Where Does the Energy Come From When a Magnet Moves Over a Paperclip?

[Jobrag]

My apologies for posting something that I'm sure people will say has been answered before, but a trawl through the results of a search of magnets didn't come up with a satisfactory answer.
If I hold a permanent magnet a few centimeters over my desk and slowly move it across a paperclip the paper clip will jump up to the magnet thus gaining potential energy, where does that energy come from?

 

[Dale]

From the energy density of the magnetic field, which is slightly reduced by the paper clip.

 

[Jobrag]

DaleSpam
So if I repeat the action often enough the magnet will become de-magnetised?

 

[Antiphon]

Probably not. You have to give the energy back when you pull off the clip.

 

[Jobrag]

Antiphon
Sorry I don't get that, when I pull the paperclip off the magnet my muscles do work on the paperclip but how does that put energy back into the magnet?

 

[mishrashubham]

Antiphon
Sorry I don't get that, when I pull the paperclip off the magnet my muscles do work on the paperclip but how does that put energy back into the magnet?

You give energy back to the paper clip not the magnet. This is called potential energy. Note that under the influence of a non contact force, an object has potential energy. We usually talk about gravitational potential energy but it can also be due to other forces, in this case magnetic force.

 

[Jobrag]

mishrashubham
If you go back to Antiphon's original post he is suggesting that somehow energy is put back into the magnet when the paper clip is pulled off I was questioning that. When the paperclip jumps up to the magnet in the first place it acquires PE what I want to know is where does that energy come from?

 

[Dale]

DaleSpam
So if I repeat the action often enough the magnet will become de-magnetised?

No, as Antiphon said, when you remove the clip you do work on it, which increases the field again (ideally back to the original field).

OK, first you need to understand that energy is stored in electric and magnetic fields:
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/engfie.html
http://farside.ph.utexas.edu/teaching/em/lectures/node89.html

Now a paperclip is ferromagnetic, meaning that when it is under the influence of an external magnetic field its magnetic domains become aligned in an anti-parallel direction. This anti-parallel arrangement reduces the total magnetic field and thus does work. When you remove the clip the magnetic domains return to their disorganized state and stop canceling out the magnet's field, thus restoring the field and requiring work.

This is why a magnet has a limited "carrying capacity", it can pick up 1 paperclip, or maybe 50, but not 5000. At some point the field is essentially completely canceled out and there is no more energy to lift another paperclip.

 

[mishrashubham]

mishrashubham
If you go back to Antiphon's original post he is suggesting that somehow energy is put back into the magnet when the paper clip is pulled off I was questioning that. When the paperclip jumps up to the magnet in the first place it acquires PE what I want to know is where does that energy come from?

The potential energy you are talking about, as I already said, is gravitational potential energy. However it also loses potential energy that comes from the magnetic field. Like DaleSpam said, understand that energy is stored in fields.

 

[sophiecentaur]

mishrashubham
If you go back to Antiphon's original post he is suggesting that somehow energy is put back into the magnet when the paper clip is pulled off I was questioning that. When the paperclip jumps up to the magnet in the first place it acquires PE what I want to know is where does that energy come from?

Correction - the paper clip Loses Potential Energy as it 'falls' towards the magnet. The guy who made the magnet in the first place effectively put lots of negative energy into the magnet, relative to the rest of the world. Things moving towards the magnet lose potential.

 

[Dale]

Correction - the paper clip Loses Potential Energy as it 'falls' towards the magnet.

I know you understand this, but just for clarification, it loses magnetic potential energy and gains gravitational potential energy.

 

[sophiecentaur]

Yes - this stuff is really fraught ain't it?

 

[pallidin]

My apologies for posting something that I'm sure people will say has been answered before, but a trawl through the results of a search of magnets didn't come up with a satisfactory answer.
If I hold a permanent magnet a few centimeters over my desk and slowly move it across a paperclip the paper clip will jump up to the magnet thus gaining potential energy, where does that energy come from?

If this helps: As the paper clip is drawn towards the upper magnet, the magnet will experience a downward "pull" directly proportional the mass of the paper clip.