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.
