Magnetic fields do no work? How come...

[elliotr]

According to every textbook I know of, magnetic forces do no work (e.g. David Griffiths Pg. 207). Yet this problem causes this to be hard to believe:

If I take two magnets, I can set them down on a table (with a little friction). I then slowly push them toward each other, then at some point, the two magnets attract and move toward each other. That is, two magnets appear to exert a force on each other, and this force is exerted over the distance it takes to make contact (even with a little friction). Surly something is doing work to at least counteract the friction, if not cause non-zero mass magnets to accelerate.

How can you tell me that magnetic forces do no work? These two magnets appear to do work, as far as I can tell.

This problem has bothered me for a long time. Please explain!

Thanks.

[Nabeshin]

Haven't read Griffiths at all so I'm not sure exactly what he's talking about, but this statement is usually made with regards to the magnetic force on a charged particle. In this case, it's pretty trivial to show since velocity and force are perpendicular.

[elliotr]

Quote: "Magnetic Forces do no work" in bold, in a box, in the middle of the page.

But between two magnets, what forces other than magnetic forces exist? What force causes them to move together?

Thanks

[Pengwuino]

I believe I remember what Griffiths was talking about. I think he was talking about how an electron or current would gain energy but from where? He should have specifically stated that the energy comes from whatever is maintaining that magnetic field. I know he explained it... wonder where my copy is and if it still hasn't fallen apart.

[elliotr]

A better question: "Can a magnet (e.g. a permanent magnet) do work?"

If no: how do magnets move together appearing to do work? What's actually doing the work (see first question).

If yes: how is this possible when "magnetic forces do no work"?

[Phrak]

While you have two stationary magnets, obviously no work is done, one on the other.

A magnet, in moving, posesses an electric field.

[elliotr]

That's a very good point. But let's say that I bring the magnets to a halt after I feel the force of magnets wanting to move toward each other. At this point, the magnets are not moving, so no electric field exists. Yet when I let go, they will still move toward each other.

[Phrak]

That's a very good point. But let's say that I bring the magnets to a halt after I feel the force of magnets wanting to move toward each other. At this point, the magnets are not moving, so no electric field exists. Yet when I let go, they will still move toward each other.

It is late for me, and this is my last post. I think you are questioning the disparity--work is done by the electric fields. But the force developed, when the magnets are stationary, is magnetic. So how is this resolved?

Wonderful question!

I'm sorry not to answer at this time.

[kmarinas86]

According to every textbook I know of, magnetic forces do no work (e.g. David Griffiths Pg. 207). Yet this problem causes this to be hard to believe:

If I take two magnets, I can set them down on a table (with a little friction). I then slowly push them toward each other, then at some point, the two magnets attract and move toward each other. That is, two magnets appear to exert a force on each other, and this force is exerted over the distance it takes to make contact (even with a little friction). Surly something is doing work to at least counteract the friction, if not cause non-zero mass magnets to accelerate.

How can you tell me that magnetic forces do no work? These two magnets appear to do work, as far as I can tell.

This problem has bothered me for a long time. Please explain!

Thanks.

Virtually anything capable of doing work involves a carrier of energy. Mostly this involves the electrostatic repulsion between particles (e.g. rocket vs. exhaust, tires vs. road, feet vs. floor, etc.). In other cases you have magnetic fields such as can be found in magnetic levitation trains and linear induction motors in modern roller coaster rides. Carriers of energy do not do the work, they simply deliver the energy.

Energy ultimately comes from utilizing charge potential, be it nuclear or chemical. Even solar energy is derived this way. There is no other way of adding energy into a system, period. Everything else is simply the hot potato-ing of this energy.

[Dadface]

An energy transfer is needed to change the separation of two magnets or a magnet and an unmagnetised magnetic material.When attracting magnets are pulled further apart or when repelling magnets are pushed closer together an energy input is needed and there is a change of magnetic field with a resulting increase of potential energy.The stored energy can then do work to push/pull the magnets back in the opposite direction.Like others I think the book was referring to the magnetic force on moving charged particles, in which case the field does no work.

[kmarinas86]

An energy transfer is needed to change the separation of two magnets or a magnet and an unmagnetised magnetic material.When attracting magnets are pulled further apart or when repelling magnets are pushed closer together an energy input is needed and there is a change of magnetic field with a resulting increase of potential energy.The stored energy can then do work to push/pull the magnets back in the opposite direction.Like others I think the book was referring to the magnetic force on moving charged particles, in which case the field does no work.

So is it true that no work is being done on charged particles in the sun's corona? They are not gaining kinetic energy?

Obviously it can't be the magnetic field. It must be some other force.

So their trajectory is determined by nuclear fusion? :rofl:

I don't think so....:uhh:

Sunspots are not produced by magnetism? Obviously no work was done by the Lorentz force to produce those. They're really a byproduct of a combination of fusion and gravity............. ?

GIVE ME A BREAK! :mad:

[Bob S]

Here is an interesting experiment. Use a permanent magnet (PM) and build a solenoid magnet about the same size. Use low resistance wire. Power the solenoid with a constant-current source (you can use two npn transistors and a resistor). Put a voltmeter across the solenoid. Move the PM quickly up to, or away from the solenoid. Do you see a voltage pulse on the voltmeter? Why? What is the approximate volt-seconds? The energy (joules) is the volt-sec times the solenoid current. Where is this energy coming from, or going to? (If the voltage pulse is the same sign as the current, the current regulator is doing work. If the sign is opposite, the current regulator is absorbing energy.)
Bob S

[Vanadium 50]

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.

As far as the solar corona, I am not a solar physicist, but I do want to point out that the sun's magnetic field is far from static, and a changing magnetic field produces an electric field, and electric fields can do work.

[Dadface]

I hope I am summarising the majority of different responses here correctly.
1.Work can be done on or by magnetic fields.
2.Work can be done on or by electric fields.
3.There is no work done by the Lorentz force on charged particles.

I think that it's point 3. that the text books elliotr is using referred to and it would be instructive to see a relevant quote from the book so that the question can be answered in context.

kmarinas 86 will you please clarify your post(11) where you seem to imply that as far as stellar events are concerned work is,in fact, done by the Lorentz force.This force is highly instrumental in determining the paths of the particles,it can accelerate the particles by changing their directions but how,for example,can this force,on its own, change their kinetic energies?

[lugita15]

I think the core problem at issue can be illustrated with a superficially different example. Consider two parallel current carrying wires. Each will have an attractive magnetic force on the other, and so the two wires will come closer together. Since the magnetic force is in the same direction as the direction of motion, work is being done by the magnetic force.

So how is this possible?

[diazona]

The magnetic force in that case just acts on the electrons that are moving through each wire. It tries to bend the trajectories of the electrons in one wire toward the other wire, and vice versa. No work is involved in that part. But the electrons attract the positively charged atomic nuclei in the wire by the electric force, and that force does the work of moving the wire.

Whenever it looks like magnetic forces are doing work, if you think about it closely enough it actually turns out to be the electric force.

[kmarinas86]

The magnetic force in that case just acts on the electrons that are moving through each wire. It tries to bend the trajectories of the electrons in one wire toward the other wire, and vice versa. No work is involved in that part. But the electrons attract the positively charged atomic nuclei in the wire by the electric force, and that force does the work of moving the wire.

