Video: Knowing Key Terms Related to Magnetism

Which of the following correctly describes what is meant by a “hard magnetic material”? [A] A hard magnetic material is a material which is magnetic but not malleable. [B] A hard magnetic material is a material which loses its induced magnetism quickly. [C] A hard magnetic material is a material which is magnetic and has a high melting point. [D] A hard magnetic material is a material which does not lose its induced magnetism easily. [E] A hard magnetic material is a material which has a high density.

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Video Transcript

Which of the following correctly describes what is meant by a “hard magnetic material”? a) A hard magnetic material is a material which is magnetic but not malleable. b) A hard magnetic material is a material which loses its induced magnetism quickly. c) A hard magnetic material is a material which is magnetic and has a high melting point. d) A hard magnetic material is a material which does not lose its induced magnetism easily. e) A hard magnetic material is a material which has a high density.

So, in this exercise, we want to figure out which of these five descriptions correctly describes what is meant by a hard magnetic material. Say that we had a chunk of just this sort of material. And we put it in an external magnetic field. We can recall that the reason a magnetic material is called hard is because it takes a while, perhaps a long amount of time, after it’s been put in an external field to generate its own internal magnetic field. And, at the same time, if we turn off that external field, then the field that’s been induced in the material itself will take a long time to dissipate. It will persist for quite some time.

This is one reason why permanent magnets are made out of hard magnetic material. The question now is which of our five answer options matches this description of a hard magnetic material. We see that option a refers to a material that is magnetic but not malleable. A malleable material is one whose shape can be easily changed. That’s not related to whether a magnetic material is hard or not. Answer option a isn’t the one we’ll choose.

Option b speaks of a material which loses its induced magnetism quickly. But, as we’ve seen, that’s the opposite of what a hard magnetic material does. Answer option b is a better description of a soft magnetic material. So we won’t choose that for this definition. Option c talks about a material which is magnetic and has a high melting point. Melting point though, whether high or low, is unrelated to whether a magnetic material is hard or soft. Option c isn’t our choice either. Option d talks about a material which does not lose its induced magnetism easily. That’s a match for our description of a hard magnetic material. It’s one whose magnetic field persists for a long time, even without an externally applied field.

So we’re leaning towards option d. But just to see what option e says, let’s read that one as well. This option says a hard magnetic material is a material which has a high density. But density is a property that’s unrelated to whether a magnetic material is hard or not. So we won’t choose option e for our answer. Option d is our final answer. A hard magnetic material is a material which does not lose its induced magnetism easily.

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