- #1
da_willem
- 599
- 1
It is well known the magnetic force cannot do work. What causes a magnet to move in the precense of another magnet? Does the magnetization of the two magnets diminish in the proces?
Reality_Patrol said:There's a little bit of mixing apples with oranges going on here.
The statement that "magnetic forces do no work" comes from
considering the Lorentz Force law. This is the expression used to obtain the
equations of motion for an eletrically charged particle
(like an electron). Let's call this the apple.
Now for the orange, magnetic fields can do work on magnetic
dipoles (permanent magnets, electromagnets, fundamental
particles with intrinsic dipole moments...). In which case the working
force can be derived from the magnetic potential as Crosson has indicated
above. That expression leads to a fairly different looking "force law"
from the well-known "Lorentz Law" and it has no official name.
da_willem said:But isn't it true that also non-intrinsic magnetic moments like current loops obey the force law dervied from the potential
[tex]U=-\vec{\mu} \cdot \vec{B}[/tex]
So it cannot have anything to do with the 'intrinsicality' of the dipole. Anyways this would be strong evidence for the intrinsic nature of the magnetic moment of the electron, which is still debated.
So the question remains how come there is this force
[tex] F= \nabla (\vec{\mu}} \cdot \vec{B})[/tex]
that can do work?
A current loop is a dipole, which is nothing more than moving charged particles which can be described by the Lorentz force law which cannot do work, so where is the solution to this paradox? Has it got something to do with the inhomogeneity of the magnetic fields, with quantum mechanics, or what?
It is not the magnetic field doing work in the example you gave. As I recall, it is another force in the system that is doing the work. It may appear to you that it is the magnetic force doing work but it is actually another force (force of conductor on charge I think) that is doing work. There was an article on this topic in AM. J. Phys. If anyone wants to read it let me know and I'lll dig up the reference.joshuaw said:Well, i believe that I have to disagree with this idea some what. If you were to take a naturally magnetic material and let us assume that it is strong enough to move a paper clip. If you bring the material close to the paper clip, the clip will move. The clip feels a force and moves a distance. Now many would argue(Griffiths) that the work was provided by another source. But, last time I checked it is the magnetic fields that caused the clip to move. Since the magnetization is "intrensic", the magnetic domains give the property needed to move the clip. So I guess he would argue that the magnetic moments have done the work.
pmb_phy said:It is not the magnetic field doing work in the example you gave. As I recall, it is another force in the system that is doing the work. It may appear to you that it is the magnetic force doing work but it is actually another force (force of conductor on charge I think) that is doing work. There was an article on this topic in AM. J. Phys. If anyone wants to read it let me know and I'lll dig up the reference.
Pete
seratend said:A good example is the moving wire with a current under a magnetic static field. The magnet loose no energy (just momentum). The wire starts moving in order to keep the orbital momentum of the electrons compatible with the current path in the wire and the magnetic field. Now, because the wire starts moving, it takes its energy from the electrons and not from the magnetic field.
Seratend
da_willem said:So the kinetic energy of the wire remains constant because as it accelerates the current decreases by just the right amount to allow for the increase in velocity of the wire?
Seeda_willem said:I would definitely want to read some more on this subject, so if you could...
pmb_phy said:See
Work done on charged particles in magnetic fields, Charles A. Coombes, Am. J. Phys. 47(10), Oct. 1979
I thought I had two but I must have been thinking about something else.
Pete
Magnets interact through their magnetic fields. Opposite poles, such as north and south, will attract each other while like poles will repel each other. This is due to the alignment of the magnetic domains within the magnets.
When two magnets are brought close together, their magnetic fields interact with each other. The magnetic domains in each magnet will align to either attract or repel each other, depending on the orientation of the poles.
Yes, magnets can lose their magnetization over time. This is known as demagnetization and can occur due to various factors such as exposure to high temperatures, strong magnetic fields, or physical shock.
Temperature can affect magnetization in two ways. When a magnet is heated to its Curie temperature, it will lose its magnetization and become demagnetized. On the other hand, cooling a magnet can increase its magnetization, making it stronger.
No, magnets do not interact with all materials. Only ferromagnetic materials, such as iron, nickel, and cobalt, are attracted to magnets and can be magnetized. Other materials, such as wood or plastic, are not affected by magnets.