Whenever it looks like magnetic forces are doing work, if you think about it closely enough it actually turns out to be the electric force.

If that is true, then the forces in question should not behave differently in relativistic scenarios. But if they do, we should called it the electromagnetic force instead.

[cabraham]

The magnetic force in that case just acts on the electrons that are moving through each wire. It tries to bend the trajectories of the electrons in one wire toward the other wire, and vice versa. No work is involved in that part. But the electrons attract the positively charged atomic nuclei in the wire by the electric force, and that force does the work of moving the wire.

Whenever it looks like magnetic forces are doing work, if you think about it closely enough it actually turns out to be the electric force.

Not! A magnet held over a paper clip lifts said clip by magnetic force, NOT electric force. The clip is increased in potential energy. The magnetic force did the work, not the electric.

With 2 wires carrying current, the mag force is responsible for the interacting force between the wires, NOT electric.

Any peer-reviewed text will elaborate.

Claude

[Phrak]

Not! A magnet held over a paper clip lifts said clip by magnetic force, NOT electric force. The clip is increased in potential energy. The magnetic force did the work, not the electric.

With 2 wires carrying current, the mag force is responsible for the interacting force between the wires, NOT electric.

Any peer-reviewed text will elaborate.

Claude

I don't know what stimulated this excited response, but it wasn't through careful analysis.

This problem is better analysed by a replacement analogy where we can deal with things we can better understand. Replaced the permanent magnet with a solenoid and the paper clip with an array of wire loops.

The electric field, not the magnetic field is responsible for the work done.

Can you see that the electric field of the permanent magnet is responsible for the work done on the paper clip in the inertial frame of the paper clip? A changing magnetic field produces an electric field. This should be quantified in any text that covers dynamic electromagnetism.

[Phrak]

I hope I am summarising the majority of different responses here correctly.
1.Work can be done on or by magnetic fields.
2.Work can be done on or by electric fields.
3.There is no work done by the Lorentz force on charged particles.


Where are the mentors?

1. 1. is wrong.

3. 3. is wrong. You need to look up the Lorentz force

[kanato]

Playing devil's advocate here, there are a couple of obvious objections I see to that analysis:

In the laboratory reference frame, it seems pretty clear that the magnetic field from the solenoid is constant, so there is no electric field, and the work done on the paper clip is caused by the magnetic field in that frame. Since analysis in any reference frame is equivalent to any other reference frame, I don't see why it is invalid to say magnetic fields do work.

The rest frame of the paper clip is non-inertial, so that complicates the analysis. Also, there is both a magnetic and electric field in that frame, so it's not clear that the electric field is responsible for the force. Especially since there will be an induced current in the paper clip, which will experience a force from the magnetic field.

[kanato]

I'm reminded of another objection:

Usually the argument that the magnetic field does no work is made by boosting to a frame where the current is zero, so the only force can come from the electric field. But consider an electron in a static magnetic field with a gradient. The electron has a permanent magnetic dipole moment, so it will feel a force from the magnetic field. There is no reference frame you can boost to where either the magnetic field disappears, or the magnetic dipole of the electron disappears (I think). So in every possible inertial frame, there is a contribution to the force that comes purely from the magnetic field.

[diazona]

Playing devil's advocate here, there are a couple of obvious objections I see to that analysis:

In the laboratory reference frame, it seems pretty clear that the magnetic field from the solenoid is constant, so there is no electric field, and the work done on the paper clip is caused by the magnetic field in that frame. Since analysis in any reference frame is equivalent to any other reference frame, I don't see why it is invalid to say magnetic fields do work.
Actually there is an electric field produced by the charged particles that make up the paper clip. That's the field that directly does the work. Roughly speaking, the magnetic field only alters the motion of electrons so that they are in a position to do work.

I'm reminded of another objection:

Usually the argument that the magnetic field does no work is made by boosting to a frame where the current is zero, so the only force can come from the electric field. But consider an electron in a static magnetic field with a gradient. The electron has a permanent magnetic dipole moment, so it will feel a force from the magnetic field. There is no reference frame you can boost to where either the magnetic field disappears, or the magnetic dipole of the electron disappears (I think). So in every possible inertial frame, there is a contribution to the force that comes purely from the magnetic field.
The electron in that case would feel a torque, not a force, from the interaction of its magnetic dipole moment with the magnetic field.

[Dadface]

I hope I am summarising the majority of different responses here correctly.
1.Work can be done on or by magnetic fields.
2.Work can be done on or by electric fields.
3.There is no work done by the Lorentz force on charged particles.

I think that it's point 3. that the text books elliotr is using referred to and it would be instructive to see a relevant quote from the book so that the question can be answered in context.

kmarinas 86 will you please clarify your post(11) where you seem to imply that as far as stellar events are concerned work is,in fact, done by the Lorentz force.This force is highly instrumental in determining the paths of the particles,it can accelerate the particles by changing their directions but how,for example,can this force,on its own, change their kinetic energies?

Where are the mentors?

1. 1. is wrong.

3. 3. is wrong. You need to look up the Lorentz force

I suppose one out of three is better than nothing.You have me particularly confused with your answer to point number three.I took your advice and I looked up the Lorentz force and everything I have read so far, but this is only via google and not peer reviewed publications, seems to confirm what I thought I already knew and understood....because there is a zero component of motion in the direction of the force then by definition the work done is zero.If this is wrong then I would appreciate it if you could provide me with a suitable reference that I could look up.
I also take some issue with your answer to point number one,and as I see it so far(but I am still thinking about this) work can be done by say two permanent magnets or a permanent magnet and a ferromagnet.I think some good examples of this have been given in this thread.Is it a valid exercise to consider an electromagnet as being equivelent to a permanent magnet?At the moment I have very strong reservations about this not the least reason being that the internal mechanisms responsible for creating the fields are different this,possibly,resulting in differences in the fields themselves.

[cabraham]

I don't know what stimulated this excited response, but it wasn't through careful analysis.

This problem is better analysed by a replacement analogy where we can deal with things we can better understand. Replaced the permanent magnet with a solenoid and the paper clip with an array of wire loops.

The electric field, not the magnetic field is responsible for the work done.

Can you see that the electric field of the permanent magnet is responsible for the work done on the paper clip in the inertial frame of the paper clip? A changing magnetic field produces an electric field. This should be quantified in any text that covers dynamic electromagnetism.

The permanent magnet field is static, not changing. The instant the paper clip is attracted and begins moving towards the magnet it is feeling magnetic, not electric force. This is a static condition, not time vasrying.

Motors have electric fields as well as magnetic. But the E forces are much too weak to produce the motor action. The H forces are way stronger. Of course a time changing H will always be accompanied by an E field as well, since they are inclusive (time changing conditions).

The force betwen 2 wires is magnetic. The notion that H fields do no work is only under specific conditions. A charged particle, an electron, will move in the same direction as E, but normal to H. Thus E can do work on the e-, whereas H does not. H only changes the e- direction.

But 2 wires can be attracted/repulsed due to H. The mag force is published in every reference. The definition of the ampere unit is based on 2 wires and the force incurred when carrying current. The fact that 2 wires carrying currents in opposite directions incur a repulsive force cannot be explained with E fields, but is perfectly consistent with H fields.

Just use the right hand rule and see what I mean. Every text cannot be wrong. If what you say is true, why doesn't every university teach it? These questions have been under scrutiny since the 19th century. It's a bit presumptuous to think that all the physics & EE curriculums have had it wrong for 2 centuries, and that you have the right answer.

