Question

A material specimen exhibits a small, negative magnetic susceptibility ($\chi < 0$) at room temperature. When placed inside a non-uniform magnetic field, this specimen experiences a net force pushing it. How will this material behave?

• A. \( \text{It will be strongly attracted toward the stronger fields.} \)
• B. \( \text{It will be weakly attracted toward the stronger fields.} \)
• C. \( \text{It will be weakly repelled away from the stronger field regions.} \)
• D. \( \text{Its properties change completely above its Curie temperature.} \)

Hint:

Diamagnetism is a fundamental property found in all materials, but it is often covered up by much stronger paramagnetic or ferromagnetic effects. Unlike those two classes, a material's diamagnetic response remains completely independent of temperature changes.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Magnetic susceptibility (\(\chi\)) is a dimensionless proportionality constant that describes how much a material becomes magnetized when placed in an external magnetic field.
Specifically, it relates the magnetization \(\vec{M}\) of the material to the applied magnetic field strength \(\vec{H}\) via the relation \(\vec{M} = \chi \vec{H}\).
Based on the sign and magnitude of \(\chi\), materials are classified into three primary groups:
1. Diamagnetic: \(\chi\) is small and negative (\(-1 \le \chi<0\)). The induced magnetic moment opposes the external field.
2. Paramagnetic: \(\chi\) is small and positive (\(0<\chi<1\)). The moments align weakly with the field.
3. Ferromagnetic: \(\chi\) is very large and positive (\(\chi \gg 1\)). Moments align strongly and permanently.
A negative susceptibility implies that the material creates an internal field that pushes back against the external field.
Step 2: Detailed Explanation:
The problem states the material has a small, negative susceptibility (\(\chi<0\)). This instantly identifies the specimen as a diamagnetic material.
Diamagnetism is an inherent property of all materials caused by the change in the orbital motion of electrons when an external field is applied.
According to Lenz's law at the atomic level, these changing orbits induce a magnetic moment that directly opposes the source of the magnetic field.
When this diamagnetic specimen is placed in a non-uniform magnetic field, the field strength varies across space.
The repulsive interaction between the external field and the induced opposite dipole leads to a mechanical force.
Because the induced dipoles are "fighting" the field, the material naturally seeks out the region where the "pressure" from the magnetic field lines is lowest.
Consequently, the specimen moves from regions of higher magnetic field intensity (stronger fields) toward regions of lower intensity (weaker fields).
This migration is observed as a weak repulsion away from the stronger parts of the magnetic field.
Option (D) refers to ferromagnetism, as diamagnetic properties are generally independent of temperature and do not have a Curie point.
Step 3: Final Answer:
The specimen, being diamagnetic, will be weakly repelled away from the stronger field regions of a non-uniform magnetic field.