# Energy in a permanent magnet question

• posted

Suppose I have a permanent magnet and attract a nail a height h from the ground. If the nail has mass m then I have done mgh of work (potential energy) which must come from the field. If I then attract a second nail the magnetic attraction will be less since I have used energy when attracting the first one - which is still attached. If I keep doing this there is little energy left to lift anything more.

I then pull off the nails which requires energy from me. That energy must be released back into the field since I can then replicate the experiment. Is this right? So energy is stored in permanent magnets.

• posted

Here's how I would think about it.

There is energy in the magnetic field. The energy density is B^2/ (2*mu sub zero) (MKS units) If you integrate this over all space you get the total energy in the field. Sticking bits of iron in the field reduces the B field outside the pieces of iron and so the total energy of the system has decreased. So sure there is some energy in any magnet. (I trust this is *not* leading to some perpetual motion gizmo.)

George H.

• posted

Magnets don't have anything to do with energy. They exert a force. The energy comes from whatever is moving/holding the magnet or the object.

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Failure does not prove something is impossible, failure simply
indicates you are not using the right tools...```
• posted

No, it's not right.

But you don't want to know anyway.

The energy stored in a magnet is called magnetization work. If you extract it, you will de-magnetize the magnet.

Coercitivity Susceptibility Remanence Degaussing

w.

• posted

They certainly store energy. It takes energy to magnetize one, and the nail experiment is one way to recover some of that energy.

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John Larkin                  Highland Technology Inc
www.highlandtechnology.com   jlarkin at highlandtechnology dot com   ```
• posted

No, Mista.

If you pull out energy (regardless how), the magnet would de-magnetize.

w.

• posted

No.

• posted

Not really,

It took you an amount of energy to set up this scenario :

...When the nail jumps to the magnet, it's just paying you back what you've spent out

So, tell this experiment to Obama and explain to him that the "no free lunch in America" still works, and it's now biting him

• posted

Not really,

It took you an amount of energy to set up this scenario :

...When the nail jumps to the magnet, it's just paying you back what you've spent out

So, tell this experiment to Obama and explain to him that the "no free lunch in America" still works, and it's now biting him

_________________________________________________

Or better yet see if you can get some of that green energy funding from the dept of energy. Let us know if it works.

• posted

So, when the magnet lifts a nail, where did the energy come from? Do you believe in conservation of energy?

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John Larkin                  Highland Technology Inc
www.highlandtechnology.com   jlarkin at highlandtechnology dot com   ```
• posted

It does demagnetize -- the more nails you put on, the less magnetic field there is available to the outside world. Then, when you pull the nails off, you re-magnetize it.

And no, I don't mean "demagnetize" in the way that you were thinking -- and no, that's not my problem.

```--
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.```
• posted

If that where true a motor with permanent magnets would stop working very quickly.

A magnet exerts a force. A magnet can attract things but it takes the same amount of energy to remove those things so the nett output is 0.

```--
Failure does not prove something is impossible, failure simply
indicates you are not using the right tools...```
• posted

Because you are kook who doesn't know what a friend is.

• posted

Right. The captured nails shunt the field locally, leaving less external field to lift future nails.

A thin coating of infinite-mu stuff would soak up all available external-to-magnet energy, and shield the magnet perfectly.

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John Larkin                  Highland Technology Inc
www.highlandtechnology.com   jlarkin at highlandtechnology dot com   ```
• posted

.> >HardySpicer wrote:

I know from practical experience it takes energy to demagnetize them as well -- perhaps as much or more to randomize the domains as it took to align them.

• posted

When an apple falls does the earth use up some of it's gravity?

• posted

That statement makes no sense. A motor gets its power from an external power source, not from the energy stored in the magnet.

There's energy stored in the lockwashers too, but it doesn't get used up.

So I guess you would argue that a spring exerts force but never stores energy. Then it follows that it takes no energy to compress a spring.

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John Larkin         Highland Technology, Inc

jlarkin at highlandtechnology dot com```
• posted

From a pure COE perspective, one should extract energy from a magnet to demagnetize it. Lifting nails actually does that, some.

Hmmm, suppose you dissolve a magnet in acid. Where does the magnetization energy go? It must come out as heat... there's (almost!) nowhere else for it to go. That would make an interesting science project experiment.

Similarly, if you heated charged and uncharged magnets to the curie temperature, the charged magnet would require less heat. Maybe.

We need a good physical chemist to comment here.

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John Larkin         Highland Technology, Inc

jlarkin at highlandtechnology dot com```
• posted

Yes. Each nail stuck to the magnet shunts some of the field, reducing external field, so there's less energy available to lift future nails.

A magnet can't yank up an infinite mass of nails.

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John Larkin         Highland Technology, Inc

jlarkin at highlandtechnology dot com```
• posted

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