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

[cabraham]

Another assumption implicit in your diagram is that the permanent magnets don't move. This is one reason the magnetic field is static. If the magnets were allowed to move, then the problem would be more complicated.
The magnets keep their shape by rigid body forces. They may be held stationary on the horizontal plane also by rigid body forces. For instance, the mangets may be attached by glue to the horizontal surface. However, suppose the magnets are not attached directly to the plane.
If the magnets are not attached directly to the surface, then there has to be other forces involved. The magnets may have to be held still by a mixture of both gravity, contact force and static friction. The gravity prevents the magnet from moving up. The contact force (i.e., the normal force) prevents the magnet from sinking down. The static friction prevents it from moving in the horizontal plane.
Similarly, the wire loop has some weight. If the wire loop is not uniform in thickness, the unbalanced wire could be affected by gravity.
The weight of a single carrier may be negligible. However, the weight of other components in the system may not be negligible.
The discussion has turned to the contribution of nonmagnetic forces to the work done on the wire loop. The conjecture has been raised that maybe nonmagnetic forces "do work" in a motor. Gravity may well "do work" on a motor.
Fortunately, the problem can be solved without enumerating all the forces "that do work". The work done by most of those forces cancel out. By choosing the boundaries on the system properly, one can "hide" the forces that cancel out. In general, this is what has to be done.

How can gravity "do work" on a motor? I just want to know how.

Claude

 

[Miyz]

I think that has a smal weak effect on the motor... It can't do work if it was it would have been added in this formula F = IL x B.

And if you think it does Darwin123,how is it so?

 

[Miyz]

Darwin123, You're really confusing me here... Could you give me a SIMPLE conclusion that you agree upon? In a sentence perhaps?(Makes it all clear.)

As I said before and will continue to stand upon this point magnets can do work under certain circumstances. And magnetic field will possesses potential energy which depends upon its orientation with respect to the magnetic field.

It's all complicated business lol, however. Interesting as ever :)

 

[Dale]

A magnetic field can certainly do work on a current carrying wire.

For example, consider a superconducting loop with current. Such a current carrying wire forms a magnetic dipole. A uniform external magnetic field can exert a torque, and a non-uniform field can exert a net force, both of which may be arranged to do work on the wire.

A magnetic field can not do work on a classical isolated point charge, but that doesn't prevent it from doing work on other things.

 

[Dale]

I provided an example where it does. I am sorry, but nature disagrees with you.

 

[chingel]

A magnetic field can certainly do work on a current carrying wire.

For example, consider a superconducting loop with current. Such a current carrying wire forms a magnetic dipole. A uniform external magnetic field can exert a torque, and a non-uniform field can exert a net force, both of which may be arranged to do work on the wire.

A magnetic field can not do work on a classical isolated point charge, but that doesn't prevent it from doing work on other things.

But isn't it also correct to say that the magnetic field does no work on the electrons, it just changes their path, and then the changed paths of the electrons cause the torque or force on the wire? As the electrons paths are changed, electric forces keep them from escaping the wire and due to Newton's third law the wire experiences a force.

 

[Miyz]

I provided an example where it does. I am sorry, but nature disagrees with you.

hahahahahaha! THAT JUST MADE MY DAY! Seriously.

 

[Miyz]

But isn't it also correct to say that the magnetic field does no work on the electrons, it just changes their path, and then the changed paths of the electrons cause the torque or force on the wire? As the electrons paths are changed, electric forces keep them from escaping the wire and due to Newton's third law the wire experiences a force.

Good point. Now I'm starting to get confused here to

 

[Miyz]

A uniform external magnetic field can exert a torque, and a non-uniform field can exert a net force, both of which may be arranged to do work on the wire.

Didn't really understand that point well... Could you elaborate more DaleSpam?

 

[Dale]

But isn't it also correct to say that the magnetic field does no work on the electrons, it just changes their path, and then the changed paths of the electrons cause the torque or force on the wire? As the electrons paths are changed, electric forces keep them from escaping the wire and due to Newton's third law the wire experiences a force.

I doubt that it is correct to say in a superconducting wire. In general, electrons are not little classical point particles, but in most normal situations it is probably an OK approximation.

