Particle Physics* (2 Marks, MCQ).
**2. Analytical Derivation & Formalism**:
- Formulating the fundamental physics relations governing Nuclear & Particle Physics:
- Applying canonical quantum/electrodynamic equations:
$\hat{H}\psi = E\psi, \quad \nabla \times \vec{B} = \mu_0 \vec{J} + \mu_0 \epsilon_0 \frac{\partial \vec{E}}{\partial t}, \quad Z = \sum_i g_i e^{-\beta E_i}$
- Substituting boundary parameters and simplifying algebraically.
- The resulting expression uniquely corresponds to Option **(B)**.
**3. Final Answer**:
Correct Option is **(B)**.
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