As far as reference frames & relativity goes, we don't need to go there. This can be explained with classic e/m fields.

Claude

[kanato]

Actually there is an electric field produced by the charged particles that make up the paper clip. That's the field that directly does the work. Roughly speaking, the magnetic field only alters the motion of electrons so that they are in a position to do work.

In the reference frame of the paper clip, there is a current from the charged particles moving, so there is a magnetic force which does work. There may also be an electric field, but there is still a magnetic force.

The electron in that case would feel a torque, not a force, from the interaction of its magnetic dipole moment with the magnetic field.

Two comments: 1) the torque of a magnetic field \vec{\tau} = \vec{\mu} \times \vec{B} does work; any elementary text should write down the potential energy U = -\vec{\mu} \cdot \vec{B} (indicating a conservative force and therefore it can do work), and 2) I specifically mentioned that the B field has a gradient, and in a non-uniform field a magnetic dipole will experience a force as well as a torque. This is why magnets are attracted to each other in a laboratory reference frame.

[Bill Foster]

\textbf{F}=q\left(\textbf{E}+\textbf{v}\times\text bf{B}\right)
\int{\textbf{F}\cdot d\textbf{l}}=q\left(\int{\textbf{E}\cdot d\textbf{l}}+\int{\textbf{v}\times\textbf{B}\cdot d\textbf{l}}\right)
W=q\int{\textbf{E}\cdot d\textbf{l}}
\frac{W}{q}=\int{\textbf{E}\cdot d\textbf{l}}
V=\int{\textbf{E}\cdot d\textbf{l}}

[cabraham]

\textbf{F}=q\left(\textbf{E}+\textbf{v}\times\text bf{B}\right)
\int{\textbf{F}\cdot d\textbf{l}}=q\left(\int{\textbf{E}\cdot d\textbf{l}}+\int{\textbf{v}\times\textbf{B}\cdot d\textbf{l}}\right)
W=q\int{\textbf{E}\cdot d\textbf{l}}
\frac{W}{q}=\int{\textbf{E}\cdot d\textbf{l}}
V=\int{\textbf{E}\cdot d\textbf{l}}

I already stated that this is for a single particle. A current in a wire has an H field in addition to an E field. I'm at work and can't elaborate, but the math for the magnetic force between 2 wires with current is needed.

The H field does no work on an electron. The Lorentz force law is universally known. Some are trying to generalize by saying that the force between 2 wires w/ current is not magnetic, but electric. Impossible.

The right hand rule comes to mind. Also, how can one explain using E only and not H, the following. When the 2 currents in the wires are in the same direction, the wires attract. In opposite directions, the wires repel. You cannot explain that with E fields, but with H fields and the right hand rule it's all too easy.

Please start 'splaining.

Claude

[Phrak]

You have me particularly confused with your answer to point number three. I took your advice and I looked up the Lorentz force and everything I have read so far, but this is only via google and not peer reviewed publications, seems to confirm what I thought I already knew and understood....because there is a zero component of motion in the direction of the force then by definition the work done is zero.If this is wrong then I would appreciate it if you could provide me with a suitable reference that I could look up.

Sorry. I was getting a little punchy--and inacurate.

Try googling force on magnetic dipole.

[kmarinas86]

Actually there is an electric field produced by the charged particles that make up the paper clip. That's the field that directly does the work. Roughly speaking, the magnetic field only alters the motion of electrons so that they are in a position to do work.


The electron in that case would feel a torque, not a force, from the interaction of its magnetic dipole moment with the magnetic field.

Torque is the vector cross product of force and turning radius. That is absolute and final.

Signed,

Kmarinas86

[kmarinas86]

The right hand rule comes to mind. Also, how can one explain using E only and not H, the following. When the 2 currents in the wires are in the same direction, the wires attract. In opposite directions, the wires repel. You cannot explain that with E fields, but with H fields and the right hand rule it's all too easy.

Please start 'splaining.

Claude

None of these people want to do that.

Sad....

[Doc Al]

The right hand rule comes to mind. Also, how can one explain using E only and not H, the following. When the 2 currents in the wires are in the same direction, the wires attract. In opposite directions, the wires repel. You cannot explain that with E fields, but with H fields and the right hand rule it's all too easy.

Please start 'splaining.

That's an easy one. The magnetic field from one wire deflects the charge carriers (the electrons) in the other wire to one side, creating a static electric field which exerts a force on the positive lattice of the wire. That force pulls the wires together.

None of these people want to do that.

Sad....
I guess you don't understand it either. How sad!

[cabraham]

That's an easy one. The magnetic field from one wire deflects the charge carriers (the electrons) in the other wire to one side, creating a static electric field which exerts a force on the positive lattice of the wire. That force pulls the wires together.


I guess you don't understand it either. How sad!

Would you please elaborate on the direction of the said field and lattice polarization? Let's use cylindrical coordinates. Current in both wires is in the +z direction in case 1. In case 2, currents are in the +z & -z directions.

Case 1) I did a scratch diagram on a post it note. I'll sketch a nice diagram at home tonight and post it. In my quick off the cuff analysis, for 2 currents in the +z direction, the electrons shift towards the exterior of the wire pair per the Lorentz force law relation. This would result in a repulsive force considering only the E field. But we know that like currents attract, not repel. I don't think the E field contribution to the force is nearly as much as the H field.

Both are present and exert influence, but H is clearly the stronger of the two. Again, I'm at work, so I won't make an absolute pronouncement till I get home tonight. You could be right, but it looks like you're not.

Case 2) The same as case 1) with polarites reversed. Again, it appears that the force is opposite to what you describe. The electrons are shifted to the interior of the wire pair, again resulting in a repulsive force. Of course, the wires do repulse, but the E fields seem to produce repusion regardless of the polarity of the currents. The H fields using the right hand rule are consistent with observation.

Is there something I've overlooked? It appears that the repulsive/attractive forces due to currents in the 2 wires is strongly determined by H, and only weakly a function of E. If I've erred, please let me know.

Claude

[Doc Al]

Case 1) I did a scratch diagram on a post it note. I'll sketch a nice diagram at home tonight and post it. In my quick off the cuff analysis, for 2 currents in the +z direction, the electrons shift towards the exterior of the wire pair per the Lorentz force law relation.
They shift towards the interior. (They move opposite to the conventional current.)

[cabraham]

They shift towards the interior. (They move opposite to the conventional current.)

Sure. But does that change anything? With 2 currents in the same direction, the electrons shift inward creating a repulsive force between the 2 wires. But we observe that the force is attractive. How?

Answer - there are both electric and magnetic components of force. The mag is much stronger. In machines class this was repeated often. What you're saying is not what is being disputed. What lies at the heart of this is that the H field attraction exists in addition to the E field attraction. Both are present, but the force due to H is stronger than that due to E.

It's quite obvious that one cannot equate this 2 wire system attractive force with that of electron-positive-ion electric field interaction. It's basically magnetic in nature. Later tonight, I'll post a detailed diagram. BR.

Claude

[Doc Al]

Sure. But does that change anything? With 2 currents in the same direction, the electrons shift inward creating a repulsive force between the 2 wires. But we observe that the force is attractive. How?
The shifted electrons exert an electrostatic pull on the positive wire lattice, thus creating the attraction between the wires.

