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

[Miyz]

I agree. The real issue here is how to draw the boundaries of our interacting systems. The boundary of an interacting system determines which are the internal forces and which are the external forces.
This is why I said that the issue was one of semantics. No boundary was drawn in the diagram. So different people here may be envisioning different subsystems. There are several composite bodies in the problem. What everyone says is correct, but they are talking about different composite bodies.

I guess we truly need to breakdown things to the quantum level to understand how energy,force,work is all processed in this system. Interesting topic.

 

[cabraham]

What do you mean by the SNF? &"important to emphasize that the currents in the field & armature windings (rotor & stator if you prefer), control the value of magnetic force, which results in electric & SNF matching the mag force."
Could you clarify this a bit more(Simpler way), I do understand the "tethered" part. But not sure what you're talking about hee...



Hence the idea of opposing and attracting magnetic fields.
E forces... Whats SN forces?

I thought by applying the magnetic force on the electron directly, an indirect force is applied on the proton and neutron?

The tethering was explained in the link I referred to, from a thread in 2009. If you review the link then I will clarify if needed. SNF is the "strong nuclear force", which tethers the neutron & proton together. I brought it up simply because some suggest that the "E forces" are "really doing the work". I just wanted to remind all that an E field cannot move a neutron. The neutrons are roughly half the mass of any element/compound, except those with hydrogen. So neutron forces/motion cannot be explained with B or E forces, only SNF.

Have I helped? BR.

Claude

 

[Miyz]

The tethering was explained in the link I referred to, from a thread in 2009. If you review the link then I will clarify if needed. SNF is the "strong nuclear force", which tethers the neutron & proton together. I brought it up simply because some suggest that the "E forces" are "really doing the work". I just wanted to remind all that an E field cannot move a neutron. The neutrons are roughly half the mass of any element/compound, except those with hydrogen. So neutron forces/motion cannot be explained with B or E forces, only SNF.

Have I helped? BR.

Claude

Claude, Based on what you said and Darwin123. Is it safe to say: Magnetic field's can do work under certain circumstances? ONLY in the presence of both E forces + N forces can magnetic fields do work.

In a sense they are... So as E force, So as the N force.

They all were triggered in this even or "action" to do work. I would say this in a general simpler way:

Magnetic force can do work with the presence of E,force + N,force.
Electric force can do work with the presence of M,force + N,force.
N force can do work with the presence of M,force + E,force.

In a sense the motor effect... A simple effect CAN NEVER occur without the upper 3 rules.
They are all dependent on each other. without one of them. Nothing would happen.

Simple forces added up together to give the total work.

 

[cabraham]

Claude, Based on what you said and Darwin123. Is it safe to say: Magnetic field's can do work under certain circumstances? ONLY in the presence of both E forces + N forces can magnetic fields do work.

In a sense they are... So as E force, So as the N force.

They all were triggered in this even or "action" to do work. I would say this in a general simpler way:

Magnetic force can do work with the presence of E,force + N,force.
Electric force can do work with the presence of M,force + N,force.
N force can do work with the presence of M,force + E,force.

In a sense the motor effect... A simple effect CAN NEVER occur without the upper 3 rules.
They are all dependent on each other. without one of them. Nothing would happen.

Simple forces added up together to give the total work.

That sounds pretty reasonable to me. Several of us examined this exhaustively, & collectively we ascertained what you just said. Ultimately, though, I believe that the actual work is done by the power source driving the motor. Although energy can be stored in fields, then released to another entity, it is the power source, i.e. the wall outlet power mains, that is providing ALL the energy/power.

In a motor, of course all 3 forces work in unison, but it is safe to say that the magnetic forces, i.e. B/H, are the quantities that literally control the motor action. SNF & E tag along like an obedient shadow, but magnetic is in charge. Using my 3 ball analogy, a magnet lifts 3 tethered balls, steel, wood, & rubber. The magnet cannot lift wood and/or rubber, but it lifts them indirectly by lifting the steel ball directly, & relying on the E & SN tether forces to lift the rubber & wood balls.

But let us not forget, the magnet must provide a lifting force equal to the combined weight of all 3 balls. If each ball weighs a pound force, then the mag force must be 3 lbf. But the E & SN forces are only equal to the weight they carry. If the steel ball is on top, then E & SN provide 2 lbf, while the mag is 3 lbf. So the magnetic force is definitely the prime mover but it still relies on help from E & SN.

Interesting question. These puzzles get us thinking. I want to thank everybody here for being polite. These types of questions usually end up in a mud wrestling match. Everybody on this thread conducted themselves very well, & I take my hat off to all. I hope future discussions can be this civil. Best regards.

Claude

 

[Darwin123]

Claude, Based on what you said and Darwin123. Is it safe to say: Magnetic field's can do work under certain circumstances? ONLY in the presence of both E forces + N forces can magnetic fields do work.

