How do magnetic fields do no work?

[varga]

You have zero understanding... without providing sources or sound arguments.

Ughh, look at your argument. I wanted to ignore your intrusion as you failed to specify anything in particular and even less make any actual arguments, but since I got some negative points regarding my previous post, as there is apperantly some "misinformation" there, then I decided to respond after all. To you I say, please WHAT is it you actually disagree about? I'll be happy to provide sources and answer all your questions, but what is it you are talking about? And to everyone else, I apologize for any misinformation and any false or ambiguous statements I made here, I did so only because it was written in my books and is my own (mis)understanding, but I will stand corrected as soon as someone actually points out what is it I am wrong about.

 

[varga]

While much of the subsequent discussion of microscopic currents in a magnetized solid may be valid, it takes us out of the practical realm--in fact, truly correct treatment of microscopic magnetization is not even classical, but requires quantum mechanics.

Let me say that I'm glad to finally see someone I agree with... at everything but this above. I do not think there is anything in QM that can come even close to describe (explain) orbital magnetic moment.

 

[marcusl]

I just found this post by Vanadium50 in another thread
https://www.physicsforums.com/showthread.php?t=347539"
that does a better job explaining my point than I am doing:

I don't like teaching the meme "magnetic fields do no work." It is true, but it is not useful.

It's clearly true for a single charged particle: the Lorentz force law has the magnetic force perpendicular to the direction of motion, so the dot product of force and displacement is always zero. It's also clearly true that magnets can do work on each other.

The solution to this apparent contradiction is that complex objects like magnets are made up of many charges, and these charges exhibit both electric and magnetic forces on each other, and if one does the calculation carefully enough, it can be shown that the work actually comes from these (usually internal) electric forces.

So what do you gain by thinking about things this way? To my mind, very little: you're trading a relatively simple calculation - say the torque on a magnetic dipole in a magnetic field - for a very complicated one involving internal electric forces. This seems like a poor trade. Note that I am not arguing that "magnetic fields do no work" is not true. I am arguing that it is not useful. It's (relatively recent) overemphasis is not, in my mind, a good thing.

Thanks V50. I wish I could have put it so clearly.

 

[Studiot]

Here is a challenge to those who say a magnetic field can do no work.

Design a non-magnetic system that will operate my solenoid door bell.

The requirements are that no part of the doorbell may be altered, except the magnetic generator may be removed if desired.

I maintain that there is no known force that can replace the magnetic coupling to transfer energy from the battery to the plunger.

 

[Frame Dragger]

Here is a challenge to those who say a magnetic field can do no work.

Design a non-magnetic system that will operate my solenoid door bell.

The requirements are that no part of the doorbell may be altered, except the magnetic generator may be removed if desired.

I maintain that there is no known force that can replace the magnetic coupling to transfer energy from the battery to the plunger.

You could reverse the circuit so that the doorbell only goes off when the circuit is broken. In that case you could use a spring and maybe the Strong or Weak force. Granted, you'd need a particle accelerator in your doorknob, but I think that's quite fashionable.

Anyway, you could just design one out of clockwork. Press the plunger against a piezoelectric element which sparks a tiny gap firing the circuit... etc. Of course, you lose the solenoid...

... Oh wait, nothing can be altered except the removal of a system. How is that conducive to DESIGNING a system?

 

[Studiot]

Since I have disassembled my solenoid in anticipation of a flood of non magnetic couplings and whilst I am waiting for all the geniuses (or genii?) here I am employing a man with a hammer. He told me his name was J Arthur Rank.

Seriously can you replace the magnetic coupling with some other sort of coupling?

The point is the solenoid transfers energy, magnetically to a mechanical striker and bell.

 

[kcdodd]

It is important to know what is doing work so you know where energy is coming from. Take the common example of a magnetic dipole "falling" in a magnetic field. You might say the magnetic field is doing work. However, you would be wronge. Why? Because the energy came from the dipole itself (the dipole actually *changes* as a result). If you said the magnetic field did work, in *addition* to the magnetic dipole changing because of the motion, then you have failed to conserve energy (you counted it exactly twice). So you have to choose one or the other.

We know how magnetic dipoles function in the presense of non-uniform magnetic fields, so you have one to choose empericaly. And you guessed it, the dipole energy did the work. So, magnetic field does not.

 

[Studiot]

What does this have to do with my doorbell?

The primary energy source is a battery.
This energy flows in (energises Mr Sulu) the primary circuit.

The only connection to the bell hammer motor is magnetic.

So energy is transferred from the primary circuit to the bell hammer motor by a magnetic field.

 

[marcusl]

Here is a challenge to those who say a magnetic field can do no work. Design a non-magnetic system that will operate my solenoid door bell.

Good sarcasm, but I'm afraid you and the posters immediately following you have missed the technical content of this thread.

I do not think there is anything in QM that can come even close to describe (explain) orbital magnetic moment.