Answer - there are both electric and magnetic components of force. The mag is much stronger. In machines class this was repeated often. What you're saying is not what is being disputed. What lies at the heart of this is that the H field attraction exists in addition to the E field attraction. Both are present, but the force due to H is stronger than that due to E.
The electrostatic force on the electrons is equal to the magnetic force.

[qsa]

Question

Since magnetic forces can do no work, what force IS doing the work when a bar magnet causes a paper clip to jump off a table and stick to the magnet?

Asked by: Steven Leduc

Answer

The original assumption that a magnetic field can do no work is incorrect. A magnetic field has an energy density that is equal to the magnetic induction (B) squared divided by twice the permeability (mu sub zero). If you were to sum (integrate) this energy of the magnet over all of its field before it picked up the paper clip and compared it to the same sum after you picked up the paper clip, you would discover that there was a loss of field energy. The paper clip has in effect 'shorted out some lines of magnetic flux'.

How much energy was lost? If you took hold of the paper clip and pulled it out to such a distance that the magnetic pull was insignificant, the work you did in this process would exactly equal the amount of energy lost when the clip was on the face of the magnet. When you picked up the clip with the magnet the clip was accelerated toward the magnet acquiring kinetic energy. This kinetic energy will equal, ignoring air drag, the loss of magnetic energy in the field. This kinetic energy will be dissipated in the form of heat on impact of the clip with the magnet.

For further understanding of the energy in a magnetic field, you may want to study magnetic fields in solenoids. See the Reference below.

Physics, Volume 2 by Halliday and Resnick

Answered by: Robert Gardner, M.S., Retired Physicist

[Doc Al]

Question

Since magnetic forces can do no work, what force IS doing the work when a bar magnet causes a paper clip to jump off a table and stick to the magnet?

Asked by: Steven Leduc

.............
Answered by: Robert Gardner, M.S., Retired Physicist
Note that this "answer" doesn't really address the question of what force does the work. :wink:

[qsa]

Note that this "answer" doesn't really address the question of what force does the work. :wink:

I thought I should just respond to the original question since it was side stepped into another
detail. As to exactly what force, it seems more complicated and have to be analysed with QED and quantum vs classical (since magnets are classical objects) and other detail configurations.

[cabraham]

The shifted electrons exert an electrostatic pull on the positive wire lattice, thus creating the attraction between the wires.


The electrostatic force on the electrons is equal to the magnetic force.

But these same electrons repulse each other. The electrons are on the interior hence the 2 wires are more influenced by the repulsion as the electrons are closer together than the positive lattice charge. The electrons are not anchored to anything. What you describe makes no sense if you draw the diagram. Although the electrons attract the latice in their own wire, this surface density of electrons in each wire push the wires away. Also, just as the electrons attract the lattice, so does the lattice attract said electrons. Which is more of an "anchor", the lattice, or electrons? It has to be the lattice. The lattice is much more massive and less mobile than the free electrons which are very low in mass. The electrons in each wire are drawn towards their own lattice indicating repulsion, and the electrons in each wire repulse each other as well. But observation shows attraction!

Also, if the currents are opposite, the electrons are on the exterior. The lattice positive charges repulse. Of course the electrons on the exterior attract the lattice charge, but again these electrons are not anchored. The lattice charges also attract the electrons inciting an attractive force. We know that the wires repulse with opposite currents.

It can only be magnetic forces accounting for observed phenomena, not electric. Your scenario makes no sense at all. Have you taken a course on motors/generators? This issue is old. Any M/G course, aka "energy conversion" explains these questions in detail.

Case closed.

Claude

[cabraham]

Question

Since magnetic forces can do no work, what force IS doing the work when a bar magnet causes a paper clip to jump off a table and stick to the magnet?

Asked by: Steven Leduc

Answer

The original assumption that a magnetic field can do no work is incorrect. A magnetic field has an energy density that is equal to the magnetic induction (B) squared divided by twice the permeability (mu sub zero). If you were to sum (integrate) this energy of the magnet over all of its field before it picked up the paper clip and compared it to the same sum after you picked up the paper clip, you would discover that there was a loss of field energy. The paper clip has in effect 'shorted out some lines of magnetic flux'.

How much energy was lost? If you took hold of the paper clip and pulled it out to such a distance that the magnetic pull was insignificant, the work you did in this process would exactly equal the amount of energy lost when the clip was on the face of the magnet. When you picked up the clip with the magnet the clip was accelerated toward the magnet acquiring kinetic energy. This kinetic energy will equal, ignoring air drag, the loss of magnetic energy in the field. This kinetic energy will be dissipated in the form of heat on impact of the clip with the magnet.

For further understanding of the energy in a magnetic field, you may want to study magnetic fields in solenoids. See the Reference below.

Physics, Volume 2 by Halliday and Resnick

Answered by: Robert Gardner, M.S., Retired Physicist

Very well stated. This is not even a legitimate debate. Any close examination of all forces reveals as plain as day that the E fields exert much less force than the H fields. The observed forces are consistent with the H field influence, and counter that of the E field. It's clear that E has the small influence and H is what plays the much larger role.

This is so open and shut, there is no reason to continue.

Good job and thanks for your input.

Claude

[Phrak]

Very well stated. This is not even a legitimate debate. Any close examination of all forces reveals as plain as day that the E fields exert much less force than the H fields. The observed forces are consistent with the H field influence, and counter that of the E field. It's clear that E has the small influence and H is what plays the much larger role.

This is so open and shut, there is no reason to continue.

Good job and thanks for your input.

Claude

The given explanation is a lot of mush that says nothing. I could take this reasoning and say that a magnet equally attracts aluminum.

Secondly, the OP question does not belong in the classical physics folder where magnetic dipoles are made of charge, and where magnetic fields do no work on charge.

[kanato]

I could take this reasoning and say that a magnet equally attracts aluminum.

No you couldn't.. that explanation requires the assumption that the permeability of the material that makes up the paperclip be different than the permeability of vacuum, otherwise the magnetic energy is the same regardless of where the paperclip is located. The permeability of aluminum is the same as that of vacuum for 4 significant figures or so.

[Phrak]

No you couldn't.. that explanation requires the assumption that the permeability of the material that makes up the paperclip be different than the permeability of vacuum, otherwise the magnetic energy is the same regardless of where the paperclip is located. The permeability of aluminum is the same as that of vacuum for 4 significant figures or so.

Yah. You're right. I'm batting 0 for 3 in this thread. My excuse is ...well, nevermind, I haven't got one.

Where does permiability come from?

[kanato]

Where does permiability come from?

It depends on the details of the material, but for a nonmagnetic metal (like aluminum), the first approximation of the susceptibility \chi_m = \mu / \mu_0 of the material would be Pauli paramagnetism.

That's an easy one. The magnetic field from one wire deflects the charge carriers (the electrons) in the other wire to one side, creating a static electric field which exerts a force on the positive lattice of the wire. That force pulls the wires together.

Is there a similar explanation for the Stern-Gerlach experiment? In my mind this is a strong indicator that the magnetic field can do work, since it is the force applied to the permanent magnetic dipole of the electron that is responsible for this effect, and the two different beams show up only due to the two different directions of this dipole.

