Where does the energy go - magnetism question.

In the non magnetic piece, only friction (and a little air resistance) would cause the slowing.

In the magnetic piece, the earth’s magnetic field would also interact by pulling the magnet’s south pole toward the north, additionally slowing the rotation.

 

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Because the friction is too high.

Interacts in the same way as a pendulum in the gravitational field, and stops from the same cause as the pendulum.

 

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No, it's a "field" of photons, as all electromagnetic radiation is. I put "field" in quotes because in near-field electric and magnetic fields, the photons are often virtual i.e. they are the force carriers, but in many cases cannot be detected as individual photons.

 

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However, it would also speed up the rotation during 50% of the cycle. The net effect would be zero.

 

You may be right. In fact, you probably are right.

 

Have you confirmed the experiment?
I still think my answer is right. "A moving magnetic field would affect electrons within that field. In the process of shifting electrons there is a resistance and resistance creates heat. So the answer is the energy is lost as heat."
Where as a non-magnetic piece would not have a field interferring with the paths/positions of electrons in the surrounding matter and hence spins longer.

 

If metal is nearby, the changing magnetic field would induce eddy currents in the metal. These eddy currents would produce magnetic fields that would oppose the magnet's field reducing the angular momentum of the spinning magnet.

Was this acually done in the bathroom near the tub?

 

I had some leftover tiles used to waterproof the wall the bathroom and wanted as smooth and flat surface as was available with ordinary household goods I don’t have any leftover glass. The experiment is the same in the garden as it is on a wooden kitchen table away from metal influence as I can make it.

The magnet is circular with a diameter of 6mm and a height of 3mm. It is placed on the tile with the circular edge on tile giving the magnet tangential contact with the surface of the tile.
The North/South poles are on the flat surface of the magnet.

 

Try a brick-shaped magnet.
Put some oil on the contact surface to reduce friction.
Place the magnet with pole N of magnet, oriented to the earth's magnetic South Pole.
See if it starts to slide toward the earth's magnetic South Pole.

 

That seems improbable as the N/S attraction strength would have to be unbalanced. Though I am not sure what methods/experiments can be done to show that all static magnetic fields are conservative. Moving magnetic fields might be unbalanced due to the approx 120 degree angle of attraction N/S poles and the path taken to move past each other.

Actually the question arose as I was trying to think of a way of measuring with household objects the strength of a magnet by using the momentum of one object hitting another. However the earths N/S field seems to affect this.

 

Agreed - provided there were metal nearby with mobile electrons to shift.

 

The question was a good one, what pays for the energy drawn from the earth's magnetic field? (Assuming you could really show that was what's going on here.) He was aware of friction and heating, but those are losses. He was really asking about energy transfer. To answer him, you have to discover the source of the energy in the earth's magnetic field. Assuming no losses, how does that source pay for any energy added or subtracted by a guy in a white coat playing with magnets and taking notes?

The answer is this. The energy source is the dynamo within the earth. It has mechanical energy in the form of inertia. The inertia acting upon the magnetic dipole moments of the liquid iron core raise a field, similar to turning a generator with a hand crank. Place a load across the generator, and the crank feels a drag. Add power to the terminals of the generator, and it will become a motor and turn by itself.

I'm skeptical of what the earth's field is really doing to the magnet, since, as it spins, it has one orientation in which north opposes north and south opposes south, and this would appear to slow the magnet. However, 180° later, north opposes south and south opposes north, so there is a tendency to accelerate the magnet for a moment. Each 360° revolution would have zero net effect. (The integral over 360° of the sin is 0).

Regardless of that, in a theoretical sense, energy drawn from the field would tend to slow the dynamo, and energy added would tend to speed it up. I believe this was where the question was directed.

 

A magnet has its own magnetic field . When this magnet is spinned in some other magnetic field ; it is a case of interaction between two magnetic fields . When two magnetic fields interact ; they generates a force-field or gravity-field . Here the magnet is spinning against this force-field . So, when the magnet is aligned ; its potential energy will rise .

So this case doesnt violate conservation of energy .

 

ooh..oh...The adequate response to this thread is post No.22. :shrug:

 

I think this is right, there is a zero net effect on the magnet from the Earth...however that's only true while the rotational energy is sufficient to overcome the tendency to align with the poles. In other words, a non-magnetized piece of metal would spin until its energy was completely lost to friction, while a magnet would abruptly stop at a point much earlier in its energy decline at the point when its remaining rotational energy is transferred to the Earth's rotation.

As an analogy, consider two rolling balls with the same initial speed; the first rolls on a flat surface _________ while the second rolls up and down gradients /\/\/\/\/\/\/\/. The second ball will on average travel less distance even though (in theory) the gradients would produce a net zero effect on its velocity.

Expanding on this, it is possible in either case for the magnet or the gradient-travelling ball to travel longer than its counterpart. It all depends on initial conditions and velocities.

 

ACTUALLY, the torque exerted by the earth's magnetic field on a cheap compass needle, doesn't slow it down very quickly. I can't post a link.

Search youtube for CNMFaa

 

The under-dampening is just due to the needle's weak magnetic properties, which lowers the energy threshold below which the needle cannot continue to make a full rotation.

 

The video isn't available yet. In it I just swiped a magnet past a small compass. The compass needle got up to a good spin. The earth's magnetic field wasn't much of a "brake" on the spinning needle. Of course, the field of the compass needle is a weak one.

 

You know, there are some analogies to "taking or giving energy to the earth". Each time there is an impact, or traction, or any force exerted on the earth's surface's the earth responds with equal and opposite reaction. (Makes you wonder what would happen if one day every one just started pushing in the same direction...)

And yes, the magnetic force between the magnet and the earth's field have a net zero effect until that last faction of a spin, when it tries to make that last turn against the wind so to speak and is blown back. That would be the case where the inertia of the magnet, coupled through its field, is just slightly less than the inertia of the earth, coupled through its field. That would seem to be the moment of netl energy transfer.

Also, in the case of "lodestone", the energy of the earth's dynamo was tapped just as molten iron began to cool and crystallize, and the intermolecular forces were at odds with the earth's field, and depending how successful that turned out, the lodestone could be just enough well enough magnetized to serve as a compass pointer.