Difficulty: Easy
Correct Answer: 35% cobalt
Explanation:
Introduction:
Permanent magnet materials must exhibit high coercivity and good retentivity. Historically, cobalt steels and later AlNiCo and rare-earth magnets have been used for strong permanent magnets. This question targets a classic composition range for cobalt steel magnets used in engineering contexts.
Given Data / Assumptions:
Concept / Approach:
Cobalt addition to high carbon steel significantly improves magnetic hardness (coercive force). Traditional references describe cobalt steels for permanent magnets containing roughly one-third cobalt by weight, i.e., about 35%. This composition yields improved ability to retain magnetization compared with plain carbon steel.
Step-by-Step Solution:
Verification / Alternative check:
Classic materials texts list cobalt steels with ~35% cobalt for permanent magnets, preceding the extensive use of AlNiCo and rare-earth magnets. This aligns with the option given.
Why Other Options Are Wrong:
15% / 20%: Lower cobalt content typically does not deliver the same coercivity as the established 35% composition.
45%: Higher than common cobalt steel compositions cited for traditional permanent magnets, impacting cost and properties.
Common Pitfalls:
Confusing cobalt steel compositions with AlNiCo alloys; assuming any cobalt addition is sufficient without considering the established optimum ranges.
Final Answer:
35% cobalt
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