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Homework Help: How to determine material of metal (magnetism)

  1. Jul 7, 2012 #1
    1. The problem statement, all variables and given/known data
    Three bars of metal are known to be brass iron and steel. A magnet was used to distinguish between them. (the metal bars are placed under the south pole of a bar magnet and are used to pick up tacks)
    Metal 1 picked up 10, metal 2 picked up 6 and metal 3 picked up 2.
    (a)Which metal is which material and why?
    (b)If the metal 1 was removed from the bar magnet and was directly used to attract the tacks, what will be the change?

    2. Relevant equations


    3. The attempt at a solution
    (a)The answer given is 1: Iron 2: Steel 3: Brass. But I have no idea how to derive it besides guessing. Here is what I don't understand:
    I'm not sure whether the magnetic field of the magnet when having the piece of brass (which does not affect the magnetic field) will be stronger than having a piece of iron in place of it. Because in the piece of iron, the magnetic field (from the magnet) will pass through it and in doing so some of the magnetic field's strength will become diminished (in order to magnetise the bar by realigning the domains some energy is used) so when the magnetic field leaves the iron, it is reduced. While for the brass, although the strength of the magnet remains the same, the field will also become weaker once it reaches the tack (as the distance between the fields becomes greater) hence the ability to attract also becomes weaker. So I don't know how to compare the two metals. I think that steel has to be the "decider" but i really don't know how to tackle this question at all.

    (b) There will be more tacks without metal 1. This is because with metal 1 (assuming that it is iron) will produce a weaker magnetic field than just the magnet. In metal 1, some of the magnetic field from the magnet will be used to realign the magnetic domains of the metal, hence some of the magnetic field is lost. While when just the magnet is used, none of the magnetic field is lost hence just using the magnet will be able to pick up more tacks than when metal 1 is in front of it.

    For (a) I'm hoping to get a detailed explanation because I'm really curious about it. as for (b) am i right in that explanation? Thanks for the help PhysicsForums! :smile:
  2. jcsd
  3. Jul 7, 2012 #2
    Hi sgstudent :smile:

    for (a) Do you know about ferromagnetic substances? What do they do? Which among these is a purely ferromagnetic substance?
  4. Jul 7, 2012 #3


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    Science Advisor

    Iron is, of course, a metallic element. Steel is an alloy of iron and carbon (and perhaps a few other minor ingrediants depending on the type of steel). Brass is an alloy of copper and zinc.
  5. Jul 7, 2012 #4
    True. Brass still has a tiny amount of iron present in it, about 0.4-0.6 percent, if memory serves me right. That explains the two tacks picked up.
  6. Jul 7, 2012 #5
    Actually what the question was for a soft magnetic material, a hard magnetic material and a totally non magnetic material to be placed in front (at my syllabus we don't cover magnetic permeability so we consider them to be the same as air). So when brass (considered non magnetic) is placed in front the magnetic field will simply be as if air was in front of it. Hence, once it reaches the tacks the field strength will be weaker (the distance between the magnetic field is wider). However, for iron though the magnetic field pass through the iron, some of the strength of the field is used to realign the magnetic domains of the iron. So when the field reaches the tacks, the field is also weaker. This confuses me because both of the field from the iron and brass becomes weaker when they reach the tacks so I don't know how to compare them.

    Also is my (b) correct?

    Thanks for the help!
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