Describe what steps must be undertaken in order to convert an unmagnetized iron rod into a permanently magnetized state. As part of your answer, explain what a magnetic domain is and how it responds to the steps described.
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Final Answer
  1. Apply a strong external magnetic field.
  2. Apply a bit of heat.

Solution video

OpenStax College Physics for AP® Courses, Chapter 22, Problem 4 (Test Prep for AP® Courses)

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Video Transcript
This is College Physics Answers with Shaun Dychko. We have an unmagnetized iron rod and it consists of these magnetic domains which are chunks of the iron that have all the same magnetic orientation of the atoms and then there's another chunk that has an orientation in a different direction so I am drawing the direction of the north polarity of the domain here and this one's north down, this one's north up into the left, this one's north directly to the right, this one's north directly upwards and because the domains have magnetic polarity in all different directions, they tend to cancel each other out so that the overall magnetism of this iron rod is zero, which is to say it's unmagnetized. Now to fix that or to turn it into a magnet, we'll do two things: number one is we apply a strong external magnetic field that will pass through this iron rod so maybe if there's an electromagnet or something that's creating this strong externally applied magnetic field and this will tend to orient these domains to all be pointing along this external magnetic field and in order to help it do that... in order to help the domains move, we'll apply little bit of heat so number (1) is apply strong external magnetic field and step number (2) is apply a bit of heat. Now not too much because if you make too much heat here then all these atoms will be so energized, they will just be scrambled and moving in all random directions and the thermal movement will be so strong that it'll ignore this external magnetic field that's applied. But if there's a bit of heat and not too much, it will tend to loosen up these domains, make them jiggleable and they can jiggle a bit easier and you know, they become a bit more fluid; I don't mean that in the sense that they are liquid or anything or gas or anything but I just mean to say that they are easier to orient when they have a bit of heat applied to them and so they will tend to orient more easily to this externally applied magnetic field and then take the field away, take the heat away and the domains will tend to stay aligned and that's what makes it into a magnet.