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sridhar_n
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We all know that the work done by the magetic field is zero. But, how does the magnetic field lift iron objects?
It's funny he couldn't answer this because the answer is pretty obvious. A magnet won't move another magnet unless someone first does the work of pushing them close enough together that their attraction draws them together. It's like a golfball; it won't fall down into the hole unless someone putts it over the edge of the hole.Gza said:It's funny, I asked my physics professor the same type of question reguarding why exactly a permanent magnet can move another magnet without violating the principal of the magnetic field not being able to do work, and he didn't have an answer.
I'm not sure what you're saying here. The set up sounds like this to me: you have a wire carrying current and a permanent magnet sitting still near the wire. You are saying the magnetic field from the magnet is changing the kinetic energy of the electrons?Chi Meson said:If the object is bound within a solid (as in electrons moving through a wire) then this force can become constant in direction.
zoobyshoe said:It's funny he couldn't answer this because the answer is pretty obvious. A magnet won't move another magnet unless someone first does the work of pushing them close enough together that their attraction draws them together. It's like a golfball; it won't fall down into the hole unless someone putts it over the edge of the hole.
Once your magnets are stuck together, you won't get any more of this apparent work out of them unless you do the work of pulling them apart. In the end you are doing all the work, not the magnets.
With the magnets in space, they will accelerate toward each other then stop dead when they finally contact. There is force and motion here, but not all force and motion equals work.ZapperZ said:I think you missed the whole point of the question here. If you have two magnets floating in space, there is a force of attraction between the N-pole of one magnet onto the S-pole of another. This alone will cause the two to move towards each other, which by basic definition, is work done on one by the other. This shows that there's a force acting on the two that actually does work.
zoobyshoe said:With the magnets in space, they will accelerate toward each other then stop dead when they finally contact. There is force and motion here, but not all force and motion equals work.
Work
Address:http://www.batesville.k12.in.us/physics/PhyNet/Mechanics/Energy/Work.html
It is a tricky situation with the magnets, but it looks to me that, despite the force and motion, you end up with 0 change in kinetic energy for both magnets.
It would be work if A. the magnets accelerated each other and then somehow kept going, or B. if they were already in motion and exerted force to stop each other.
As it is, the force they exert is an accelerating force and the stopping is a matter of the resultant collision.
I believe the case of two magnets in space most resembles the 4th example at that site, where a force is exerted on the wall, but the wall doesn't experience any net motion. In that case, no work is done.
zoobyshoe said:Don't give yourself an aneurism, ZapperZ.
Gza said:You both brought up interesting points, but I think this argument stems from a disagreement with regards to semantics. To simplify the question, from my point of view: as a thought experiment, place a magnet in free space, and hold it fixed. Now place the N pole of another magnet aligned with the original magnet's N pole. This magnet obviously accelerates away, and my question is why?
Since work is defined as:
[tex]W = \int \vec{F} \cdot \vec{ds} [/tex]
and [tex]\vec{v} = \frac {\vec{ds}}{dt}[/tex]
obviously implying the velocity of an object to be in the same direction of ds; while v remains perpendicular to the force from the B field from the cross product in the relation:
[tex] \vec{F_B} = q\vec{v} \times \vec{B}[/tex],
and any beginning student in physics can tell you that any force perpendicular to the displacement of an object does no work. So the question is, what exactly is doing the work to change the kinetic energy of the magnet?
Gza said:You both brought up interesting points, but I think this argument stems from a disagreement with regards to semantics. To simplify the question, from my point of view: as a thought experiment, place a magnet in free space, and hold it fixed. Now place the N pole of another magnet aligned with the original magnet's N pole. This magnet obviously accelerates away, and my question is why?
Since work is defined as:
[tex]W = \int \vec{F} \cdot \vec{ds} [/tex]
and [tex]\vec{v} = \frac {\vec{ds}}{dt}[/tex]
obviously implying the velocity of an object to be in the same direction of ds; while v remains perpendicular to the force from the B field from the cross product in the relation:
[tex] \vec{F_B} = q\vec{v} \times \vec{B}[/tex],
and any beginning student in physics can tell you that any force perpendicular to the displacement of an object does no work. So the question is, what exactly is doing the work to change the kinetic energy of the magnet?
A magnetic field does no work on a moving charge. But there are other forces in nature besides this Lorentz force. If there were magnetic monopoles, they would attract and repel each other with a law exactly analogous to Coulomb's law for electric charges. Since there are only dipoles, one can derive the force law between two magnets directly from this "Coulomb's Law" for magnetic "charges". It is a Different Force from the Lorentz force.Gza said:So the question is, what exactly is doing the work to change the kinetic energy of the magnet?
Since there are only dipoles, one can derive the force law between two magnets directly from this "Coulomb's Law" for magnetic "charges". It is a Different Force from the Lorentz force.
Why? A dipole is a combination of two opposite poles. The force between two dipoles is derivable from the force between each one of the 4 charges. It is immaterial whether or not the dipoles can be separated into individual charges. Secondly, a permanent magnet with its little individual dipoles all lined up is to a very good approximation a system where there are charges of type n at one end and charges of type s at the other. Think of two magnets: one of all n charges and the other of all s charges, occupying the same solid, but that the n charges are ever so slightly displaced w.r.t. the s charges. In between the N and S poles of the magnet, all the n's and s's cancel, leaving a surface at the S end with left over s charges, and the surface at the N end with left over n charges.Gza said:But to me at least, it seems you must assume the existence of monopoles for this to work.
The more I think about it, the less I think the free magnet will accelerate away.Gza said:as a thought experiment, place a magnet in free space, and hold it fixed. Now place the N pole of another magnet aligned with the original magnet's N pole. This magnet obviously accelerates away, and my question is why?
A magnetic field is created when electrically charged particles move or spin. In iron objects, the electrons are arranged in such a way that they create a small magnetic field. When an external magnetic field is brought close to the iron object, the two fields interact and align in the same direction. This alignment causes the iron object to be pulled towards the external magnetic field, resulting in the lifting of the object.
The source of the magnetic field that lifts the iron object can be either a natural magnet, such as a lodestone, or an artificial magnet, such as a bar magnet or an electromagnet. These magnets have a strong magnetic field that can attract and lift iron objects due to their alignment of electrons.
No, only iron and some other magnetic materials, such as cobalt and nickel, can be lifted by a magnetic field. This is because these materials have electrons that can easily align in the presence of a magnetic field, while other non-magnetic materials do not have this property.
The strength of the magnetic field needed to lift an iron object depends on the size and weight of the object. Generally, the larger and heavier the object, the stronger the magnetic field needs to be. Stronger magnets, such as rare-earth magnets, can lift heavier objects compared to weaker magnets.
When the iron object is lifted, the magnetic field of the object becomes aligned with the external magnetic field. This creates a stronger magnetic field around the lifted object. However, once the external magnetic field is removed, the iron object's magnetic field returns to its original state, and the object falls back to the ground due to gravity.