## Does magnetic field do any work..

 Quote by burashka I am not sure I understand. The energy doesn't have to come from "somewhere". It is simply conserved at all moments in time.
The kinetic energy has to come from "somewhere." In order for the kinetic energy of the iron to go up, the energy of some other subsystem has to go down. That's what conservation of energy means. That's what I'm saying I want to understand, what other energy subsystem goes down? I don't understand how is this not clear to you.

 Quote by burashka But you have to account for all energy, including the energy of EM fields. In the example of a magnet and a nail, the magnet makes an H field around itself and that field has energy (and also mass according to E=mc^2). When the nail starts moving, it aquires some kinetic energy but the EM fields arond the objects also change and the total energy is conserved. It is however, not easy to see in detail how this happens. So it is better to consider the end-points, when everything is static.
You don't have to account for all the energy, only the energy subsystems that change significantly. I have a very hard time believing that there is significant radiation moving the weakly magnetized iron. The speeds here are such that v <<<< c, so any effect that is $$\beta^3$$ should be quite negligible. So I'm assuming that it's negligible.

That lagrangian you wrote down is not the related to the system I'm describing. You have charges in there, but there is nothing charged here. The magnet and the iron are both neutral.
 To simplify: http://en.wikipedia.org/wiki/dirac_bracket Wikipedia makes magnetic force a little clearer. Dirac bracket is a bit more direct than your explanation.
 That lagrangian you wrote down is not the related to the system I'm describing. You have charges in there, but there is nothing charged here. The magnet and the iron are both neutral.[/QUOTE] Any macroscopic object is made up of electrons, protons ane neutrons. Except for neutrons, these are charged particles. And if the object is magnetized, they surely move around with relativistic velocities. As you must know, magnetization is a relativistic effect. That's why it is usually so week. Well, it is not so week if it is caused by spins. But spins themselves are relativistic and can be only understood properly from the Dirack equation. And yes, the energy is in the field. When a system of charged particles move, the energy of the particles + the field must be conserved. When you turn on a lignt bulb, where do you think those 100Watts go? Same with magnetism.
 GUT advocates, please don't give me *that* look: perhaps we have a simpler explanation for monopole/dipole conversion than Dirac and Hawking allow: Rip apart a proton and we see UUD quarks. Couple UD and we have a -1/3 monopole. Couple DD and we have a -4/3 monopole. Etc. Free leptons also present as monopoles in lower dimensions Su/SUn>1d. Your thoughts?

 Quote by burashka That lagrangian you wrote down is not the related to the system I'm describing. You have charges in there, but there is nothing charged here. The magnet and the iron are both neutral.
Note that we're talking [I]fields[I]. EM fields are stronger than gravity here, depending on temperature. Note also that EM fields have strange effects on superconductors.

Why?

Magnetic domains like to stay aligned until we heat the magnet, disordering the domains. When we just heat the field, we see a concurrent increase in the field (Curie's Second Law).

Any macroscopic object is made up of electrons, protons ane neutrons. Except for neutrons, these are charged particles. And if the object is magnetized, they surely move around with relativistic velocities. As you must know, magnetization is a relativistic effect. That's why it is usually so week. [Sp: weak]

Observe conservation of energy: add energy to a field, the field strength increases: E=2/pi\RT.

Well, it is not so week [sp.weak] if it is caused by spins. But spins themselves are relativistic and can be only understood properly from the Dirack [sp: Dirac] equation.

And yes, the energy is in the field. When a system of charged particles move, the energy of the particles + the field must be conserved.

Finally, we concur.

When you turn on a lignt bulb, where do you think those 100Watts go?

Let's presume that we're talking HID lighting, among the most efficient in current use.
E=I/R (Ohm's law) gets us on the right track. Most of the energy, alas, goes straight to ground. Only about 2.5% is emitted as heat, photons, and scalar field. Just because a light draws 100W, does not mean that you get 100W out. Also, observe the 4th root law for em radiation. Light decreases in intensity by the fourth root of its distance from the source.

Back to magnetic fields. Inductance laws still appply.

Same with magnetism.[/QUOTE]
 When you turn on a lignt bulb, where do you think those 100Watts go? Let's presume that we're talking HID lighting, among the most efficient in current use. E=I/R (Ohm's law) gets us on the right track. Most of the energy, alas, goes straight to ground. Only about 2.5% is emitted as heat, photons, and scalar field. Just because a light draws 100W, does not mean that you get 100W out. Also, observe the 4th root law for em radiation. Light decreases in intensity by the fourth root of its distance from the source. Actually, I was thinking of the most inefficient bulb you can imagine. 100Watts go all to radiation (thermal and otherwise) since there is no other significant cannel apart from some weak heat conductivity. I don't understand what you mean by "go to the ground". The Joule's law applies to the filament and the energy is consumed from the power station by the filament. Then it heats up and starts to radiate.

Mentor
 Quote by burashka When you turn on a lignt bulb, where do you think those 100Watts go? Only about 2.5% is emitted as heat, photons, and scalar field. Just because a light draws 100W, does not mean that you get 100W out.
Only about 2.5% would go to light in an incandescent (without checking, that sounds about right), the rest does, in fact, go to heat. All 100 Watts is consumed and eventually it all ends up as heat (even the light, if your window shades are closed).

 Quote by russ_watters Only about 2.5% would go to light in an incandescent (without checking, that sounds about right), the rest does, in fact, go to heat. All 100 Watts is consumed and eventually it all ends up as heat (even the light, if your window shades are closed).
And where does the heat go, do you suppose?
 "And where does the heat go, do you suppose?" There are 3 main methods. Diffusion (spreading out via direct contact), convection (movement due to flow induced by density differential), and radiation (light). The big problem with lightbulbs is that they emit much more infrared than visible light.

 Quote by fedaykin "And where does the heat go, do you suppose?" There are 3 main methods. Diffusion (spreading out via direct contact), convection (movement due to flow induced by density differential), and radiation (light). The big problem with lightbulbs is that they emit much more infrared than visible light.
Yes, exactly. In fact, most of the energy goes straight to radiation. I think I have said previously "visible and otherwise". Of course, we consider here the filament only. For the tiny wire that heats up inside the bulb, the mechanisms of heat transfer related to heat diffusion and convection (presumably, but the gases inside the bulb) are absolutely insignificant.

The point was that radiation must be included in the energy balance. Whenever electrodynamic interaction occurs and the EM energy of the initial and final states differ, you must take the radiation into account. Of course, this does not need to be visible radiation, or even infrared.
 "Most of the energy, alas, goes straight to ground. Only about 2.5% is emitted as heat, photons, and scalar field. Just because a light draws 100W, does not mean that you get 100W out." The lightbulb dissipates the power the flows across it. $$P=I^2*R$$ None of the power dissipated by the bulb goes to ground. If a lightbulb draws 100W, and there is not reactive power (lightbulbs usually have very little), you will get 100W out. You might read up a bit on electric circuits. Most of the 100% (100W) is given out as heat, either through infrared or thermal conduction.