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Questions about magnets.

  1. Oct 5, 2005 #1
    I was thinking of building something but I first must know a little bit more about magnets.

    First of all where could I buy good magnets of all sorts, with different strengths. Are there any stores like home hardware or renodepot that are known for selling magnets? Would they sell very strong magnets to anyone? And how strong are the strongest magnets?

    Another thing is say I bought a normal rectangular magnet. Would it have a negative and positive side or are some magnets positive and some negative. Hahaha I know this is a stupid question but I am just making sure I always thought that every magnet has a north and south + and -.

    Lastly I want to know if I put a metal cube in the middle of stationary magnets one being very strong and the other being somewhat weak would the metal only be attracted to the strong one or would it also be attracted to the weak one?
  2. jcsd
  3. Oct 5, 2005 #2
    You can buy plenty of magnets. I'm personally not aware of where you can buy them locally, but ebay tends to have a lot of all shapes and sizes. Hobby stores might have some. You can buy very strong magnets. So strong that they are dangerous and could result in severe injury or even death in extreme circumstances. Magnetic materials like NeFeB are amongst the strongest magnetic materials. I've seen these on ebay as discs as large as 6" in diameter and 2" thick. This is can be a very dangerous object.

    Magnets always have a north and south pole. This is one of Maxwell's equations [tex] (\nabla \cdot \vec{B} = 0) [/tex]. There are no magnetic monopoles.

    The metal cube could be attracted to both, repelled by one attracted to the other or repelled by both (depends on the alignment of the magnets to the material). There would be some force on it due to each magnet as long as the material was affected by magnetic fields (ferromagnetic, etc).
    Last edited: Oct 5, 2005
  4. Oct 5, 2005 #3
    If you were to cut a long magnet in between where you think the south and north poles are, you would end up with two magnets, each with a N and S pole.
  5. Oct 5, 2005 #4


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    As 2nd poster said, you can buy magnets on ebay, that's where I buy all mine. http://www.wondermagnet.com/ is a great place to find a wide array of amazing magnetic materials and expirements. I highly recommend looking at that site.
    A great search in physics is looking for the magnetic monopoles, meaning ones that have only a negative, or only a postitive, they are predicted by many a theory, but none have ever been observed. Any magnet you will see has two poles, I think in particle accelerators sometimes quadrupole magnets are used, with four poles.

    The metal in the middle will be attracted to both, but the stronger magnet will suck in the metal.

    If you cut a ferromagnet period, each piece will have a north and south pole.
  6. Oct 5, 2005 #5
    What kind of application? I would recommend looking into magnets of the neodyium iron boron variety. Just search for them on ebay, you should see a lot for sale there. I bought a 2x2x1 inch neodyium magnet that i think lifts over 200 pounds of steel. I've tested a little under 200 pounds so far with it. It cost about 50 dollars on ebay.
  7. Oct 6, 2005 #6


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    If we're bragging about the strength of our magnets...
    I bought 28 N40 magnets, about 1/2 in. diameter, on ebay for VERY cheap. I forgot how much though, less than 25 dollars.
  8. Oct 6, 2005 #7
    I once saw a 6x1.5 inch cylindrical magnet on ebay, i think around 200 hundred dollars? If I had an extra two hundred dollars, and perhaps an application for it I'd like to get one of those. I guess the optimal geometry for the neodymium magnet is a 1/8 "pancake" or cylindrical height/diameter ratio, I remember reading that somewhere.
  9. Oct 6, 2005 #8
    Thanks for all the information. It seems that the internet is probably the best place to buy magnets. I have a question about repulsive forces. Just how strong of a repulsive force is produced by these magnets you guys are talking about?
  10. Feb 10, 2012 #9


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    Staff Emeritus
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    You cannot make a magnetic motor using permanent magets only. You must be able to switch the poles. (I guess you could physically turn the magnets over, but this is much easier to accomplish using electromagnets)

    Also, this thread hasn't been responded to in over 6 years. In the future please start a new thread instead of responding to very old ones.
  11. Feb 10, 2012 #10
    The thread I responed to was the only one I had a desire to ask about. I have no need to respond to something I have no interest in!
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