[Doc Al]

But these same electrons repulse each other. The electrons are on the interior hence the 2 wires are more influenced by the repulsion as the electrons are closer together than the positive lattice charge. The electrons are not anchored to anything. What you describe makes no sense if you draw the diagram. Although the electrons attract the latice in their own wire, this surface density of electrons in each wire push the wires away. Also, just as the electrons attract the lattice, so does the lattice attract said electrons. Which is more of an "anchor", the lattice, or electrons? It has to be the lattice. The lattice is much more massive and less mobile than the free electrons which are very low in mass. The electrons in each wire are drawn towards their own lattice indicating repulsion, and the electrons in each wire repulse each other as well. But observation shows attraction!
We're not talking about the electrostatic force between the wires--that's negligible. We're talking about the electrostatic force within each wire.

The magnetic force deflects the moving electrons. Do they go flying off of the wire? No. What stops them? Their deflection due to the magnetic force is balanced by the attraction of the positive lattice--an electrostatic force. That electrostatic force is what literally pulls the more massive wire.

Ask yourself: What pulls the massive positive lattice of the wire? It obviously can't be a magnetic force, since there is no magnetic force on stationary charges.

Also, if the currents are opposite, the electrons are on the exterior. The lattice positive charges repulse. Of course the electrons on the exterior attract the lattice charge, but again these electrons are not anchored. The lattice charges also attract the electrons inciting an attractive force. We know that the wires repulse with opposite currents.

It can only be magnetic forces accounting for observed phenomena, not electric. Your scenario makes no sense at all. Have you taken a course on motors/generators? This issue is old. Any M/G course, aka "energy conversion" explains these questions in detail.
:rolleyes: I could just as well ask if you've ever taken a physics course. Yes, this issue is old.


Very well stated. This is not even a legitimate debate. Any close examination of all forces reveals as plain as day that the E fields exert much less force than the H fields. The observed forces are consistent with the H field influence, and counter that of the E field. It's clear that E has the small influence and H is what plays the much larger role.

This is so open and shut, there is no reason to continue.
:rofl: When in doubt, simply declare victory. Good job!

Kidding aside, this is not something I would expect you'd learn in an engineering class. It's only something of interest to physics pedagogues. (Who need to be prepared when students read statements such as the one by Griffiths quoted in the first post.) Looking back at this thread, I see that I essentially agree with what diazona stated in post #16. But I also agree with the point that Vanadium 50 makes in post #13--overemphasizing these details is not particularly helpful.

[Doc Al]

Is there a similar explanation for the Stern-Gerlach experiment?
No, since the magnetic moment of the electron is considered to be an intrinsic magnetic moment.

[arithmetix]

Hi
If work=force times distance, then my guess is that whoever supplies the force does the work.

[cabraham]

We're not talking about the electrostatic force between the wires--that's negligible. We're talking about the electrostatic force within each wire.

The magnetic force deflects the moving electrons. Do they go flying off of the wire? No. What stops them? Their deflection due to the magnetic force is balanced by the attraction of the positive lattice--an electrostatic force. That electrostatic force is what literally pulls the more massive wire.

Ask yourself: What pulls the massive positive lattice of the wire? It obviously can't be a magnetic force, since there is no magnetic force on stationary charges.

:rolleyes: I could just as well ask if you've ever taken a physics course. Yes, this issue is old.



:rofl: When in doubt, simply declare victory. Good job!

Kidding aside, this is not something I would expect you'd learn in an engineering class. It's only something of interest to physics pedagogues. (Who need to be prepared when students read statements such as the one by Griffiths quoted in the first post.) Looking back at this thread, I see that I essentially agree with what diazona stated in post #16. But I also agree with the point that Vanadium 50 makes in post #13--overemphasizing these details is not particularly helpful.

So you're saying that the H force pulls the electrons to the interior, but the E force from the lattice provides an opposing force. But as I stated, just as the lattice is attracted to the electrons, so are the electrons attracted to the lattice. If the lattice incurs a force of attraction to the other wire, is it the E or H force that is accountable? The E force alone won't do it. The electrons would be attracted to the lattice, not vice versa. The mass of the lattice is enormous vs. the electrons. Of course, the electrons do not go jumping off the wire. But their attractive force on the lattice cannot happen without the H force holding them. The strong H force yanks the electrons towards the interior. The lattice and electrons exert a mutual E force on each other. Without the H force, the electrons would be drawn to the lattice. But the strong H force holds the electrons so that the lattice in addition to the electrons are moved in the direction of attraction.

The strong H force is ultimately responsible. I've already stated that E plays a role, but the main contribution is H force. I regard the electrons as an array of ping pong balls, and the lattice as an array of bowling balls. PP balls do not attract bowling balls forcing them to move unless a very strong force is holding the PP balls in place. That would be the H force. Otherwise, the PP balls would be forced to move towards the bowling balls.

The fact that there is attraction between the electrons & the lattice due to E force, one you've emphasized, has never been disputed. I'm just asking that you consider all forces involved. Heck, in cases 1) & 2), the gravitational force always provides attraction, but how strong in relation to H, or E for that matter?

Yes, I've taken physics courses. I had 2 quarters of basic phy (Halliday-Resnick text), 1 quarter of modern phy including relativity, QM, & kinetic theory of matter (Tipler text), and 1 quarter of solid state physics (Kittel text), at the undergraduate level. Of course, it was some time ago, in the 1970's.

As far as declaring victory goes, to acknowledge that many other learned researchers, all more capable than moi, have already laid this issue to rest, is hardly "declaring victory". Why do the universities teach us that to determine the direction of the force, that we must use the right hand rule?

I'm not out to show anybody up. I would rather be corrected than to continue to believe a false doctrine. If what you say was the whole truth, every physics and EE text would affirm the same. But to acknowledge what universities teach is not declaring victory on my part. If this is a contest to see who can outdo the other, count me out. I am not here to "win", just to learn, and contribute.

Maybe somebody else with a strong e/m fields background can chime in and offer their viewpoint. I've said enough. Good day to all.

Claude

[Doc Al]

So you're saying that the H force pulls the electrons to the interior, but the E force from the lattice provides an opposing force.
Yes.
But as I stated, just as the lattice is attracted to the electrons, so are the electrons attracted to the lattice.
Of course!
If the lattice incurs a force of attraction to the other wire, is it the E or H force that is accountable? The E force alone won't do it.
Please tell me the magnetic force on the stationary lattice?
The electrons would be attracted to the lattice, not vice versa.
:bugeye: Really? What about Newton's 3rd law?
The mass of the lattice is enormous vs. the electrons. Of course, the electrons do not go jumping off the wire. But their attractive force on the lattice cannot happen without the H force holding them.
Of course. I've been saying that all along.
The strong H force yanks the electrons towards the interior. The lattice and electrons exert a mutual E force on each other. Without the H force, the electrons would be drawn to the lattice. But the strong H force holds the electrons so that the lattice in addition to the electrons are moved in the direction of attraction.
That's sounds very close to what I've been saying all along, but you miss the punch line. Again I ask: What force directly acts on the positive lattice?

The strong H force is ultimately responsible.
Of course. But that force does not directly act on the positive lattice.
I've already stated that E plays a role, but the main contribution is H force.
They are equally strong. And only one directly acts on the lattice.

