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A question about permanent magnets

  1. Jan 19, 2007 #1
    Does a permanent magnet ever loose it's energy or decrease over time? I read somewhere that a "fixed" magnet's energy will dissipate in time as it does work. I don't know what they mean by fixed.

    Also, will a permanent magnet work without the earth's energy (eg. in space).
     
  2. jcsd
  3. Jan 19, 2007 #2

    ranger

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    The short answer is yes it will. Here is a post that sums it up:

    https://www.physicsforums.com/showpost.php?p=1195930&postcount=5
     
  4. Jan 19, 2007 #3
    Under what conditions will a permanent magnet loose it's magnetic force:

    1: I have read that the magnet will lose some energy when it "hits" the metal item. Is this the only way it will lose it's energy?

    2: Will it degrade by simply sitting?
     
  5. Jan 19, 2007 #4

    ranger

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    I thought the quote in my first reply addressed number 1. The domains will be realigned by heat, shock, or the influence of a demagnetizing magnetic field - permanent magnets have a characteristic called coercivity, which is basically its ability to withstand being demagnetized.

    For number 2, if you remove all the external aspects that conspire to knock the domains out of alignment, is there any reason for the domains to be realigned?
     
  6. Jan 19, 2007 #5
    What does "domain" mean here?

    Also, when you say heat, is there a certain threshold or referencepoint? For example does room temperature count or are we talking higher temps?
     
    Last edited: Jan 19, 2007
  7. Jan 19, 2007 #6

    ranger

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    More info on domains can be found here:
    http://hyperphysics.phy-astr.gsu.edu/hbase/solids/ferro.html#c4

    Yes there is a certain threshold called the curie point. At temperatures above the curie point, the material loses its ferromagnetic characteristic. For a list of curie temperatures read this:
    http://en.wikipedia.org/wiki/Ferromagnetism
     
  8. Jan 19, 2007 #7
    A permanent magnet looses energy very slowly over time. Thats the point though, its supose to keep its magnetic feild
     
  9. Jan 19, 2007 #8
    Even without the external aspects that ranger listed above?
     
    Last edited: Jan 19, 2007
  10. Jan 19, 2007 #9
    Does all this mean that as long as the domains stay in their origional position the magnet will not be affected?
     
  11. Jan 19, 2007 #10

    ranger

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    If none of these external factors are present, then there isnt any reason for the domains to get realigned.
    Yes.
     
    Last edited: Jan 19, 2007
  12. Jan 19, 2007 #11
    Thanks much everybody.
     
  13. Jan 19, 2007 #12
    Oh, I have one more :):

    Do permanent magnets rely on the earth (will they work in space)?
     
  14. Jan 19, 2007 #13

    ranger

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    It will also work in space (free space). Everything has measurable permeability.
     
  15. Jan 19, 2007 #14
    What exactly happens when these domains change direction. Is the something physicaly occuring inside the domains? I'm finding it hard to find info on this.
     
  16. Jan 20, 2007 #15
    Many motors use permanent magnets (usually rare earth strong type). In this cases, there a few things that can demagnetize them, and thus must be avoided:

    -extreme temperature (Currie temperature I think?)
    - Too high of current (and thus magnetic field from the coils)
    - damage from dropping, bashing, or even disassembling the magnets.

    So, although permanent magnets can quickly become demagnetized, in such an application, you have to know at what temperature, current, etc this occurs, and design your system to avoid it.
     
  17. Jan 20, 2007 #16
    We have already went over that MedievalMan. Nonetheless your input is appreciated. My last question is right on top of yours.
     
    Last edited: Jan 20, 2007
  18. Jan 20, 2007 #17
    Nevermind, I have found a good resource.
     
  19. Jan 28, 2009 #18
    If I were to put two opposing magnets next to each other of equal strength would either be affected? I understand that if one were stronger it would destabilize the other.
     
  20. May 24, 2009 #19
    how about if you built an electric generator with "permanant" magnets?
    how long would it last? i'm trying to cut down on costs on energy but i need some alternatives so far i have solar energy, and thinking most about kinetic energy(magnets) how long would it stay permanent and will we be alive by the time it dies?
     
  21. Aug 14, 2009 #20
    I was wondering about that too. Would the magnets loose their power if under constant collision of opposite fields that they would experience in a motor. Based on what is being said here, i am guessing that they should retain their power, but then again they might not. I need to confirm this because magnets are expensive.
     
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