Can a magnetic fields/forces do work on a current carrying wire?

[Miyz]

Thanks DaleSpam,

 

[cabraham]

I mention the superconductor because it gets rid of a lot of the "smokescreens" that people try to put up in asserting that a magnetic field cannot do work. It shows that it is not impossible for a magnetic field to do work. Given that it is not impossible then I have no qualms about saying that the magnetic field in a motor does work on the wire.

The only formula which justifies the contrary applies only for classical point particles and is not a general law of nature.

Sounds reasonable.

Claude

 

[Miyz]

Sounds reasonable.

Claude

I guess you and I stand corrected huh Claude?

 

[vanhees71]

It is very clear that the power of any electromagnetic field on charges is given, according to Poynting's theorem by
$$P(t)=\int_{\mathbb{R}^3} \mathrm{d}^3 \vec{x} \; \vec{E}(t,\vec{x}) \cdot \vec{j}(t,\vec{x}).$$
Of course a motor does work, but it's the electric field according to the above equation.

 

[Philip Wood]

I believe that my earlier post (13) shows in some detail how this work done by the electric field (post 75) appears as work done on the wire as the wire moves. This electric field is set up by the battery connected across the wire. Throughout this thread I don't think there's been nearly enough emphasis on the battery as the source of the work that's done when the wire moves in the magnetic field.

[Incidentally, for a wire of cross-sectional area A, lying in the ±x direction and carrying current I, vanhees's formula yields

Work done per unit time in length $\Delta x$ of wire = $(A\Delta x) E_x (\frac{I}{A}) = I E_x \Delta x = -I \Delta V$,

which is rather familiar!]

 

[harrylin]

[..] As I said before and will continue to stand upon this point magnets can do work under certain circumstances. [..]

After skimming through this discussion, I wonder if this is the main point of misunderstanding. In your original post you referred to a permanent magnet. I don't believe that this magnet cools down in the process, and it certainly has no energy source. Obviously it does not output energy in the process.
Another, related possible point of confusion that I think has been mentioned earlier is that work is typically done through several intermediates, for example you can pull on something heavy with a rope, supporting the force with your feet on the street. Does one then say that the rope does work, or that the street does work? I think that that isn't a common way of formulating things; the permanent magnet acts like the street.

 

[Miyz]

After skimming through this discussion, I wonder if this is the main point of misunderstanding. In your original post you referred to a permanent magnet. I don't believe that this magnet cools down in the process, and it certainly has no energy source. Obviously it does not output energy in the process.
Another, related possible point of confusion that I think has been mentioned earlier is that work is typically done through several intermediates, for example you can pull on something heavy with a rope, supporting the force with your feet on the street. Does one then say that the rope does work, or that the street does work? I think that that isn't a common way of formulating things; the permanent magnet acts like the street.

Magnets, are no energy source. However, a source of force. That can do work in certain orientation example: MOTOR.

If you'd disagree please use equation's to back you're opinion. Because its a known fact the magnets can do work on a dipole(Repel/attract). Now in the case of a motor its stator that is winded up with coil wires generates a magnetic field and acts as a magnet(dipole) thus is attracted/repeled by the magnetic field of the magnets.

F = IL x B

If you break any system that is doing work, its just applying forces. In some complicated physical systems they apply MULTIPLE FORCES just like a motor.
DaleSpam gave out a good point, so as Claude, and Darwin123.
If you're still not convinced I'd recommend studying this matter more.

Miyz,

 

[Miyz]

Ow yea and don't Skimm since you might have skipped a lot of good info.

 

[vanhees71]

Sigh! The equation, I've given is exact (within classical Maxwell theory). A nice paper about this question is the following one. The classical part of it precisely answers the question discussed here on hand of a simple example:

http://link.aps.org/doi/10.1103/PhysRevE.77.036609

 

[Miyz]

Sigh! The equation, I've given is exact (within classical Maxwell theory). A nice paper about this question is the following one. The classical part of it precisely answers the question discussed here on hand of a simple example:

http://link.aps.org/doi/10.1103/PhysRevE.77.036609

I noticed Maxwell's equations are relevant to this topic. What was his theories about this matter that some find it to be absurd?