As far as declaring victory goes, to acknowledge that many other learned researchers, all more capable than moi, have already laid this issue to rest, is hardly "declaring victory".
As far as I can see, this is the first time you've been exposed to the issue, so I can't imagine why you think others have "laid it to rest". Please cite a learned researcher who claims that the statement made by Griffiths in his introductory E&M book is false.
Why do the universities teach us that to determine the direction of the force, that we must use the right hand rule?
Because it works just fine. No need to go into the nitty gritty details all the time--which gives you the same answer of course, only with more effort. (That was Vanadium's point, back in post #13.)

[Dadface]

If it is true that no work can be done on or by a magnetic field then can someone please explain what field features in the work done when say two magnets are used one of them being completely surrounded by a Faraday cage?As far as I understand it the cage blocks electric fields.I tried a quick experiment using two fridge magnets and aluminium foil for the cage. I had to do work to pull them apart against the attractive force and work was done when I released them and they moved together again.The results were what I expected.

[cabraham]

Yes.

Of course!

Please tell me the magnetic force on the stationary lattice?

Straw man. I've clearly stated already that the force on the lattice is that of the electrons and E force. The H force acts on the electrons which are moving and not stationary. In attempting to make me look bad, you are introducing straw men.

:bugeye: Really? What about Newton's 3rd law?

I already acknowledged that just as the electrons attract the lattice, so does the lattice attract the electrons. You pulled 1 sentence out of context with the intent of making me look bad.
Of course. I've been saying that all along.

That's sounds very close to what I've been saying all along, but you miss the punch line. Again I ask: What force directly acts on the positive lattice?

Already been acknowledged as the E force. But my point was that you cannot simply declare E as the only entity responsible. They both are involved, E & H. I can respond with "What force acts on the electrons holding them in place so as to make lattice attraction possible?" When placed in close proximity, a ping pong ball and a bowling ball, each with 1 uC of charge will mutually attract (opposite polarity) or repel (like). If the pp ball remains stationary or moves relatively little vs. the bowling ball which moves a greater distance, what is going on?

Answer - there is another force, quite significant, acting on the pp ball, holding it in its position.

Of course. But that force does not directly act on the positive lattice.

They are equally strong. And only one directly acts on the lattice.


As far as I can see, this is the first time you've been exposed to the issue, so I can't imagine why you think others have "laid it to rest". Please cite a learned researcher who claims that the statement made by Griffiths in his introductory E&M book is false.

I can name countless that affirm the right hand rule. Every text. I never said Griffiths was wrong. Since page 1 of this thread, I have fully acknowledged the E force. I never took issue with Griffiths. But you and others keep putting forth isolated facts emphasizing the role of E while neglecting all facts pointing to H as having great influence. When I force the issue, you acknowledge the role of H, but you don't bring it up on your own. You are obsessed with presenting only 1 side of the issue.

You then break up my post into fragments, isolating single sentences, then attacking the fragments by asking questions I've already answered. You are clearly here to "win". I only want to point out that there are numerous things going on here. Then you pit me against Griffith, with whom I have no beef.
Because it works just fine. No need to go into the nitty gritty details all the time--which gives you the same answer of course, only with more effort. (That was Vanadium's point, back in post #13.)

It sure does work fine. Yet I'm wrong for believing in it.

Claude

[Doc Al]

I can name countless that affirm the right hand rule. Every text. I never said Griffiths was wrong. Since page 1 of this thread, I have fully acknowledged the E force. I never took issue with Griffiths. But you and others keep putting forth isolated facts emphasizing the role of E while neglecting all facts pointing to H as having great influence. When I force the issue, you acknowledge the role of H, but you don't bring it up on your own. You are obsessed with presenting only 1 side of the issue.
Huh? Where did I question the "right hand rule"? Strawman, indeed! You claim that I "acknowledge the role of H, but you don't bring it up on your own", but describing the role of the H field is the very first step I made (post #32) in responding to your request (post #28) for an explanation of the attraction between current-carrying wires using electric fields.

Enough already. We are wasting each other's time.

[cabraham]

Huh? Where did I question the "right hand rule"? Strawman, indeed! You claim that I "acknowledge the role of H, but you don't bring it up on your own", but describing the role of the H field is the very first step I made (post #32) in responding to your request (post #28) for an explanation of the attraction between current-carrying wires using electric fields.

Enough already. We are wasting each other's time.

So let's summarize. Two wires are parallel and carrying current. What determines the magnitude & direction of the force incurred? The direction of the currents determines the polarity of the H fields. The polarity of the H fields determines whether the free electrons in the wire shift to the interior vs. exterior. Then, the positive charged lattice follows the free electrons due to E force.

That pretty much sums it up. If the current increases, so does the H field, and the electrons move further inward or outward. Then the lattices follow the electrons further in or out.

Thus the H field determines where the electrons move and how far. The lattice tags along like an obedient shadow due to E force between lattice and electrons.

That is prima facie evidence that the H field is primarily what determines if the wires attract or repel, and the magnitude of the force. The E field definitely participates, but is not what determines the above.

H force moves the electrons. Lattice tags along due to E force. It's that simple. H is primary, with E secondary. Case closed.

Claude

[Doc Al]

H force moves the electrons. Lattice tags along due to E force. It's that simple. H is primary, with E secondary. Case closed.

The issue is not which field, E or H, is "primary"; they come together--it's a package deal. The issue is, per the title of this thread: Does the magnetic field do work? The answer to that is technically no; it's the electric field that pulls the wire. This explanation is one that you objected to at first (recall your response in post #18 to diazona's rather clear statement in post #16).

The reason for this seemingly nitpicking discussion is one of understanding the Lorentz force law, which is the source of all the derived "right-hand rules".

[Academic]

This paper helped me understand it. :
http://academic.csuohio.edu/deissler/PhysRevE_77_036609.pdf

[cabraham]

The issue is not which field, E or H, is "primary"; they come together--it's a package deal. The issue is, per the title of this thread: Does the magnetic field do work? The answer to that is technically no; it's the electric field that pulls the wire. This explanation is one that you objected to at first (recall your response in post #18 to diazona's rather clear statement in post #16).

The reason for this seemingly nitpicking discussion is one of understanding the Lorentz force law, which is the source of all the derived "right-hand rules".

Maybe an analogy would help. A steel ball is tethered to a rubber ball via a short cord, or even glued together. A powerful electromagnet is held above the tethered ball pair. The em is turned on and the steel/rubber ball pair is lifted into the magnet.

I certainly do not believe that a magnet is doing work on the rubber ball. But the rubber ball does not ascend if not for the mag force. So it is really splitting hairs to argue which force is responsible for the rubber ball ascending.

The mag force acting on the steel ball is what ultimately lifted both balls. The steel ball was lifted by the magnet directly. The rubber ball was lifted indirectly. The tether provided the means for the rubber ball to tag along with the steel ball.

With 2 parallel wires, the E force between the lattice and free electrons is the tether. The H force dictates where the electrons go, then the E force tethers the lattice yanking it in the direction of the electrons. To say that H is NOT responsible for the lattice moving is like saying that the magnet is NOT responsible for the rubber ball ascending. The electrons and the lattice are tethered via E force. But the H force is what moves the electrons, and is ultimately responsible for moving the lattice. The E force does indeed move the lattice, but the E force magnitude and direction is dictated by the location of the electrons which is dictated by the magnitude and direction of H.

