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Questions about magnets

  1. Jul 18, 2004 #1
    im only 15 n very interested in magnets, so i have many questions. can magnets have different strengths, and if so, how is this strength messured? What would happen to a magnet if it cannot turn or twist to make the oposite poles attract, and was stuck with like poles close together? thats all for now, but im sure there will be more, anything you can tell me, or any website u can give me to help educate me will be very much appriciated. Thank you
     
  2. jcsd
  3. Jul 18, 2004 #2
    (I assume your only asking about permanent magnets, not electromagnets.)

    Yes. You can have 20 different magnets, all the same size and shape, but all with different strengths. The difference in strength is mostly dependent on what particular material is used in the magnet. The first man made magnets were made of hardened steel. By today's standards these weren't very strong, and they lost their magnetism relatively quickly. Now magnets are made of all kinds of interesting alloys that allow them to have incredibly strong fields and stay magnetized just about indefinitely.
    I bet someone else can answer this in better detail than me.
    If you took two bar magnets and held them together north pole to north pole, and south pole to south pole, you would be adding the strength of the poles together to make stronger north and south poles. You would have to superglue them together to get them to stay like that, or clamp them together somehow, because they will repell each other, of course.

    Does being forced together, like pole to like pole, affect the life of the magnet? I don't believe so, because that is what a magnet is in the first place: each pole is already a collection of microscopic north and south poles that are being forced to remain side by side. The microscopic magnets that are being held in place, like pole to like pole, are called domains. If you want to look at things at an even smaller level, then people start talking about the concept of the dipole.

    I think if you do a google search on magnetic domains and magnetic dipoles you would find enough material to keep you busy for a long time.

    -Zooby
     
  4. Jul 20, 2004 #3
    where would i be able to buy magnets of different strengths?
     
  5. Jul 20, 2004 #4
    If you are looking for something like I mentioned above: magnets that are the same dimensions as each other but different strengths, then I don't know.

    Magnets in general, shouldn't be too hard to find. I believe Radio Shack carries a couple different shapes of ferriceramic magnets. Edmund Scientific used to sell a kind of "grab bag" of different shapes of magnets to experiment with.

    You can always just google "permanent magnets sales" and see what comes up.

    Ceramic magnets can be weak or strong, but rare earth magnets are generally the strongest (and most expensive). There is also the "Alnico" (Aluminum-Nickel-Cobalt) magnet which you find alot of.
     
    Last edited: Jul 20, 2004
  6. Jul 20, 2004 #5
    Measuring Magnetic Strength


    I am not an expert in magnetism but I do know that the strength of a magnet is measured in Tesla (Symbol = T). You probably know that a magnet is surrounded by a magnetic field, which is an imaginary field in which a magnetic material will experience a force. The Tesla is actually a measure of the strength of that field.

    Tesla was a scientist who lived at the same time as Thomas Edison (the inventor of the light bulb). He also used magnets to engineer generators alongside Thomas Edison. While the latter was working on DC (Direct Current) generators, Tesla was working on AC (Alternating Current - which is what is used today) generators.

    There is a documentary that is worth watching, called Tesla-Master of Lighting. I think it's from PBS.

    Hope that sheds some light onto your inquests :smile: ,
    Crumbles
     
  7. Jul 20, 2004 #6
    Crumbles,

    The field surrounding a magnet is not imaginary. It is an authentic magnetic field. I think you may be confused about the concept of the field and the lines that are used to illustrate the directions of force in the field. Alot of people maintain there is no reality to the "lines of the magnetic field". Whether or not that is true depends on how literally you take the lines.
     
  8. Jul 20, 2004 #7
    You said it, it all depends how literally you take the lines. I believe they should be referred to as imaginary because they are not actually THERE. You only know of their existence by their effects. But well, that is more philosophy rather than physics.
     
