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Magnet pole pieces...

Discussion in 'General Electronics Discussion' started by Externet, Feb 1, 2019.

  1. Externet

    Externet

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    Aug 24, 2009
    Not exactly electronics; please delete if improper.

    A magnet exhibits its field as demonstrated with iron filings aligned between its poles.
    Say you have an ordinary bar magnet 1cm x 1cm square, 10cm long

    Such magnet (N,S at left) fitted with long, straight, parallel pole pieces (upper ==== and lower ====) ; will have a constant field along/between them, near or distant from the magnet ?
    Or, at which location A,B,C the placement of a piece of iron between the pole pieces should be attracted the most ?

    =======A===================B=====================C=
    N
    S
    =======A===================B=====================C=
     
  2. duke37

    duke37

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    Jan 9, 2011
    My guess is that the field will be strongest at the magnet and will be weaker as you move along the pole pieces. The lines of force appear to have a tension so will concentrate near the magnet and be much weaker at C/C.
     
    poor mystic and Cannonball like this.
  3. davenn

    davenn Moderator

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    Sep 5, 2009

    you don't need a pole piece for a bar magnet ... they are only usually used on horseshoe magnets
     
  4. ChosunOne

    ChosunOne

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    Jun 20, 2010
    The force on (presumably unmagnatized) iron is always toward the ends of the bar(s). There is no force drawing iron anywhere except toward an end. The magnetic field is from end to end: No magnetic force emanates from the bar between the poles.

    This is easy enough to test: You can buy 100 small disc magnets (Search eBay for 3 X 3 mm Neodymium Disc magnet) for a few dollars and stack to make a very long bar magnet that is strong at the ends but won't even pick up a small paper clip in the middle. In fact, I suggest you do just that: Experiment with them to see exactly how magnets work. There might be a surprise or two there for you.
     
    Cannonball likes this.
  5. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    By using the extended pole pieces you have essentially created a "horseshoe" magnet. Almost all the magnetic flux from the original bar magnet will now be directed into these two pole pieces, where it will have little external effect. Instead, the flux (in air) from the original bar magnet now appears at the far ends (C in your diagram) of the two extended pole pieces. The strength of the original magnetic field (near A in your diagram) in air will be less than the strength in air (near C in your diagram) because of the existence of magnetic reluctance in the magnetic path from A to C. The amount of reluctance (a scalar quantity) depends on properties of the material and its dimensions. More information can be found here and here.

    @ChosunOne offered you an excellent suggestion: purchase a stack of small neodymium disk magnets and experiment with them. I would also suggest you experiment with various magnetic materials to get a practical "feeling" for how magnetic fields are produced and directed.

    Try annealing (in an oven) ordinary steel bolts and steel coat-hanger wire to change their magnetic properties from relatively "hard" to relatively "soft". You can do this by heating the material to a red-heat glow and then allowing the material to cool slowly in air. Do not rapidly "quench" the hot material in water or oil. If you don't want to use an oven, most magnetic materials can be heated to a red glow by passing a sufficient current (AC or DC) through them. The current will range from a few hundred to a few thousand amperes, depending on the dimensions of the object and how and where electrodes are attached.

    The important thing to do is to have fun while learning about magnetic fields, but be very careful around powerful rare-earth magnets, such as those made from neodymium or samarium cobalt alloys and frequently salvaged from hard disk drives. These magnets can come together with considerable force, sometimes chipping and creating magnetic shards that will cause havoc if they wind up in the wrong places. Cleanup and recovery can sometimes be accomplished with steel wool, but better to not create strong magnetic shards in the first place.
     
    Cannonball likes this.
  6. ChosunOne

    ChosunOne

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    Jun 20, 2010
    What hevans 1944 said,. Plus, sorry I didn't think to mention this before, since I suggested buying lots of small neodymium disc magnets, but these small powerful magnets are NOT TO BE USED around children young enough that there's a risk of ingestion. Swallowing a magnet is LOTS more dangerous than swallowing other small items like buttons and coins, etc. Magnets in a digestive track while there is also something else iron/steel like, e.g., a washer, can cause the pair to pinch together, sometimes through one or more membranes. Bottom line, DON'T let small kids play with small magnets. Especially since they're shiny, keep them out of sight when littles are around.

    Other things to watch out for: Don't let magnets near credit cards, magnetic tape, or watches with steel moving parts. One dose of strong magnetism can ruin them.
     
    hevans1944 likes this.
  7. Thalmazo

    Thalmazo

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    Feb 10, 2019
    Speaking of pole pieces, does anybody have a good explanation of how a humbucker works?
     
  8. davenn

    davenn Moderator

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    Sep 5, 2009
    poor mystic likes this.
  9. hevans1944

    hevans1944 Hop - AC8NS

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    Sure, what @davenn said. And the next time you visit the northern panhandle of Floridia, be sure to drop by Destin and visit Fudpucker's Beachside Bar and Grill for a family-friendly humbuckin' good ol' time, courtesy of their house band.:cool:
     
    davenn likes this.
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