In a sense they are... So as E force, So as the N force.

They all were triggered in this even or "action" to do work. I would say this in a general simpler way:

Magnetic force can do work with the presence of E,force + N,force.
Electric force can do work with the presence of M,force + N,force.
N force can do work with the presence of M,force + E,force.

In a sense the motor effect... A simple effect CAN NEVER occur without the upper 3 rules.
They are all dependent on each other. without one of them. Nothing would happen.

Simple forces added up together to give the total work.

And gravitational forces. Let us not forget those! Furthermore, you have to say what it does work on. I think that you are asking whether a magnetic field can do work on an electric charge or electric current.
I think the following is valid, but demonstrating it may sometimes be difficult.
ONLY in the presence of both E forces + N forces+gravitational forces can magnetic fields do work on an electric charge or electric current.

 

[cabraham]

And gravitational forces. Let us not forget those! Furthermore, you have to say what it does work on. I think that you are asking whether a magnetic field can do work on an electric charge or electric current.
I think the following is valid, but demonstrating it may sometimes be difficult.
ONLY in the presence of both E forces + N forces+gravitational forces can magnetic fields do work on an electric charge or electric current.

Other than gravity, we seem to have reached a unanimous agreement. But gravity is too weak to be concerned with as it is many orders of magnitude smaller than E, M, or N. Nonetheless, we cannot deny its presence, nor that of "W" (weak force", if such materials are present, but I don't think beta decay is present in motor materials).

Thanks again to all. BR.

Claude

 

[Miyz]

Ultimately, though, I believe that the actual work is done by the power source driving the motor. Although energy can be stored in fields, then released to another entity, it is the power source, i.e. the wall outlet power mains, that is providing ALL the energy/power.

When you break that down you'd realize that what the energy source is really just supplying the E forces within a distance. Already energy has been transferred on the wire, and work has been done.

I believe then the E force + SN force would interact with the M forces. We all should break down the concept of Work in, and Work out. Its more understanding when you would say the Forces in and the Netforce out. If you see where I'm going with this... It simply draws a better picture you can visualize it and understand everything more clearly then saying energy In and the energy converted(With respect to all laws, that's just my own way of understanding the relationship of energy,work,force all together).

Interesting question. These puzzles get us thinking. I want to thank everybody here for being polite. These types of questions usually end up in a mud wrestling match. Everybody on this thread conducted themselves very well, & I take my hat off to all. I hope future discussions can be this civil. Best regards.
Claude

Thanks! We should raise questions similar to this! To understand the truth about it more!

 

[Miyz]

Thank you all again for you're efforts!

 

[chingel]

It is important to note that magnetic fields apply forces only on moving charged particles, and for example in the the case of the electric motor, not on the wire itself but on the electrons.

I found this picture from google:

Consider a stationary particle that is on the path of the moving charged particle after it has moved through the magnetic field (for example where the arrow points that is with the text "Path of the particle"). The charged particle would collide with the stationary one after having gone through the magnetic field. Note that the magnetic field does no work on the charged particle, gives it no additional kinetic energy, only changes its direction. If there were no magnetic field, the particles wouldn't collide, but the magnetic field makes them collide and "makes" the second particle move. So it would seem that the magnetic field did work on the other particle, but it didn't, it simply redirected the first particle to hit something it otherwise wouldn't.

The energy to do the work came from the kinetic energy of the first particle, yet without the magnetic field the work couldn't not have been done on the other particle. Nevertheless, the magnetic field did no work, it simply redirected a particle's path.

In the case of the electric motor, the magnetic field would also simply change the paths of electrons, but would not give them any additional energy. The kinetic energy for the moving wire would come from the kinetic energy the electrons had before.

 

[Miyz]

It is important to note that magnetic fields apply forces only on moving charged particles, and for example in the the case of the electric motor, not on the wire itself but on the electrons.

I found this picture from google:

Consider a stationary particle that is on the path of the moving charged particle after it has moved through the magnetic field (for example where the arrow points that is with the text "Path of the particle"). The charged particle would collide with the stationary one after having gone through the magnetic field. Note that the magnetic field does no work on the charged particle, gives it no additional kinetic energy, only changes its direction. If there were no magnetic field, the particles wouldn't collide, but the magnetic field makes them collide and "makes" the second particle move. So it would seem that the magnetic field did work on the other particle, but it didn't, it simply redirected the first particle to hit something it otherwise wouldn't.

The energy to do the work came from the kinetic energy of the first particle, yet without the magnetic field the work couldn't not have been done on the other particle. Nevertheless, the magnetic field did no work, it simply redirected a particle's path.

In the case of the electric motor, the magnetic field would also simply change the paths of electrons, but would not give them any additional energy. The kinetic energy for the moving wire would come from the kinetic energy the electrons had before.

