conservative or nonconservative. That's the question...

[NobleWolf]

Hello everybody.

I've been lurking around in this forum for several months and this is my first post.

Countless times I've read that the magnetic force is conservative but today I found the following:

"While the material here does not address the issue, magnetism is not a conservative force yet various quantities (such as the amount of field passing through a unit of area) are conserved"

in a web page of the... ?NASA!

here:
http://pwg.gsfc.nasa.gov/istp/outreach/ed/back/1.html

Any comments?

best wishes
NobleWolf

[tinu]

I suppose it refers to magnetism in a general approach (electromagnetic theory, Maxwell). Indeed, you may easily find that magnetism is not conservative in such systems but electro-magnetism is.
Whether I am wrong in the above assumption, the given statement would be very bold indeed!

Have a nice day,
Tinu

[ken_nyus]

Is magnetism only conservative when looking at attraction?

If you are working with repulsion, I don't see how anyone can call it conservative, after the push there is no pull to take the energy back.

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[tinu]

Is magnetism only conservative when looking at attraction?

If you are working with repulsion, I don't see how anyone can call it conservative, after the push there is no pull to take the energy back.

Nope. Conservative refers to closed loops, hence both to attraction and repulsion.
Being conservative means that the work done in a closed loop is always zero, no matter what shape the loop has, neither anything else matters. In practice it is not always easy to check it because of friction and other external influences. But that?s what it means (mathematically/theoretically speaking): that if you move something magnetically interacting (ferro/dia/para) in a static magnetic field on a closed loop, you do not have to spend energy by the time when again reaching the starting point but you will not gain any energy either. It is the same in gravitational field: moving a weight up and down will never result in more energy. (Except for the Lead Out theory, of course  )

Note the word ?static? used above. If the field is not static, you may gain some mechanical energy. (You may loose some too, depending on the particular setup). But you have to pay for modifying the field. When you gain some mechanical energy ? that?s usually what is commonly referred as electrical motors.  In the other case one can speak of electrical generators.

One example where the universe pays is in gravitational slings used for some space vehicles. But there is no free lunch? In that case, the planet used for sling will slow down. Imperceptible, of course, but still?
So, if someone can find a magnet that significantly changes its own field by itself, in an oscillatory manner, I?d say FE is piece of cake. Unfortunately my magnets don?t. 

Hope it helps,
Tinu

[ken_nyus]

Thanks tinu.

I also had the thought after posting, that even in repulsion, I may think I have more energy after the push, but if I want to close the loop and get another push I will use the same energy to get back into position for the next push. I can understand this if I think of a magnet in repulsion as if it were a spring, I have to compress the spring first to get the push second.