  9. Jul 20, 2004 #8
    The thing is you can't be confident there is nothing there.
    When people say the lines aren't literal what they mean is that the effects you see that indicate there are lines aren't sure proof of lines at all. There is equally, no sure proof there aren't any lines.And, it isn't really a matter of philosphy, but semantics. You can't even start to look at the question untill you decide what you mean by a "line", and what you mean by "real".

    At any rate, your concept of the lines as purely imaginary lead you to mistakenly refer to the magnetic field as imaginary. Magnetic fields are real. They affect everything you put into them: glass, plastic, gas, metal, electrons, protons, everything.
     
  10. Jul 20, 2004 #9
    Crumbles, I think the world you're looking for is "invisible", not "imaginary". If a field is imaginary, that would be something we concocted to make a problem easier to solve. I believe that Gaussian surfaces would fall under this category.

    Also, I always thought Westinghouse was the champion of AC, not Tesla. I'm probably wrong though.
     
  11. Jul 20, 2004 #10
    conkermaniac, yeah, invisible would probably be a better term for the magnetic field. And you are also right about Westinghouse because Tesla worked for Westinghouse. He first joined forces with Edison but the latter wasn't keen on AC generators claiming AC current was dangerous!! At that point Tesla left Edison and joined Westinghouse and pioneered the AC generator.

    http://www.energyquest.ca.gov/scientists/tesla.html
    http://205.243.100.155/frames/tesla.html


    zoobyshoe, I don't think that proof that there are lines or that there are no lines is of any importance. The lines are just visualised as a tool to aid scientific analysis. Just like you use the complex number i in calculations to help you solve your equations. Just because i is an imaginary number does not mean that i^2 is imaginary too!
     
  12. Jul 20, 2004 #11
    Tesla did not quite Edison in a dispute about AC verses DC. He quit because Edison promised him, verbally, a $10,000 bonus if he could solve a particular problem with the DC generators, but when Tesla had solved it Edison claimed he'd only been joking about the bonus. Tesla quit in disgust and actually spent a year digging ditches rather than work for Edison. Westinghouse sought Tesla out later after he found patents for AC equipment that had been granted to Tesla which he thought he could use.
    They are more real than that. There are, in fact, distinct linear pathways in magnetic fields that are prefered by stuff like plasma. This doesn't become obvious untill you look at huge magnetic fields like the ones around planets and the ones in the sun. I shall quote a post I made in another thread:

     
    Last edited: Jul 20, 2004
  13. Jul 20, 2004 #12
    I found out the hard way not too long ago how powerful magnets can be made these days. I was working with a ring of magnets we use to stabilize plasma. They needed cleaning. Unfortunately, they were incredibly, insanely strong.

    It's difficult to describe what it was like working with magnets of this power. They were each only about the size of a box of ciggarettes (slightly wider), but they would slide violently towards each other over distances of more than a foot (creating sparks when they smashed together), and once together you couldnt get them to even slide against each other, they were pressed so tight. If two did stick together, you had to be very ingenious to get them apart.

    I had several accidents. The first time (and this should have clued me in that things were more dangerous than I supposed) I had two flip together and close on my finger. The initial slap was extremely painful, but what was really disconcerting is when I found I lacked the strength to separate them. I had a piece of wooden board nearby I used as leverage to pry them apart; If I hadn't, they would have gone on crushing my finger, because I couldnt get them off.

    I won't go on about how I actually cleaned them (though it was pretty amusing) but it all culminated when I was trying to restore them in their original configuration in the ring. It was extremely unstable until you got the last one on, I didn't take precautions I should have.... well, and 4 of them collapsed catastrophically in a shower of sparks around two of my fingers. A car door slamming on your fingers would be much preferable to that.

    Anyhow, enough of my ramblings :approve:
     
  14. Jul 20, 2004 #13
    I believe the distinct linear pathways in magnetic fields you/the site is referring to is the field line directions as you would draw them or as they would appear if you do the classic iron filing scattering over a sheet of paper with a magnet underneath. This in no way proves that the field lines are real. They are something we visualise, like I said to aid in understanding magnetism... You seem to take the meaning of imaginary as meaning 'non-existent'. I guess it all depends on what your interpretation if imaginary is. But anyway, it is stupid to keep arguing such a nugatory point. Whichever way you think of it, magnetic behaviour remains what it is, which is why I say this is a Philosophy issue rather than Physics.

