Question

Which of the following is true for hard magnetic materials:

• A. both coercivity and retentivity are high
• B. coercivity is high and retentivity is low
• C. both coercivity and retentivity are low
• D. coercivity is low and retentivity is high

Hint:

Think of the terms literally: a "hard" magnet is hard to magnetize but also hard to demagnetize. This directly translates to high coercivity. To be a strong permanent magnet, it must also hold a lot of magnetism, which means high retentivity.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Magnetic materials are classified as "hard" or "soft" based on their hysteresis loop properties. Hard magnetic materials are suitable for making permanent magnets.
Step 2: Detailed Explanation:
For a material to be a good permanent magnet, it must satisfy two key criteria:

High Retentivity (or Remanence): After being magnetized by a strong external field, the material should retain a large amount of its magnetization when the field is removed. This property is called retentivity.
High Coercivity: The material must be difficult to demagnetize. It should resist demagnetization from stray magnetic fields, thermal effects, or mechanical shock. The measure of this resistance is coercivity, which is the strength of the reverse magnetic field required to bring the magnetization back to zero.
Materials with both high retentivity and high coercivity are called hard magnetic materials. Their hysteresis loop is wide and tall. In contrast, soft magnetic materials (used in transformers and electromagnets) have low coercivity, making them easy to magnetize and demagnetize.
Step 3: Final Answer:
Hard magnetic materials are characterized by both high coercivity and high retentivity.