 

[harrylin]

Magnets, are no energy source. However, a source of force. That can do work in certain orientation example: MOTOR. If you'd disagree please use equation's to back you're opinion.[..]
DaleSpam gave out a good point, so as Claude, and Darwin123. [..]

Today vanhees gave you the equation you asked for, and also philipwood and I gave you good points. The most pertinent one is just your first sentence here: Permanent magnets are no energy source. That means that they do not give off energy, and most physicists mean with "doing work" that a system provides energy to one or more other systems.

Compare: http://www.lightandmatter.com/html_books/lm/ch13/ch13.html#Section13.1
The tractor does work, but the rope does not.

 

[Dale]

On the contrary, the magnetic field does store energy. That energy can be used to do work, every bit as much as the energy stored in a battery or a capacitor can.

http://en.wikipedia.org/wiki/Magnetic_field#Energy_stored_in_magnetic_fields

 

[Miyz]

On the contrary, the magnetic field does store energy. That energy can be used to do work, every bit as much as the energy stored in a battery or a capacitor can.

To aid DaleSpam's point look at this.

 

[Miyz]

The most pertinent one is just your first sentence here: Permanent magnets are no energy source. That means that they do not give off energy, and most physicists mean with "doing work" that a system provides energy to one or more other systems.

They don't "give off energy" they have magnetic fields that have potential energy. A wheel has not energy? But its the main source for transferring force, for work to be done, that eventually "TRANSFERS" energy.

 

[Miyz]

Compare: http://www.lightandmatter.com/html_books/lm/ch13/ch13.html#Section13.1
The tractor does work, but the rope does not.

That WHOLE idea is irrelevant to this topic.

 

[harrylin]

They don't "give off energy" they have magnetic fields that have potential energy. A wheel has not energy? But its the main source for transferring force, for work to be done, that eventually "TRANSFERS" energy.

After one turn the potential energy is identical - no change over one cycle. So, as Darwin already pointed out in post#30, it reduces to a disagreement about the meaning of words. In physics language the rope behind the tractor and the permanent magnet in the motor do no work - that has nothing to do with equations, just with definitions.

That WHOLE idea is irrelevant to this topic.

That is, the definition of work and many explanations of how to deal with it "is irrelevant to this topic"... Well then, good luck!

 

[Dale]

A nice paper about this question is the following one. The classical part of it precisely answers the question discussed here on hand of a simple example:

http://link.aps.org/doi/10.1103/PhysRevE.77.036609

Thanks, I will give it a read before making more assertions about magnetic fields and work.

 

[Dale]

In physics language the rope behind the tractor and the permanent magnet in the motor do no work - that has nothing to do with equations, just with definitions.

I would say that whether or not the rope does work depends on where you arbitrarily draw your system boundary.

 

[harrylin]

On the contrary, the magnetic field does store energy. That energy can be used to do work, every bit as much as the energy stored in a battery or a capacitor can.

http://en.wikipedia.org/wiki/Magnetic_field#Energy_stored_in_magnetic_fields

Miyz seems to think that you were talking to me. I did not mention the magnetic field, as my comment was on a post about the magnet. And sure the magnetic field can act like a spring. Moreover, a spring can do work. However, there is over one full turn of the motor no change in the field. The magnetic force of the OP is a force between the magnet and the coil, and the coil's field energy is provided by the current source.

Note: it seems fair to assume that the coil's magnetic field increases the permanent magnet's field energy - if so, then in that sense one could say that these fields do work, just after they were made/increased.

 

[harrylin]

I would say that whether or not the rope does work depends on where you arbitrarily draw your system boundary.

I would say that such arbitrary conventions are not to be preferred. As described there, the system that looses the energy is the one that does the work. Anyway, I have no interest in discussions over words and supposedly that wasn't the purpose of this topic.