Um, if you say that magnets do no work... Check this out

1 - Yes they can,
2 - Why?
3 - Convinced?

If you still need some more "worded" detail check theses links out: 1, 2,

As I said before and I will say this again. Magnets/magnetic fields/etc... Are not understood fairly well... I mean so many attention went for complex idea how the most simplest products of nature and most important forces have been underrated...

 

[cabraham]

It is important to note that magnetic fields apply forces only on moving charged particles, and for example in the the case of the electric motor, not on the wire itself but on the electrons.

I found this picture from google:

Consider a stationary particle that is on the path of the moving charged particle after it has moved through the magnetic field (for example where the arrow points that is with the text "Path of the particle"). The charged particle would collide with the stationary one after having gone through the magnetic field. Note that the magnetic field does no work on the charged particle, gives it no additional kinetic energy, only changes its direction. If there were no magnetic field, the particles wouldn't collide, but the magnetic field makes them collide and "makes" the second particle move. So it would seem that the magnetic field did work on the other particle, but it didn't, it simply redirected the first particle to hit something it otherwise wouldn't.

The energy to do the work came from the kinetic energy of the first particle, yet without the magnetic field the work couldn't not have been done on the other particle. Nevertheless, the magnetic field did no work, it simply redirected a particle's path.

In the case of the electric motor, the magnetic field would also simply change the paths of electrons, but would not give them any additional energy. The kinetic energy for the moving wire would come from the kinetic energy the electrons had before.

But we're discussing forces between current carrying wires, akin to motor operation. We're not discussing collisions among particles. It has already been stated unanimously that a single charged particle, or several, moving through a magnetic field can only have its momentun changed, not its kinetic energy.

The rebuttal directly before this one by Miyz addresses what we've been talking about. When 2 current loops interact resulting in a net torque, we are examining the work done, forces involved, etc. Please review the link I gave earlier. Rather than repeat what has already been discussed, I will elaborate if you still have questions. Best regards.

Claude

 

[Miyz]

But we're discussing forces between current carrying wires, akin to motor operation. We're not discussing collisions among particles. It has already been stated unanimously that a single charged particle, or several, moving through a magnetic field can only have its momentun changed, not its kinetic energy.

The rebuttal directly before this one by Miyz addresses what we've been talking about. When 2 current loops interact resulting in a net torque, we are examining the work done, forces involved, etc. Please review the link I gave earlier. Rather than repeat what has already been discussed, I will elaborate if you still have questions. Best regards.

Claude

He should review you're thread and ours. Then he would understand our common conclusion about magnets doing work or not.(Again, THEY DO but UNDER CERTAIN circumstances)

 

[chingel]

But we're discussing forces between current carrying wires, akin to motor operation. We're not discussing collisions among particles. It has already been stated unanimously that a single charged particle, or several, moving through a magnetic field can only have its momentun changed, not its kinetic energy.

The rebuttal directly before this one by Miyz addresses what we've been talking about. When 2 current loops interact resulting in a net torque, we are examining the work done, forces involved, etc. Please review the link I gave earlier. Rather than repeat what has already been discussed, I will elaborate if you still have questions. Best regards.

Claude

A current through a wire is several charged particles moving. They feel the magnetic force, not the wire directly, and respond to the force by changing their direction and giving some of their momentum in the new direction to the wire, creating a force on the wire.

It should be similar to the collision. The direction of the electrons is changed and thus allowing them to do work on the wire by colliding with it. Otherwise the collisions would be chaotic, but the magnetic force changes the directions of the electrons in a consistent manner and allows the work to be done at the expense of the electrons kinetic energy, as in the case of a simple collision.

 

[Darwin123]

Other than gravity, we seem to have reached a unanimous agreement. But gravity is too weak to be concerned with as it is many orders of magnitude smaller than E, M, or N. Nonetheless, we cannot deny its presence, nor that of "W" (weak force", if such materials are present, but I don't think beta decay is present in motor materials).

Thanks again to all. BR.

Claude

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.

 

[cabraham]

A current through a wire is several charged particles moving. They feel the magnetic force, not the wire directly, and respond to the force by changing their direction and giving some of their momentum in the new direction to the wire, creating a force on the wire.

It should be similar to the collision. The direction of the electrons is changed and thus allowing them to do work on the wire by colliding with it. Otherwise the collisions would be chaotic, but the magnetic force changes the directions of the electrons in a consistent manner and allows the work to be done at the expense of the electrons kinetic energy, as in the case of a simple collision.

An internal collision does not do work on the wire since conservation of momentum requires that the wire moves, i.e. undergo a momentum change, only when acted upon by an external force. This force is M. But, M exerts no force on stationary lattice protons & neutrons. Hence E & SN forces provide the tether force so that when M yanks on the electrons, the protons & neutrons are tethered along by E & SN forces.

Claude

 

[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.