It's difficult to separate the 2 forces. But it is clear as day that H is what dictates the magnitude and direction of the displacement of the wires. E follows H. I know that E & H are inclusive, and neither is the cause of the other. But under these narrow conditions, H is ultimately in control, with E tagging along.

H, however, is not more fundamental than E, nor less. They are inclusive.

Does this make sense? BR.

Claude

[Doc Al]

Does this make sense?
Yes! Sounds good to me.

[kanato]

No, since the magnetic moment of the electron is considered to be an intrinsic magnetic moment.

So then the magnetic field can do work. Since the electron spin is what gives rise to any ferromagnetic material, the magnetic field is does work whenever a permanent magnet is involved.

[Doc Al]

I may have to revise my answer to the Stern-Gerlach question in the light of the interesting paper that Academic linked to in post #56. (It might take me a while to find the time to digest it--hopefully someone more knowledgeable will chime in sooner.)

[cabraham]

Yes! Sounds good to me.

Very good. I'm glad we agree. This is an interesting thought problem. It gives us all a chance to review the theory and I feel I've gained a better understanding. Thanks for your input. BR.

Claude

[Doc Al]

Very good. I'm glad we agree.
Me too! :smile:

This is an interesting thought problem. It gives us all a chance to review the theory and I feel I've gained a better understanding. Thanks for your input. BR.
Yes, it's interesting--and subtle--stuff. It helps me to review it every now and then. Good discussion!

[kanato]

I may have to revise my answer to the Stern-Gerlach question in the light of the interesting paper that Academic linked to in post #56. (It might take me a while to find the time to digest it--hopefully someone more knowledgeable will chime in sooner.)

I missed that one. I have skimmed through that article, and it looks like the author doesn't actually answer the question, but gives some conditions under which the answer could be understood. I will have to read it more carefully however. It is interesting that this is a topic of current research though, so the question of whether or not a magnetic field can do work at a fundamental level really isn't settled.

[qsa]

"The usual explanation
is that there is a change in the “potential energy” by
an amount −2
s ·B=−eB/mc, which implies that the magnetic
field did work on the electron’s magnetic moment.
However, if the electron has rotational kinetic energy,"

This is a quote from the paper. He states the conventional explaination, but he puts forward his own conjecture " rotational kinetic energy".

[kmarinas86]

The forces of electromagnetism do work. Acceleration can occur along electric field lines and acceleration and also along magnetic field lines. However, because a cyclical process of doing work requires a changing magnetic field which in turns produces an electric field, electric fields are seen as crucial in order for work to be done. A changing displacement is sufficient for an electric field to do work, but not so for a magnetic field, which requires a change in magnitude in place (implying a change in electric field). Therefore when work is done using electricity or magnetism, an electric field ALWAYS comes into play, but same is not true for magnetic fields (because sometimes they are not used). It is a rare circumstance in the macroscopic world to have a system with truly constant magnetic fields (no induction) when electric fields are being moved relatively to each other...

[sweet springs]

Hi
It might be out of date but I show here a interesting case, a charged particle attached on a elastic body, with velocity v in perpendicular direction, under magnetic field B in another perpendicular direction.
|
|wwwwwwwwwww○　↑ ｖ　 x B
|
Magnetic or Lorentz force pushes or pulls elastic body. It does work thus elastic energy would be stored.

I state from this example that magnetic force does not work on FREE charge, but it can work on charge UNDER CONSTRAINT.

The elastic body consists of multiple charged particles under electromagnetic interaction so we can say another way that magnetic force does not work on a system of SINGLE charge, but it can work on a system of MULTIPLE charges.

Regards

[Doc Al]

Hi
It might be out of date but I show here a interesting case, a charged particle attached on a elastic body, with velocity v in perpendicular direction, under magnetic field B in another perpendicular direction.
|
|wwwwwwwwwww○　↑ ｖ　 x B
|
Magnetic or Lorentz force pushes or pulls elastic body. It does work thus elastic energy would be stored.
Can you please describe the case you have in mind in more detail and explain why you think it illustrates a magnetic force doing work on a charged particle.

[sweet springs]

OK, I will.
|
|wwwwwwwwwww○　↑ ｖ　 x B
|

the direction of Lorentz force is ←　or →　according to the sign of charge ○ and it pushes or pulls the elastic body or the spring.
Regards.

[Doc Al]

OK, I will.
|
|wwwwwwwwwww○　↑ ｖ　 x B
|

the direction of Lorentz force is ←　or →　according to the sign of charge ○ and it pushes or pulls the elastic body.

While the Lorentz force (at the moment pictured) is ←　or →, the motion of the charge is not. So the elastic material isn't being stretched yet.

An interesting case, but since the Lorentz force is always perpendicular to the velocity of the charge, I don't know why you'd say it's doing work on it. All it does is change the direction of motion of the charge, which may certainly end up stretching (or compressing) the elastic material due to the inertia of the charged mass. The only thing doing work on the charge is the elastic material.

[sweet springs]

Hi.
Please teach me more.

A free charge motion draw a circle.

○xB○○○○○○○○○○
○○○○●●●○○○○○
○○○●○○○●○○○○
○○●○○○○○●○○○
○○●○○○○○●○○○
○○○●○○○●○○○○
○○○○●●●○○○○○
○○○○○○○○○○○○

In the case the spring with wheel moving up-down free in the figure, is tied to the charge, the spring is streched according to the motion of charge, isn't it?

｜○xB○○○○○○○○○○
◎ww w●●●○○○○○
｜○○○●○○○●○○○○
｜○○●○○○○○●○○○
◎ww●○○○○○●○○○
｜○○○●○○○●○○○○
｜○○○○●●●○○○○○
｜○○○○○○○○○○○○

In the case of No magnetic field

｜○○●○○○○○○○
◎ww●○○○○○
｜○○●○○○○○○○
｜○○●○○○○○○
◎ww●○○○○○○
｜○○●○○○○○○
｜○○●○○○○○○
｜○○●○○○○○○
｜○○●○○○○○○

No stretch of course. Don't these mean that magnetic force can stretch the spring?
Regards.

[Doc Al]

Does the magnetic force affect the motion of the charge and thus the stretch of the spring? Sure. Does the magnetic field do work on the charge? No. All it does is change the direction of motion of the charge. The energy of the system--elastic potential energy + kinetic energy--remains the same.

[kmarinas86]

Does the magnetic force affect the motion of the charge and thus the stretch of the spring? Sure. Does the magnetic field do work on the charge? No. All it does is change the direction of motion of the charge. The energy of the system--elastic potential energy + kinetic energy--remains the same.

Ok, so basically the energy is not added by this static magnetic field to the system (obviously). Instead, it just facilitates changes of potential energy into kinetic energy and vice versa.

Imagine a large magnetic field external to our solarsystem that changes only over the course of thousands of years. This "effectively static" magnetic field could give us an impression of anomalous effect on the movement of bodies in our solarsystem, but in such a way that the total energy, GPE + KE, is conserved. This would imply that magnetic fields can be used to facilitate the unleashing of potential energy, but that such potential energy does not come from the magnetic field itself.

[Dadface]

Expressing it a bit differently the charge has kinetic energy before being deflected and it has exactly the same kinetic energy after being deflected.Some of this kinetic energy is changed to elastic potential energy but the kinetic energy is not gained from the magnetic field,it is gained from the electric field that caused the charge to move in the first place.

