Question

Retentivity means:

• A. Resistance to magnetization
• B. Ability to retain magnetism
• C. Loss of magnetism
• D. Increase in magnetization

Hint:

For permanent magnets: High Retentivity and High Coercivity are required.
For temporary magnets (electromagnets): Low Retentivity and Low Coercivity are preferred.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question asks for the definition of "retentivity" in the context of the magnetic properties of materials.

Step 2: Key Formula or Approach:
Retentivity is a key parameter observed during the magnetic hysteresis loop ($B\text{-}H$ curve) of ferromagnetic materials.
It corresponds to the residual magnetic flux density ($B_r$) when the external magnetizing field ($H$) is reduced to zero.

Step 3: Detailed Explanation:

• When a ferromagnetic material is subjected to an increasing external magnetic field ($H$), its magnetic domains align, increasing its magnetic induction ($B$) up to saturation.

• When the external magnetizing field is subsequently reduced back to zero, the magnetic domains do not return completely to their original random orientations.

• The material retains a certain amount of magnetization even when the external field $H = 0$.

• This residual magnetic flux density is known as remanent magnetization or remanence.

• The physical capacity of a material to retain this residual magnetism is called its retentivity.

• Materials with high retentivity are used to make permanent magnets, while those with low retentivity are preferred for electromagnets and transformer cores.

Step 4: Final Answer:
Retentivity refers to the ability of a material to retain its magnetism.