 

[Dale]

I would say that such arbitrary conventions are not to be preferred.

I don't think that there are any generally-accepted conventions, nor even any typically-recommended ones, for drawing system boundaries. It may not be preferred, but I don't see any way around it.

If the rope is applying a force to the system along some distance then by the usual definition of work the rope's force is doing work on the system. Work transfers energy, it doesn't have to be the ultimate source of the energy.

 

[harrylin]

I don't think that there are any generally-accepted conventions, nor even any typically-recommended ones, for drawing system boundaries. It may not be preferred, but I don't see any way around it.

If the rope is applying a force to the system along some distance then by the usual definition of work the rope's force is doing work on the system. Work transfers energy, it doesn't have to be the ultimate source of the energy.

That's not the definition that I use, and indeed with your definition there is no way around it.
And with that definition every force pair "does work"; which makes the OP's question (see below) like trying to kick in an open door.

I finally manged to decipher it, Myiz meant of course:

"Aren't the magnetic forces in a motor one of the key factors of motion inside? I mean it makes no sense to me why in this case magnetic force can't do work on an electric charge..."

 

[Dale]

That's not the definition that I use, and indeed with your definition there is no way around it.

Oh, well I don't want to argue about semantics either, but what is the definition you use? I am not familiar with another definition, or maybe I am and just cannot recall it right now.

I think that to answer questions like this it is important to at least be clear on the definitions that everyone is using, including both definitions about general terms like "work" and definitions about the problem itself like the boundaries of the system. E.g. if the boundary of the system is drawn one way then the magnetic force is internal to the system, and if you draw it another way it is external.

"Aren't the magnetic forces in a motor one of the key factors of motion inside? I mean it makes no sense to me why in this case magnetic force can't do work on an electric charge..."

For this phrasing of the question it sounds that the magnetic force is considered an internal force.

 

[cabraham]

It is very clear that the power of any electromagnetic field on charges is given, according to Poynting's theorem by
$$P(t)=\int_{\mathbb{R}^3} \mathrm{d}^3 \vec{x} \; \vec{E}(t,\vec{x}) \cdot \vec{j}(t,\vec{x}).$$
Of course a motor does work, but it's the electric field according to the above equation.

Which electric field? Have you read the link I provided? We discussed this in detail. Please read it then comment. An electric field does NO WORK on NEUTRONS. Only strong force can do that. Also, it is impossible for any field, E, M, or SN, to do long term "work". Ultimately the power source, i.e. battery, ac mains, automobile alternator, etc., that does the continuous work. Fields store & release energy, but they are spent when doing so, & need to be replenished.

I believe E & B fields can only receive energy for storage, & upon transfer of said energy, they are spent, then replenished by the power source. I just provided the link so that we can examine & understand all pertinent forces involved.

Claude

 

[cabraham]

Today vanhees gave you the equation you asked for, and also philipwood and I gave you good points. The most pertinent one is just your first sentence here: Permanent magnets are no energy source. That means that they do not give off energy, and most physicists mean with "doing work" that a system provides energy to one or more other systems.

Compare: http://www.lightandmatter.com/html_books/lm/ch13/ch13.html#Section13.1
The tractor does work, but the rope does not.

Good point. In my linked thread the magnetic force is akin to the tractor, while the E & SN forces are analogous to the rope. I firmly believe that al 3 forces are involved, but the power source provides all the energy to the magnetic field, which gets transferred, then the power source replenishes said energy. Of course the internal combustion in the tractor engine is akin to the power source replenishing the magnetic field.

Claude

 

[harrylin]

Oh, well I don't want to argue about semantics either, but what is the definition you use? I am not familiar with another definition, or maybe I am and just cannot recall it right now. [..]

Apparently Claude uses a similar definition as the one I phrased and linked to in post #82. A rope that is not used for its elastic force merely transmits energy and is not a source of energy, so that it does no work if we use that definition.

