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

+ Magnets are permanent dipole's
Current carrying loop is considered a temporary dipole?(No electricity not magnetic field)
 Mentor Yes.
 Thanks DaleSpam,

 Quote by DaleSpam 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

 Quote by cabraham Sounds reasonable. Claude
I guess you and I stand corrected huh Claude?
 Recognitions: Science Advisor 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.
 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!]

 Quote by Miyz [..] 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.

 Quote by harrylin 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,
 Ow yea and don't Skimm since you might have skipped a lot of good info.
 Recognitions: Science Advisor 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

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

 Quote by Miyz 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_b...ml#Section13.1
The tractor does work, but the rope does not.
 Mentor 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/Magneti...agnetic_fields

 Quote by DaleSpam 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.

 Quote by harrylin 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.

 Quote by harrylin Compare: http://www.lightandmatter.com/html_b...ml#Section13.1 The tractor does work, but the rope does not.

That WHOLE idea is irrelevant to this topic.