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My question is, one of them is iron and the other is a magnet. It is forbidden to break. However, it is desired to determine the magnet without using any other objects. How can I find , which is iron or magnet?

I learned that the following method is wrong.

My method:

1) To bring the $ S-$pole of the magnet on the left to the $S-$pole of the right magnet.

2) Then bring the $ S-$pole of the magnet on the left to the $N-$pole of the right-hand magnet.

If both are attractive, then it is iron on the left.

Is my method correct?

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  • $\begingroup$ can you break the magnet with the iron :) $\endgroup$
    – Micheal Brain Hurts
    Oct 21, 2018 at 19:21
  • $\begingroup$ @user2312512851 It is forbidden to break. $\endgroup$
    – Elementary
    Oct 21, 2018 at 19:23

6 Answers 6

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Place the bar "magnet" between the poles of the horseshoe "magnet", perpendicular to the plane that contains it.

Now move the bar along its axis. If the bar is actually a magnet, the field is the strongest at the tips of the bar, so the greatest magnetization of the horseshoe bar (and therefore lowest energy state, by E = - m . B) will be roughly when either tip of the straight bar is closest to the horseshoe bar.

If, otherwise, the horseshoe bar is a magnet, the magnetization is greatest, roughly, when you maximize the volume of metal between the poles, so the lowest energy state will be when the straight bar is in the midpoint.

If the straight bar, constrained to move only along its axis, is pulled into the middle, the horseshoe bar is a magnet. If instead, either end of the bar is pulled into the middle, the straight bar is the magnet.

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If you have another magnet, one end of that will repel one end of the magnet, but it will attract both ends of the iron.

If you have another piece of iron, it will be be attracted to the magnet, but not the iron.

If you have neither, you will have to resort to destructive testing: cut one of the items in two. If the two parts attract and repel each other, you have cut the magnet. If not, you have cut the iron.

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  • $\begingroup$ it is desired to determine the magnet without using any other objects... $\endgroup$
    – Elementary
    Oct 21, 2018 at 19:20
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You can't say that using this method. Iron will be attracted by any pole of magnet. Using your method only determines that one of these is made of iron. So this is not a good idea. Instead use them on any other non magnetized object made of ferromagnetic material. The one which doesn't attract the object is made of iron.

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  • $\begingroup$ I thought of the right as a magnet. Yeah, I think I understand my mistake. $\endgroup$
    – Elementary
    Oct 21, 2018 at 18:45
  • $\begingroup$ But the question needs to be determined without using anything else. $\endgroup$
    – Elementary
    Oct 21, 2018 at 18:55
  • $\begingroup$ @Beginner it's not mentioned in the question and I don't think you can do that without using anything else. $\endgroup$
    – Mechanic
    Oct 21, 2018 at 18:57
  • $\begingroup$ Thank you so much for answer. You are right. I edited question. Thank you again for answer. $\endgroup$
    – Elementary
    Oct 21, 2018 at 19:25
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Too bad that you can't use any other objects. Otherwise, I would simply put both of the items into any nearby dirt or sand and rub them around for a minute or two. Whichever of them comes back coated in little pieces of iron--which is present in almost any amounts of natural detritus that would have broken down into those natural substances--that would be the magnet.

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Given the known conditions that one object is a magnet and one is iron, touch either end of the bar-shaped object to the curved part of the U-shaped object. If there is attraction, the bar-shaped object is the magnet. If it does not attract, the U-shaped object is the magnet.

If the bar is a magnet, either end will attract any part of the iron U, including the curve. If the U is a magnet, only the tips (not the curved part) will attract any part of the bar.

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Take the 1st (A) object, bring one end of it to the middle of the 2nd one (B). Note if there's a significant attraction.

Then, bring one end of B to the middle of the other one, also noting if there's significant attraction.

The magnet has it's magnetic field least pronounced in the middle, regardless of actual shape. If one object strongly attracts the other while touching the middle of the other one, the former one is most likely magnetic.

BTW, this riddle is old as the world itself:

https://www.quora.com/I-have-a-bar-magnet-and-an-unmagnetized-iron-bar-that-appear-identical-Using-only-those-to-objects-how-do-I-determine-which-is-the-magnet-and-which-the-plain-iron-bar

https://van.physics.illinois.edu/qa/listing.php?id=16626

https://van.physics.illinois.edu/qa/listing.php?id=20834

https://www.physicsforums.com/threads/ur-in-a-room-with-a-magnet-and-an-iron-bar-how-do-u-know-which-one-is-the-magnet.299003/

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