A question about permanent magnets

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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).
 

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  • #2
ranger
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The short answer is yes it will. Here is a post that sums it up:

'Wear' is a somewhat misleading term. Permanent magnets gradually decrease in strength, but this has very little to do with usage. The "life" of a permanent magnet depends on many factors. Naturally occuring forces conspire to knock the little domains out of alignment. But this is normally a very slow process. Temperature is a major player in this process. The higher the temperature, the faster this process will occur. Extreme heat [surpassing the curie point] will immediately randomize the domains. A sharp blow can also knock domains out of alignments, as can other nearby magnetic or electrical fields. Radiation can also knock domains out of alignment. But again, under normal conditions, neither your fridge or you will live long enough to watch the little fellow fall to the floor in exhaustion. At the quantum leve [e.g., electrons] magnetism is eternal.
https://www.physicsforums.com/showpost.php?p=1195930&postcount=5
 
  • #3
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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?
 
  • #4
ranger
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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?

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?
 
  • #5
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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?
 
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  • #6
ranger
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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?

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
 
  • #7
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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).

A permanent magnet looses energy very slowly over time. Thats the point though, its supose to keep its magnetic feild
 
  • #8
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A permanent magnet looses energy very slowly over time. Thats the point though, its supose to keep its magnetic feild

Even without the external aspects that ranger listed above?
 
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  • #9
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Does all this mean that as long as the domains stay in their origional position the magnet will not be affected?
 
  • #10
ranger
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Even without the external aspects that ranger listed above?
If none of these external factors are present, then there isnt any reason for the domains to get realigned.
Does all this mean that as long as the domains stay in their origional position the magnet will not be affected?
Yes.
 
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  • #11
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Thanks much everybody.
 
  • #12
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Oh, I have one more :):

Do permanent magnets rely on the earth (will they work in space)?
 
  • #13
ranger
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Oh, I have one more :):

Do permanent magnets rely on the earth (will they work in space)?

It will also work in space (free space). Everything has measurable permeability.
 
  • #14
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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.
 
  • #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.
 
  • #16
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We have already went over that MedievalMan. Nonetheless your input is appreciated. My last question is right on top of yours.
 
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  • #17
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Nevermind, I have found a good resource.
 
  • #18
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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.
 
  • #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?
 
  • #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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