Can a magnet's magnetic field perform work on another magnet?

harrylin

I gave you the answer in my posts #138 and #148. Did you not understand it? I will summarize it even simpler here. My only regret is that this is basic stuff from my physics curriculum which I forgot over the years - and apparently I'm not the only one with amnesia!
Once more: that turned out to be wrong - and herewith my excuses for letting myself go along with such error! Not only is the energy provided by the magnetic field, the basic textbook solution for calculating the force only accounts for the magnetic field force.

Here it is once more, with more step by step explanation:

- Magnetic fields are modeled as due to small current loops inside the magnets.
- The simplest model of two magnets consists of two current loops.
Analysis:
- Magnetic attraction is due to the inhomogeneous magnetic fields of the magnets:
http://en.wikipedia.org/wiki/Force_b...magnetic_field (skip the Gilbert model and stick to the Ampere model)
- You can find all the equations you need there and in the further links
- The only force in basic textbook calculations of magnetic attraction is this magnetic force
- It is proportional to the square of the magnetic field strengths and it is still present when the magnets are moving
- Therefore, the magnetic force pulls the magnets together (surprised? )
- It therefore does work (see: https://en.wikipedia.org/wiki/Work_%28physics%29)

cabraham

I've already stated that E forces keep the current in the loop going as well as generate the magnetic dipole. But E force is not in the right direction to produce torque, where Fm is. Fm = qvXB spins the loop. You pretty much admit that Fm is what spins the loop. If you're telling me that Fm is zero unless an E force keeps charge carriers moving around the loop, please reread my posts & you will find that I've already stated that eons ago. Without E to maintain loop current, Fm is indeed zero. I've said that since day one. You seem to arguing my case. BR.

Claude

K^2

Your question was phrased generally, so I gave a general answer. If you are talking specifically about a case where there is no external E field, then yes, the only thing that can do work is the induced E field.

A current loop placed in magnetic field precesses work done is precisely zero. Same for a dipole placed in a magnetic field. The torque is applied perpendicular to the angular momentum already carried by the loop/dipole. Therefore, the direction the loop will turn is going to be perpendicular to torque. There is no work being done on the current loop.

Think of it as a gyro in a gravitational field. As gyro precesses, the work gravity does on gyro is zero. In order for there to be a net work on the gyro, a dissipative force, such as friction, needs to be introduced. Same deal with spins precessing in magnetic field.

cabraham

Sorry, but you make no sense at all. The torque spins the loop. How can the loop turn in a direction normal to torque. We're talking very basic physics here, per Halliday & Resnick. The induced E field does not spin the loop. Refer to my diagrams. Fm is clearly what spins the loop, Fe is what clearly established loop current.

We're dealing with 3 dimensional forces, loops, rotations, moments, etc. Visualizing such is all but easy. That is why I keep nagging everyone to draw sketches. I don't think anybody is too smart to need to rely on sketches. A sketch shows us in detail what exactly is going on. Those who refuse to provide sketches are merely guessing, assuming, & arm waving. Draw a picture. Please draw a picture.

Draw a picture!

Claude

harrylin

Yes, sorry that my question wasn't clear. What makes you believe that explaining magnetic force needs a discussion of all those things such as magnetization etc, so that there would be no magnetic attraction between two electromagnets? I gave the example of two magnetic current loops, which explains the basics of magnetic attraction.
1. Sorry, there is no mention of centripetal force in any such discussion in any textbook that I know - it's assumed to be irrelevant for calculating magnetic field strength. I already gave a link to Wikipedia which cites references; in addition I can mention Fundamental University Physics, Electromagnetism, by Alonso&Finn which corroborates the Wikipedia article on the force between magnets. For electromagnets the magnetic moment is calculated from the Lorentz force F=IBL, which drives as example a galvanometer.
- http://en.wikipedia.org/wiki/Galvanometer#Operation
2. Electrical force. According to the textbooks, it does not drive the galvanometer action.
3. Presumably yes.

harrylin

This morning when I awoke I drew a picture of two current loops in my head, complete with Lorentz force vectors. That picture is of course only qualitative, but it agrees with you, Wikipedia and my textbooks.

cabraham

Thanks for your feedback. I wouldn't insist on drawing pics unless I knew first hand how helpful they are. Like I said, mentally visualizing all this stuff is not easy. Those who claim that they don't need a pic have abilities beyond mine. BR.

