Question

Which of the following substance has relative magnetic permeability $\mu_r \gg 1$ ?

• A. Aluminium
• B. Copper
• C. Lead
• D. Nickel

Hint:

Memorize the "Iron Triad" (Iron, Cobalt, Nickel) as the three quintessential ferromagnetic elements encountered relentlessly in physics exam problems.

Answer 1

The Correct Option is D

Concept:
• Materials in nature are broadly classified into three distinct categories strictly based on their inherent magnetic properties and their interaction with external magnetic fields.

• Diamagnetic materials (e.g., Copper, Lead, Water) create a weak opposing internal magnetic field. For them, the relative magnetic permeability is strictly slightly less than one ($\mu_r < 1$).

• Paramagnetic materials (e.g., Aluminium, Platinum, Oxygen) create a very weak reinforcing internal magnetic field. For them, the relative magnetic permeability is strictly slightly greater than one ($\mu_r > 1$, but close to 1).

• Ferromagnetic materials (e.g., Iron, Cobalt, Nickel) possess large crystalline domains that aggressively align with external fields, generating massive internal reinforcement. For them, the relative magnetic permeability is phenomenally larger than one ($\mu_r \gg 1$, often in the thousands).

Step 1: Analyze the given mathematical condition
The problem presents the specific mathematical inequality condition: $\mu_r \gg 1$.
The "much greater than" symbol definitively isolates the required material class.
This specific condition is the textbook hallmark exclusively characterizing ferromagnetic materials.

Step 2: Evaluate the provided substance options
We systematically examine the known magnetic classification of each option provided:
(A) Aluminium: It is a widely known paramagnetic material. Its permeability is only slightly greater than 1, failing the "much greater" test.
(B) Copper: It is a textbook diamagnetic material. Its permeability is physically less than 1.
(C) Lead: It is also strongly diamagnetic, meaning its permeability is distinctly less than 1.
(D) Nickel: It is one of the very few famous elements that are strongly ferromagnetic at room temperature (alongside Iron and Cobalt).

Step 3: Conclusion
Since Nickel is the only genuine ferromagnetic material securely listed among the choices, it uniquely satisfies the extreme condition $\mu_r \gg 1$. This perfectly aligns with option (D).