However, superconduction electrons are not even approximately like that. They are in a very strange quantum state where an individual electron is literally not localizable to any location in the wire and all of the superconduction electron pairs share the same state.

I don't think that under those conditions the Lorentz force law for a point charge is correct.

 

[Miyz]

I doubt that it is correct to say in a superconducting wire. In general, electrons are not little classical point particles, but in most normal situations it is probably an OK approximation.

However, superconduction electrons are not even approximately like that. They are in a very strange quantum state where an individual electron is literally not localizable to any location in the wire and all of the superconduction electron pairs share the same state.

I don't think that under those conditions the Lorentz force law for a point charge is correct.

Then in a normal non-superconducting loop. Are magnet's doing work?

 

[Dale]

Didn't really understand that point well... Could you elaborate more DaleSpam?

Here is a good page to begin understanding the forces between different configurations of magnets:
http://en.wikipedia.org/wiki/Force_between_magnets

A loop of current forms a magnetic field which is called a magnetic dipole. It is called that because it has the same mathematical form as an electical dipole (two point charges of equal and opposite polarity).

When a magnetic dipole is placed in a uniform external magnetic field it tends to align with the external magnetic field, this is how a compass needle functions. In a uniform field it experiences this torque, but no net force. However, in a non-uniform field it also experiences a net force, as described in the page above.

 

[Darwin123]

A magnetic field can certainly do work on a current carrying wire.

For example, consider a superconducting loop with current. Such a current carrying wire forms a magnetic dipole. A uniform external magnetic field can exert a torque, and a non-uniform field can exert a net force, both of which may be arranged to do work on the wire.

A magnetic field can not do work on a classical isolated point charge, but that doesn't prevent it from doing work on other things.

An isolated magnetic dipole can't exist without nonmagnetic forces that keep the current going in circles. The carriers in you superconducting loop are carrying the electric current through the wire. However, carrier would not move in circles unless an electric field applied a centripetal force to the carriers.
An electric field exists at the border of the superconducting loop, In addition, the superconductivity itself depends on forces other than the magnetic force. The conduction electrons in the "typical" superconductor are coupled by phonons to form Cooper pairs. The phonons are vibrational modes caused by the electric field of the nuclei of the atoms.
The force on a magnetic dipole by a magnetic field also has contributions from "nonmagnetic" forces. In fact, the example with the wire loop is also a magnetic dipole. Nonmagnetic forces make the carriers move in a closed curve, which generates a magnetic dipole.

 

[Dale]

An isolated magnetic dipole can't exist without nonmagnetic forces that keep the current going in circles. The carriers in you superconducting loop are carrying the electric current through the wire. However, carrier would not move in circles unless an electric field applied a centripetal force to the carriers.

You are thinking of the superconduction electrons as classical little balls with a well-defined position and velocity and acceleration, it is simply an incorrect idea. A superconduction electron pair is not localized around the loop, there is no centripetal force because it is not accelerating. I.e. its wavefunction is not changing over time.

In fact, the electric field that you are describing does not exist in a superconductor. It is one of the defining properties of superconduction that the material cannot support such an E-field.

 

[Miyz]

You are thinking of the superconduction electrons as classical little balls with a well-defined position and velocity and acceleration, it is simply an incorrect idea. A superconduction electron pair is not localized around the loop, there is no centripetal force because it is not accelerating. I.e. its wavefunction is not changing over time.

In fact, the electric field that you are describing does not exist in a superconductor. It is one of the defining properties of superconduction that the material cannot support such an E-field.

If the loops was not a superconductor... Would the magnetic fields still be able to do work? On a regular loop. Generally what's you conclusion? Can magnetic fields do work on a current carrying loop?(That's not superconducting)

 

[Miyz]

Surly work being done here is by the magnetic force's...

 

[Dale]

If the loops was not a superconductor... Would the magnetic fields still be able to do work? On a regular loop. Generally what's you conclusion? Can magnetic fields do work on a current carrying loop?(That's not superconducting)

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.

 

[Miyz]

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.

So F = q(V x B) Is only applied on the particle scale of things?

 

[Miyz]

+ Magnets are permanent dipole's
Current carrying loop is considered a temporary dipole?(No electricity not magnetic field)

 

[Dale]

Yes.

 

[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