However, see also my Note in post #90 (this discussion goes to fast!).

 

[Dale]

Apparently Claude uses a similar definition as the one I phrased and linked to in post #82. ... (this discussion goes to fast!).

Oops, sorry, I missed the link. You are right, it does move fast. The energy transfer definition in the link is subtly different from the F.d definition I was using, so you are correct that we were using different definitions.

However, using that definition the lightandmatter link explicitly says that the rope does work on the plow: "When the tractor pulls the plow with a rope, the rope does negative work on the tractor and positive work on the plow." (emphasis added).

Also, the definition used there clearly applies to a rope: "Work is the amount of energy transferred into or out of a system, not counting energy transferred by heat conduction." The rope does transfer energy to the weight/plow/trailer. It doesn't produce any energy, but it transfers it from the tractor to the weight and not via heat conduction.

 

[Philip Wood]

vanhees71. There's no argument about the power equation is there? Of course it is right.

 

[harrylin]

[..] However, using that definition the lightandmatter link explicitly says that the rope does work on the plow: "When the tractor pulls the plow with a rope, the rope does negative work on the tractor and positive work on the plow." (emphasis added). [..]

Ah right I missed that - thus that link actually corresponds to your definition, so it appears that yours is the more commonly used. And using that definition the reply to the title question is obviously yes: fields/forces that move a wire do work on that wire.

 

[cabraham]

Apparently Claude uses a similar definition as the one I phrased and linked to in post #82. A rope that is not used for its elastic force merely transmits energy and is not a source of energy, so that it does no work if we use that definition.

However, see also my Note in post #90 (this discussion goes to fast!).

I agree with that, but likewise the E field which tethers the stationary lattice protons to the mobile electrons merely transmits the force on the electrons from the magnetic field. Likewise SN force is also like the rope in that it transfers force to the neutrons. Both E & SN forces are akin to the rope in the tractor example.

Remember that the force integrated over the distance is the work done. The mag force must be strong enough to match the E force, plus the SN force, as well as move the electrons. But the mag field gives up energy as it transfers energy to produce torque & speed. The power source at the motor terminals replenishes this energy.

Is the mag force doing "work"? Well, in the short term, YES, in the long term NO. The power source, battery, ac mains wall outlet, etc., is doing all of the long term work. The mag force does move the rotor, but it only acts directly on electrons, but indirectly on protons & neutrons. The E & SN forces are internal tethers, like the rope in the tractor example. They are indispensable as they transmit force to protons & neutrons. The mag force is ineffective on proton & neutron.

Is the mag force doing work? Again, it stores energy then transfers it. It needs help from E & SN forces as well. Mag force participates but can't do it alone, nor long term. The power source is ultimately what does the work, not B field, not E field, not SN force. Did I help or confuse matters more?

Claude

 

[harrylin]

[..] Did I help or confuse matters more?
Claude

Without scrutinizing your arguments in detail, I think that you nicely summarized it.

 

[Miyz]

After one turn the potential energy is identical - no change over one cycle. So, as Darwin already pointed out in post#30, it reduces to a disagreement about the meaning of words. In physics language the rope behind the tractor and the permanent magnet in the motor do no work - that has nothing to do with equations, just with definitions.

That is, the definition of work and many explanations of how to deal with it "is irrelevant to this topic"... Well then, good luck!

Um, Darwin123 gave a very good statement about the forces involved in doing the work and will eventually transfer energy. Now in you're cause the tractor is the main input of force applied on the rope to lift,move,etc... When you brake the whole system up you'd find out the main "SOURCES" to transfer energy is the rope. Without the existence of any force in the system nothing is applied on the weight and no work will be done and no energy would be transferred.

In a motor what basically happens? A battery is connected to a loop supplying the input "magnetic field" when a magnet is present and oriented in a certain way it will apply a force on the loop it will rotate and torque will be created. Now who's the rope in this situation and the tractor? I think it pretty obvious isn't it?