Claude

PhilDSP

Hi Miyz,

I don't want to get embroiled in this interesting debate, but while you could *very loosely* state what you said above, to be rigorous, that statement is not true: magnetic fields are most definitely NOT the cause of electric fields as Jefimenko's analysis has shown:

http://en.wikipedia.org/wiki/Jefimenko%27s_equations

vanhees71

There is one and only one electromagnetic field, and thus some of its components are not the cause for other components. That doesn't make sense to begin with since it's a reference-frame dependent statement anyway, and there is no physical meaning in something that's frame dependent.

Further, according to Maxwell's equations, the sources of electromagnetic fields are charge and current distributions (including magnetization described effectively of a contribution to the current given by [tex]\vec{j}_{\text{max}}=\vec{\nabla} \times \vec{M}.[/tex] Admittedly, I've to think about the relativistic covariant formulation of this ;-)).

Darwin123

The magnetic field doesn't have to make the force that does the work on the electric current. It can change the direction of an electric current so that it is moving in the direction of the electric field.
The magnetic field doesn't have to cause an electric field that does the work on the electric current. The magnetic field can change the direction of the electric current so that it is in the direction of a previously existing electric field that is unchanged.
The ferromagnetic material already has loops of electric current even without a externa magnetic field applied to it. The loops of electrical current are randomly oriented at different points. The ferromagnetic material already has electric fields even without an external magnetic field applied to it. The electric fields are randomly oriented at different points.
If the electric fields and the electric currents are randomly oriented with respect to each other, the net work done by the electric fields is zero. This is the case before the external magnetic field is applied to the magnet.
When a magnetic field is applied, then there is a torque applied to the loops of electric current so that the axis of the loop is rotated. This torque does not work since the velocity of the electric charges is orthogonal to the electric fields.
I believe this is what happens in the case of two permanent magnets that are acting on each other.
The electric currents are oriented in the direction of the electric fields. When the electric currents are moving in the direction of the electric fields, the electric fields do work on the electric current.
No electric field has to be induced by the magnetic fields for the electric fields to do work. All that has to happen is that the axes of the current loops rotate. The electric fields can remain oriented precisely as they were before.
Electric current is not created. Electric field is not created. The electric current is rotated into the direction of the electric field.

gabbagabbahey

No, what I said was:

An you then claimed to have proved otherwise with some sketches you drew:

So, to clarify, which part of my quoted statement are you claiming to have disproved and why?

PhilDSP

And I have to think about the apparent disconnect between classical EM and the relativistic variant that lies in the effective redefinition of very many values such as E and B ;-) Does the Minkowski EM tensor preclude you from having separate fields where one (the former B field) deals only with the rotational aspects of EM and the other only the linear aspects of EM? Does the stepping from the transformational matrices of the Lorentz and Poincare groups into the tensor formulation force a reinterpretation of the distinction between rotation, boost and translation operations? (But this is certainly off-topic for this thread)

harrylin

Please clarify how your explanation works with two electromagnets like this (current loops 1 and 2; x = current direction into screen, 0=current direction out of screen):

1 x---0

2 x---0

I have not yet tried it, but this looks like a good example to work through, complete with numbers to calculate the force between them according to the different explanations.

K^2

I'm also talking about very basic physics. Do you understand how a gyroscope works? If not, look it up. The precession axis is perpendicular to applied torque. Gravity does zero work on a gyro.

In order to do work on an object with angular momentum (aligned with principal axis) torque vector must have a non-zero projection onto angular momentum vector. Torque due to magnetic field is always perpendicular to angular momentum of an ideal current loop or elementary dipole.

What happens to a physical loop of wire carrying a current in a magnetic field is a little different and involves electric field built up due to hall effect. It is that electric field that actually does work on the metal lattice of the wire carrying the current.


Just to make sure you understand. If you place a piece of wire carrying a current in a magnetic field, the magnetic field does not apply force on a wire. Wire isn't moving, so vxB=0. It's the electrons in the wire that are moving, get deflected to one side of the wire, causing that side of the wire to become negatively charged. That results in a net electric field in the wire, which acts now on the positively charged nuclei within the metal itself and move the wire.

harrylin

We have gone through that discussion before, perhaps you missed it - it's a mere matter of definitions. According to your definition of "work": if you pull a cart by means of a rope, do you do work on the cart or not? And if something does work on electrons which are part of a wire, does it do work on the wire or not?

If such a discussion over words is all that there is left to discuss, then I will from here on abstain from the discussion.

chingel

It is not similar to the cart pulling, because the magnetic field doesn't affect the wire, only the electrons, and it does no work on the electrons. With the cart you could argue that you do work on the rope and the rope does work on the cart, because it pulls the cart through a distance along the direction of movement, or whatnot, but the magnetic field does zero work on the electrons, which is the only thing it affects.

harrylin

Consequently, your analysis of the basic example in post #166 will be interesting.