You won't find much support for this view. Suggest you review orbital angular momentum in your QM books. It is the basis of diamagnetism, which classical physics predicts does not exist but which QM describes correctly. This is known as the Bohr-van Leeuwen theorem http://en.wikipedia.org/wiki/Bohr%E2%80%93van_Leeuwen_theorem" .

As one example, consider the contribution of free electrons to diamagnetic susceptibility in conducting solids (known as Landau diamagnetism). There is no contribution to magnetization M classically, basically because the effect of electrons executing Larmor orbits at the conductor surface is predicted to cancel that of those in the interior, to minimize global internal energy. It's a little reminiscent of eddy currents in a highly conductive solid. Quantum mechanically, on the other hand, the orbital angular momentum is quantized and cancellation cannot occur. For derivations, see books such as Harrison's Solid State Theory, Ashcroft and Mermin's Solid State Physics, or Pathria's Statistical Mechanics.

Ferromagnetism and paramagnetism are also quantum mechanical phenomena at their root. They derive from electron spin rather than orbital angular momentum, BTW.

The classical equations we have been discussing in this thread are practical bulk descriptions of microscopic electronic and atomic behavior. They can and should be used, and I continue to believe in the utility of the expressions I and others listed that give the work of one magnetic system on another. However as I said earlier, a proper microscopic view of magnetism is outside the scope of classical E&M and requires QM.

 

[Frame Dragger]

Good sarcasm, but I'm afraid you and the posters immediately following you have missed the technical content of this thread.

You won't find much support for this view. Suggest you review orbital angular momentum in your QM books. It is the basis of diamagnetism, which classical physics predicts does not exist but which QM describes correctly. This is known as the Bohr-van Leeuwen theorem http://en.wikipedia.org/wiki/Bohr%E2%80%93van_Leeuwen_theorem" .

As one example, consider the contribution of free electrons to diamagnetic susceptibility in conducting solids (known as Landau diamagnetism). There is no contribution to magnetization M classically, basically because the effect of electrons executing Larmor orbits at the conductor surface is predicted to cancel that of those in the interior, to minimize global internal energy. It's a little reminiscent of eddy currents in a highly conductive solid. Quantum mechanically, on the other hand, the orbital angular momentum is quantized and cancellation cannot occur. For derivations, see books such as Harrison's Solid State Theory, Ashcroft and Mermin's Solid State Physics, or Pathria's Statistical Mechanics.

Ferromagnetism and paramagnetism are also quantum mechanical phenomena at their root. They derive from electron spin rather than orbital angular momentum, BTW.

The classical equations we have been discussing in this thread are practical bulk descriptions of microscopic electronic and atomic behavior. They can and should be used, and I continue to believe in the utility of the expressions I and others listed that give the work of one magnetic system on another. However as I said earlier, a proper microscopic view of magnetism is outside the scope of classical E&M and requires QM.

I didn't miss the content, I simply responded to sarcasm in kind. I would say, as a science advisor you should probably at least anticipate that Studiot is going to post asking for his non-magnetic doorbell. You may as well get to explaining the issue in terms he understands before that, don't you think? It seems like a better role for you than just pointing out wit in print.

For the magnetic field not doing work, I have no question myself. I've seen the studies "seperating" dipoles in spin-ices, and doping for ferromagnetic properties in nonmagnetic materials, etc... so I'm not unclear on the QM nature of this issue. I understand that the field isn't doing work, I was simply being glib as Studiot was, and in the spirit of how repetative and inane this thread has become.

 

[Studiot]

Good sarcasm, but I'm afraid you and the posters immediately following you have missed the technical content of this thread.

I just checked back and yes the original post was still there, posted by what I took to be a mid-term student asking about one of the sweeping generalisations students are plagued with.

So often these generalisations cause students grief in later career as they can be very hard to shake when a better theory comes along or more advanced material is studied.

I don't know what the OP is studying, but offered a different worms eye view, which was acknowledged by the OP as last year's statement may well have been preparation for something in this year's curriculum.

Incidentally I am still waiting for the proponents of today's latest and greatest be-all and end-all theory to offer an alternative to magnetic energy transfer in my machine.

 

[Frame Dragger]

Oh look, he responded as predicted. Enjoy Marcusl!

 

[Repainted]

Wait so is the answer:
1. Magnetic fields never do work.
or
2. Magnetic fields do no work by themselves?

I'm guessing the answer is 2, because let's say if I have a square coil of wire, and I rotate it in a constant B-field, the magnitude of the velocity of a charge in the coil will simply be distance from the axis of rotation(r) x angular velocity(w).

Now it is possible to orient the coil such that the B-field and velocity have perpendicular components, while the Lorentz Force is directed along the coil. Hence the work integral along the coil isn't zero?

 

[berkeman]

Thread closed temporarily pending moderation.