Question: A piece of a diamagnetic material, free to move when placed in a uniform magnetic field:

Step 1 (Governing Principle): [0.5 Mark]

Diamagnetic materials are characterized by a small negative magnetic susceptibility ($\chi < 0$). When placed in an external magnetic field $\vec{B}$, they develop an induced magnetic moment in a direction opposite to the applied field, as per Lenz's Law. In a uniform magnetic field, the net force $\vec{F}$ acting on a magnetic dipole $\vec{m}$ is given by:

Since the magnetic field $\vec{B}$ is uniform, the gradient of the field is zero ($\nabla \vec{B} = 0$).

Step 2 (Step-by-Step Logical Deduction): [0.25 Mark]

• For a uniform magnetic field, the spatial derivative of the field is zero.
• The force experienced by a magnetic material in a magnetic field is proportional to the gradient of the field ($\vec{F} \propto \nabla B$).
• Since $\nabla B = 0$ in a uniform field, the net force $\vec{F} = 0$.

Step 3 (Final Concluded Answer): [0.25 Mark]

Because the net force acting on the diamagnetic material is zero, it experiences no translational motion. Therefore, the correct option is (D) does not move at all.

💡 EL10 Concept Insight: Imagine the magnetic field is like a perfectly flat, level floor; because there is no "slope" or change in the field strength, the diamagnetic piece has no reason to slide in any direction and simply stays exactly where it is.