Exploring Magnetic Material Responses
Magnetic materials interact differently when placed in an external magnetic field. This interaction depends on the material's internal structure, specifically how its atoms or molecules respond to the applied field. We can classify materials based on whether they are attracted to, repelled by, or only weakly interact with the field.
Understanding Induced Magnetic Moments
When a magnetic material is exposed to an external magnetic field (let's call it H), the atoms within the material generate their own magnetic moments. This is known as an induced magnetic moment. The key difference between material types lies in the direction of this induced moment relative to the applied field.
- In some materials, the induced magnetic moment aligns itself parallel to the applied field, strengthening the overall magnetic field within the material.
- In others, the induced magnetic moment aligns itself anti-parallel (opposing) to the applied field, weakening the overall magnetic field.
The strength and direction of this response are quantified by a property called volume magnetic susceptibility, denoted by the symbol $ \chi_v $. This value tells us how easily a material becomes magnetized and in which direction relative to the external field.
Characteristic Behavior: Opposing the Applied Field
The question specifically asks about a material where the induced magnetic moment consistently opposes the applied external magnetic field. This means the material is essentially repelled by the magnet creating the field.
Furthermore, the question states this opposition results in a small, negative volume magnetic susceptibility ($\chi_v$). A negative value for $ \chi_v $ is the defining mathematical characteristic of materials that exhibit magnetic repulsion.
Diamagnetic Materials: The Opposing Force
Diamagnetic materials exhibit the exact behavior described. When placed in an external magnetic field, the orbital motion of electrons within the atoms changes slightly due to induced currents. According to Lenz's Law, these induced currents generate a magnetic field that opposes the applied field.
- Response: The induced magnetic moment is always opposite to the applied field.
- Susceptibility: The volume magnetic susceptibility ($ \chi_v $) is small and negative. The values are typically around $ -1 \times 10^{-5} $ to $ -1 \times 10^{-6} $.
- Strength: This effect is weak and present in all materials, but it is often masked by other stronger magnetic effects (like paramagnetism) if they are also present.
- Examples: Common examples include water, copper, bismuth, gold, and mercury. These materials are weakly repelled by magnets.
Contrasting Other Magnetic Materials
To confirm why other options are incorrect, let's briefly consider them:
- Paramagnetic materials: These materials have atoms with permanent magnetic dipole moments that tend to align with the applied field. This results in a weak attraction and a small, positive magnetic susceptibility ($ \chi_v > 0 $).
- Ferromagnetic materials: These materials contain unpaired electron spins that align strongly in the same direction as the applied field, leading to a strong attraction and a *large, positive* magnetic susceptibility ($ \chi_v \gg 0 $). They can also retain magnetism after the external field is removed. Examples include iron, nickel, and cobalt.
- Antiferromagnetic materials: In these materials, neighboring magnetic moments align in opposite directions, cancelling each other out. While they interact with external fields, their susceptibility is generally small and positive, although more complex than paramagnetism. They do not exhibit the consistent opposition to the applied field characteristic of diamagnetism.
Summary Table of Magnetic Behaviors
| Material Type | Response to External Field | Volume Magnetic Susceptibility ($ \chi_v $) |
|---|---|---|
| Diamagnetic | Opposes field (Repelled) | Small, Negative ($< 0$) |
| Paramagnetic | Aligns with field (Weakly Attracted) | Small, Positive ($> 0$) |
| Ferromagnetic | Strongly aligns with field (Strongly Attracted) | Large, Positive ($ \gg 0 $) |
| Antiferromagnetic | Complex response (Weak effect) | Small, Positive ($> 0$) |
Conclusion on Material Identification
Based on the analysis:
- The requirement for the induced magnetic moment to oppose the applied field points towards diamagnetism.
- The condition of having a small, negative volume magnetic susceptibility ($\chi_v$) definitively identifies the material as diamagnetic.
Important Questions from Magnetism and Maxwell's Equations
The Curie temperature for cobalt is:
A metallic rod when placed in strong magnetic field, aligns itself at right angles to the magnetic field. The nature of material is:
Identify which of the following expressions is not Maxwell’s equation for time-varying fields.
Which of the following materials is used for the generation of ultrasonic waves by using magnetostriction effect?
Ampere circuital law can be expressed as:
