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

Concept: Magnetic susceptibility, denoted by the Greek letter \(\chi\) (chi), is a dimensionless proportionality constant that tells us exactly how easily a given material becomes magnetized when exposed to an external magnetic field. It forms the relationship: \[ \vec{M} = \chi \vec{H} \] where \(\vec{M}\) is the induced magnetization density, and \(\vec{H}\) is the applied magnetic field intensity. Based on the value of magnetic susceptibility, materials are classified into three primary groups:
• Diamagnetic Materials: Characterized by a small, negative susceptibility (\(-1 \le \chi < 0\)).
• Paramagnetic Materials: Characterized by a small, positive susceptibility (\(0 < \chi < 1\)).
• Ferromagnetic Materials: Characterized by an incredibly large, positive susceptibility (\(\chi \gg 1\)).

Step 1: Match the given property to the correct material group. The problem states that the specimen has a small, negative magnetic susceptibility (\(\chi < 0\)). This instantly identifies the specimen as a diamagnetic material.

Step 2: Analyze the structural magnetic response of this group. When a diamagnetic material is introduced to an external field lines, its atomic electrons shift their orbital paths. This induces an internal magnetic dipole moment that directly opposes the applied external field direction (Lenz's law behavior at the atomic scale). Because this induced magnetization runs counter to the external magnetic field lines, the material experiences a net mechanical force pushing it away.

Step 3: Determine the behavior inside a non-uniform spatial gradient field. In a uniform magnetic field, a diamagnetic object experiences an alignment torque but zero net translational force. However, inside a non-uniform (gradient) magnetic field, the field strength varies across different regions of space. Because the induced dipoles oppose the external field lines, the material is mechanically repelled by areas of high magnetic flux density. This creates a net translational migration force, shifting the material out of high-intensity zones into weaker field regions. Therefore, it will be weakly repelled away from the stronger field regions.