Question

The material selected for making a permanent magnet should have:

• A. Low coercivity, low permeability and low retentivity
• B. High coercivity, low permeability and high retentivity
• C. Low coercivity, low permeability and high retentivity
• D. High coercivity, high permeability and high retentivity

Hint:

Think of it this way: Permanent means it needs to be stubborn. High retentivity ensures it keeps plenty of field, while high coercivity guards it against being demagnetized by outside influences!

Answer 1

The Correct Option is D

Concept: A permanent magnet requires material characteristics that allow it to be easily magnetized initially and ensure it retains its magnetic field strength despite external disturbances.
• Retentivity measures the residual magnetization remaining in the substance when the external magnetizing field is brought down to zero.
• Coercivity measures the intensity of the reverse magnetic field required to reduce this residual core magnetization back to zero.
• Permeability represents how easily magnetic field lines can pass into and align the molecular dipoles of the substance.

Step 1: Analyze the practical requirements for permanent alignment.
To establish a powerful magnetic core field right away, the material needs a high value of magnetic permeability. Next, to keep a large fraction of that strength after removing the external magnetizing device, it needs high retentivity. Finally, to ensure the magnet doesn't lose its alignment when exposed to stray reverse fields, temperature shifts, or physical impacts, it must possess high coercivity. Consequently, materials like Alnico or steel, which display wide hysteresis loops with high values for all three parameters, are selected.