    You are right about the $10,000 bonus but it is also true that Tesla solved Edison's DC generator problem by proposing an AC generator instead, which Edison was against. Watch 'Tesla: Master of Lighting' from PBS and you'll get the whole picture.
     
  15. Jul 20, 2004 #14

    Gokul43201

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    Traditionally, the strength of a magnet is defined in terms of the "pole strength", m , where :

    [tex] B = \frac {\mu _0 m}{4 \pi r^2} [/tex]

    Nowadays, most Permanenet Magnet manufacturers tend to specify the coercivity (Hc), remanence (Br) and Energy Product (BHmax) of the material of the magnet. I have bought a few magnets and have rarely found the pole strength specified. Typically, you get the highest pole strengths (without losing durability and operating range) out of NdFeB (Neodymium-Iron-Boron) magnets.

    The Magnet Factory sells a lot of permanent magnets, but they may have a minimum order and could be a little pricey. If you want to buy a small number of magnets, try mcmaster.com and type in "magnet" in the search box.
     
  16. Jul 20, 2004 #15

    krab

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    I find that surprising. Are you taking into account that there are 2 poles of opposite strength?
     
  17. Jul 21, 2004 #16
    imaginaryadj1 a : existing only in imagination: lacking factual reality b :formed or characterized imaginatively or arbitrarily 2 : containing or relating to the imaginary unit

    Merriam-Webster's Collegiate Dictionary, Tenth Edition

    When you used the word "imaginary" to describe the magnetic field I think you are confusing its use in the term "imaginary number" with its use applied elsewhere. You can't apply it as it is used in the term "imaginary number" to any other situation. In any other situation it means : lacking factual reality, existing only in someone's mind.

    I don't find the issue of the meaning of words to be any more nugatory than any other issue that arises that might get in the way of communicating physics concepts.
    Actually, I was not right about the ten thousand dollars: it was fifty thousand;

    "Before long Tesla observed ways in which the primitive Edison Dynamos could be made to work more efficiently, even though limited to the production of direct current. He proposed a plan for redesigning them and said it would not only improve their service but would save a lot of money.

    "The astute businessman in Edison brightened at the mention of the latter, but he realized the project Tesla had described was major and would take a long time. "There's fifty thousand dollars in it for you - if you can do it," he said...

    "...It took Tesla the better part of the year to finish redesigning Edison's dynamos. When at last the job was done, he went to his boss to report complete success and, not incidently, to ask when he might recieve his $50,000.
    "Edison swept back his high back shoes from his desk and fell forward openmouthed.
    "Tesla," he exclaimed, "you don't understand our American humor."

    -Tesla: Man Out Of Time
    by Margaret Cheney

    I have seen this show, thanks. Edisons did cut any discussion of converting to AC short, but this refusal on his part to consider converting to AC had nothing to do with why Tesla quit the Edison Company. Tesla, as you can see from the quote above, was content to help Edison with his DC so long as he believed he'd be properly paid.
     
  18. Jul 21, 2004 #17
    Actually, I appreciated your story. Those are the strongest permanent magnets I've ever heard of. Very interesting.
     
  19. Jul 21, 2004 #18
    Krab or anyone, you said that the poles have opposite strength, do u mean that each has a different strength or the same but opposite strength becasue it is the opposite pole?
     
  20. Jul 21, 2004 #19

    krab

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    As Gokul said, the field decreases as 1/r^2 from a pole. However, there are always 2 poles; one positive and the other negative. That's why a magnet is called a dipole. If you add the 2 together, you get something proportional to
    [tex]1/r^2-1/(r+\delta)^2[/tex]
    which as proportional to 1/r^3.
     
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