Even in you're example the "rope" is the connection between to object to do work which is crucial, without the presents of a magnet/rope no work wold be done.

Yea and when I said its irrelevant not by definition but more into the forces acting upon object because its very simple and obvious. In a cause of motor where its all about the magnetic fields its more complex isn't it?

Oops, sorry, I missed the link. You are right, it does move fast. The energy transfer definition in the link is subtly different from the F.d definition I was using, so you are correct that we were using different definitions.

However, using that definition the lightandmatter link explicitly says that the rope does work on the plow: "When the tractor pulls the plow with a rope, the rope does negative work on the tractor and positive work on the plow." (emphasis added).

Also, the definition used there clearly applies to a rope: "Work is the amount of energy transferred into or out of a system, not counting energy transferred by heat conduction." The rope does transfer energy to the weight/plow/trailer. It doesn't produce any energy, but it transfers it from the tractor to the weight and not via heat conduction.

Makes more sense and more logical.

Ah right I missed that - thus that link actually corresponds to your definition, so it appears that yours is the more commonly used. And using that definition the reply to the title question is obviously yes: fields/forces that move a wire do work on that wire.

Woho! Now you see what I truly mean,

Thanks DaleSpam for clarifying things out!

Magnetic fields + forces are INDEED doing work in a motor, simple explanation of this as I said at the beginning of this post: When current flows thorough a wire it creates a magnetic field, and another magnetic field is present that is from the magnet, They attract,repel. Now what's doing all the work? Magnetic fields + forces.(Simple explanation to a complicated effect.)

+ There is energy within the magnetic field.

Interesting how things turned out.

 

[Miyz]

I agree with that, but likewise the E field which tethers the stationary lattice protons to the mobile electrons merely transmits the force on the electrons from the magnetic field. Likewise SN force is also like the rope in that it transfers force to the neutrons. Both E & SN forces are akin to the rope in the tractor example.

Remember that the force integrated over the distance is the work done. The mag force must be strong enough to match the E force, plus the SN force, as well as move the electrons. But the mag field gives up energy as it transfers energy to produce torque & speed. The power source at the motor terminals replenishes this energy.

Is the mag force doing "work"? Well, in the short term, YES, in the long term NO. The power source, battery, ac mains wall outlet, etc., is doing all of the long term work. The mag force does move the rotor, but it only acts directly on electrons, but indirectly on protons & neutrons. The E & SN forces are internal tethers, like the rope in the tractor example. They are indispensable as they transmit force to protons & neutrons. The mag force is ineffective on proton & neutron.

Is the mag force doing work? Again, it stores energy then transfers it. It needs help from E & SN forces as well. Mag force participates but can't do it alone, nor long term. The power source is ultimately what does the work, not B field, not E field, not SN force. Did I help or confuse matters more?

Claude

Well put Claude yet again.
True mag forces can't do anything by its own. However, the power source is supplying all that flow E & SN forces!
If the power source or let's say (Input force) is constant the mag force would constantly be doing work :)

 

[Dale]

A nice paper about this question is the following one. The classical part of it precisely answers the question discussed here on hand of a simple example:

http://link.aps.org/doi/10.1103/PhysRevE.77.036609

Hmm, I found the classical part of the paper quite convincing. Especially the ring. Using the "transfers energy" definition of work, someone could say that the magnetic field does work because it transfers energy from rotational KE to translational KE, but that is quite a stretch since the system with the rotational KE is the same as the system with the translational KE.

 

[Miyz]

Hmm, I found the classical part of the paper quite convincing. Especially the ring. Using the "transfers energy" definition of work, someone could say that the magnetic field does work because it transfers energy from rotational KE to translational KE, but that is quite a stretch since the system with the rotational KE is the same as the system with the translational KE.

Now what's you're conclusion on magnetic fields/forces? They do work? Have energy stores within them?

Whats you're new conclusion after check out the links :) ?

For everyone who was/is involved in this topic what are you're FINAL conclusions? (I'd like to see where we are all now).