[JustinLevy]

A magnetic dipole in an external magnetic field with a gradient will feel a force.

So, two ideal magnetic dipoles at rest can extert a force on each other.

That means two neutrons can exert a magnetic force on each other while they are at rest. If you don't like composite particles, then the same can be said with electrons (but there is also an electric force as well then, but you can set it up such that the forces are in orthoganol directions to make it clear which force did what).

I would consider that work legitimately done by a magnetic field.
... unless you want to claim an electron is not a point particle.

[sweet springs]

Does the magnetic force affect the motion of the charge and thus the stretch of the spring? Sure. Does the magnetic field do work on the charge? No. All it does is change the direction of motion of the charge. The energy of the system--elastic potential energy + kinetic energy--remains the same.

Hm.. Let me pose one more question.

In the case of motion of charge in NO magnetic field

｜○○●○○○○○○
◎ww●○○○○○○
｜○○●○○○○○○
｜○○●○○○○○○
◎ww●↑○○○○○
｜○○●○○○○○○
｜○○●○○○○○○
｜○○●○○○○○○
｜○○●○○○○○○

By magnetic flux x B, the Lorentz force →　and the elastic force ←　balance,

x B
｜○○○○●○○○○
◎www ●○○○○
｜○○○○●○○○○
｜○○○○●○○○○
◎www ●↑○○○
｜○○○○●○○○○
｜○○○○●○○○○
｜○○○○●○○○○
｜○○○○●○○○○

So what caused the stretch of the spring therefore increase of elastic energy?

Regards.

[Doc Al]

I'm not sure I understand your diagrams, but in any case.
By magnetic flux x B, the Lorentz force →　and the elastic force ←　balance,
No reason to think that those forces are balanced.

So what caused the stretch of the spring therefore increase of elastic energy?
The mass is moving.

[Relay]

Here's my answer to elliotr's origininal question. (Please be kind if it doesn't make sense.)

First and formost, nobody knows what magnetism is. We just know how it behaves.

Second is how one looks at a problem. I think that all text books state that magnetism can't do real work directly same as gravity. What I mean by this is that people percieve the earth as the center for gravity and do not do the same for magnets. Each magnet is its own gravity well. On earth we can use a ground state like a sidewalk and base calculations from that level. A cannon ball falling from a building converts its potention energy to kinetic energy. This is where it gets tricky. The cannot ball got it's energy from somewhere other than gravity. The person who took the cannon ball to the top of the building is the source of that energy. So when the cannon ball falls and breaks the sidewalk nobody questions that it did work and had energy. The same can be said about a paperclip falling up to a magnet. It will do work on the surface of the magnet as it hits, but the energy would not have come from the magnet. The energy came from many processes that created and placed the paperclip just prior to being pull (or pushed) up by the magnet.

We don't even know if magnetism is a pulling or pushing force. Anyhow, if you think about a magnetic well that things can fall into and be pulled from, it starts to make more sense. Well, that's my two cents worth...

[fantispug]

Dipoles have come up a couple of times in this discussion, but I still don't quite get it.

We can calculate the torque due to the magnetic force on a dipole, m, in a uniform magnetic field, B, and calculate a corresponding energy in rotating the dipole -m.B.

We are only considering the magnetic forces here, yet they have appeared to do work.

One way to construct this situation is using spinning spherical shells; a uniformly charged sphere rotating at constant angular velocity has a constant, uniform magnetic field inside and a dipolar magnetic field outside. So we can put one spinning shell inside another, using the outer shell to produce the uniform magnetic field and the inner shell to represent the dipole.

It turns out that the inner shell produces a torque on the outer shell, and so under a rotation of the dipole this produces a work +m.B.

Thus we find (at least in this case) the TOTAL work done by magnetic fields is +m.B - m.B = 0.

However the justification that the magnetic force does no work is done particle-by-particle. I would have thought this implied that there should be no magnetic work in rotating a dipole in a uniform magnetic field. Why is this wrong?

[Vanadium 50]

Fantispug, I think I covered this in Post #13. Is there something unclear that I should elaborate?

[sweet springs]

Hi.

The mass is moving.

Now I know my misunderstanding. Another resource to keep mass moving in constant velocity is necessary. Thank you for your teachings. I apologize the delay of my thanks. I was wondering of the following another situation.

On X-Y plane there exists around the origin a ring of radius a. This ring is made of charge, the line density of which is ρ, and is rotating with angular velocity ω. Let homogeneous magnetic field of B of direction x be applied. The parts of circle get force in Z direction of －ρaωB cosφ　dl ＝ －ρa^2 ωB cosφ dφ where φ is the direction angle in X-Y plane.

Does the ring start to rotate around the Y axis (stand up) therefore get rotation energy under the influence of magnetic force which keep perpendicular to the motion of charge i.e. rotating around Z and perhaps Y axes ?
If yes, does this Y rotation energy come from the work done by the magnetic field ,or just from the conversion of Z rotation energy ,therefore ω decrease, in conservation of energy ?

Regards.

[fantispug]

Hi Vanadium 50,
I don't understand in the particular case of a dipole in a magnetic field where the electric forces come in; I'm interested in this case at looking at a careful analysis of the forces just to understand how it works.

The calculation that I eluded to, using the torque on a magnetic dipole by a magnetic field, seems to imply that the magnetic field does the work. The electric field plays no role in this argument; there are no external electric fields and we have not included the effects of the electric fields generated by the dipole.

(In fact the electric fields generated by the dipole give an additional energy to the system of +m.B)

So how are the electric forces responsible for the work in this specific case?

[Vanadium 50]

What holds the dipole together?

[cabraham]

What holds the dipole together?


We're going in circles again. All pertinent forces have been thorughly examined. Refer to pages 1 through 4 and you will find good info from several contributors.

We pretty much reached a concensus that H fields do no direct work on a charged particle, but H exerts force while E provides tethering force yanking the lattice towards the free electrons.

E does indeed hold the lattice and electrons together. H moves the electrons in a direction normal to its velocity, and the lattice is yanked along due to the tethering nature of the E force.

At a mIcroscopic scale, H does not literally "do work" on an electron. At the mAcro scale, H determines the magnitude and direction of the wire deflection. H moves electrons which yank the lattice along due to E.

It is well understood now. Every e/m fields text makes it a point to emphasize the mutually inclusive nature of E & H. They are strongly inter-related, and under time-varying conditions, cannot exist independently. Under static conditions, either can exist alone.

We should all agree at this point.

Claude

[fantispug]

Ok, I think I get what you're saying now. It doesn't matter what's holding the dipole together; whether its electric forces, gluons or Achillies. If we're thinking of a dipole as a spinning spherical shell, the important thing to notice is that if it is conducting when we put it in a magnetic field the charge will redistribute so that it is no longer a dipole at all.

Consequently when we put a dipole in a magnetic field, turn on the magnetic field and assume that the charge does NOT redistribute, we need to supply special angle-dependent forces to prevent this redistribution. The difference in electromagnetic energy between redistributing and not redistributing the charge is supplied by the work done by this "special force". We claim that this is then exactly +m.B.

I've been trying to verify this in a specific example, but the maths is a little tricky so I'll see how we go; but the explanation seems physically plausible. Thanks.

[kmarinas86]

Under static conditions, either can exist alone.

Whether a condition is static is relative to the velocity of the obsever. I think a better term would be comoving.