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How do magnetic fields do no work? 
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#19
Mar1510, 02:34 AM

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#20
Mar1510, 02:44 AM

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#21
Mar1510, 02:44 AM

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Can you give full details about these rings of current, what is initial condition in terms of electron velocity in the 'primary' ring and what is observed effect in the 'secondary' ring in terms of electron displacement or velocity. What is their exact relative position, orientation and size. 


#22
Mar1510, 02:50 AM

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#23
Mar1510, 02:50 AM

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It's a little late for me varga. Besides, I want to see if experts here can rise to the challenge.
gabbagabbahey, I don't have that text. What does it say? 


#24
Mar1510, 03:01 AM

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#25
Mar1510, 03:44 AM

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[tex] \mathbf{F} = I \oint \left(d\mathbf{l}\times\mathbf{B}\right) [/tex] Which is directly from the Lorentz force by noting that the charge is Idl. In a uniform magnetic field, the net force is zero but not so in a nonuniform field. For certain problems this may not be too difficult. For example, Griffiths' gives the net force if you place a current loop directly above a solenoid such that the solenoid and current loop share the same axial axis. The calculated net force is along the axial direction. Since the charges are flowing in the plane normal to this direction, the initial force direction gives 0 work. This follows from the fact that the force is acting directly on the moving charges, which means that it must be acting along a direction that is not going to contribute to the work. While the initial force comes from the magnetic field alone in the frame of the current loop, once the current loop deflects then there is a timevarying magnetic field and thus a timevarying electric field as well. The same could be said about a single electron in a spatially varying, but time static, magnetic field. From the electron's frame of reference, the spatial variations of the magnetic field appear as a timevarying magnetic field and thus an associated timevarying electric field appears as well. EDIT: Quick thought occured to me. If we have a system where the lab frame only has a magnetic field, then given a reference frame that is traveling with velocity v, then the electric field in the reference frame is [tex] \mathbf{E} = \mathbf{v}\times\mathbf{B} [/tex] Neat huh? I think a lot of this discussion is mainly a point of pedantics. I don't think anyone is going to seriously try and work every problem from the perspective of electric field work. But I think that the underlying physics that mediate the energy from the magnetic field to a system are rather subtle and seem to cause a lot of confusion. I have not seen in the literature that the magnetic field's force can give rise directly to work. Instead, I have always read that there are subtle mediations that involve field transformations and such. 


#26
Mar1510, 03:47 AM

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1. moving charge is one magnetic field (ignoring spin dipole moment) 2. "(charge) passes through the field"  this describes the SECOND field We are talking about classical mechanics and classical electrodynamics, in this framework we can explain these experimental setups completely from our (observer) reference frame, just like we do for all the rest of classical physics, and so there is no reason to involve any other more abstract and complicated explanation, which I find flawed anyway. We can measure displacement of those parallel current caring wires and it perfectly matches to the magnitude of magnetic force as given by Ampere's force law or Lorentz force equation and BiotSavart law, so why change anything about it if it works as it is? Two parallel wires, if electric fields of electrons cancel due to superposition with positive electric fields of protons in these wires, then we are obviously left with some other force here, and what kind of force would be the one that can act in two directions in the same time anyway  the very fact that you have two independent and different displacements suggests you have two unique force vectors at work  two forces. As for that page, thank you. Unfortunately there are statements there that are wrong according to everything I encountered in my education, and although that was quite some time ago, still I'm talking about basics like vector math and Newton's laws of motion, so I must raise my objections. But I'm not surprised those wired conclusions were reached if the logic of it was derived from Maxwell's equations instead of from Coulomb's law, BiotSavart law and Lorentz force equations.  "Magnetic forces may alter the DIRECTION in which particle moves, but they can not speed it up or slow it down." 1. What "speed up" and "speed down" has to do with work done? 2. Is change of direction not displacement either with constant velocity or not? This is the same as projectile motion or free fall under gravity field. You have to split the movement into VECTOR COMPONENTS. If bullet starts its trajectory horizontal to the ground, what does gravity do? It ALTERS THE DIRECTION. How? By causing displacement in the direction perpendicular to the motion of the bullet, yet we do not say gravity field can ONLY alter the direction, because "altering direction" is what DISPLACEMENT actually is if you look at the vertical component of the motion  vector calculus. Take the Earth and its gravity field out of the equation and bullet moves straight as its gravity field has nothing to interact with and so we do not expect any gravity force, any change in velocity nor change in direction  Newton's 1st law of motion. 


#27
Mar1510, 04:08 AM

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#28
Mar1510, 04:51 AM

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#29
Mar1510, 10:58 AM

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[tex]W=\frac{1}{2}\int\vec{H}\cdot\vec{B}\kern+2pt dV[/tex] done in bringing a magnetic field up from zero. As mentioned by others, this equation can be derived by considering the electric potentials that are generated by the timevarying currents needed to create the magnetic field. Note also that, for fields created by currents, this leads directly to the engineer's statement of stored energy in terms of inductance [tex]U=\frac{1}{2}LI^2,[/tex] and its extension involving mutual inductance when multiple linked coils are involved. Still, it is most convenient to write the work done in terms of magnetic fields, as above, and not the difficult and detailed equations involving timevarying fields and electric potentials. If the fields in the equation above for work are from two coupled currents, then the derivative of work gives the force between the currents, as pointed out by Bob S. While much of the subsequent discussion of microscopic currents in a magnetized solid may be valid, it takes us out of the practical realmin fact, truly correct treatment of microscopic magnetization is not even classical, but requires quantum mechanics. In the classical world, on the other hand, problems involving magnetic matter are best handled through the magnetization M, which properly accounts for their bulk behavior. In this construct, there is no question that magnetic fields can do work on magnetized solids. To return to two cases already considered, the linear force and the torque on a dipole in a field can be integrated to find the work done as the dipole moves. The general expression for the work done in all such cases is [tex]W=\frac{1}{2}\int\vec{M}\cdot\vec{B}\kern+2pt dV .[/tex] If you want references to crosscheck Griffiths, take a look at ch. 5 in Jackson or ch. 8 of Smythe. 


#30
Mar1510, 11:17 AM

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http://academic.csuohio.edu/deissler..._77_036609.pdf I am planning for some time to clear this thing up in my head. A working solution I don't plan to defend too vigorously is that magnetic field can do work. Within CED answer is no. Reason this isn't so in real world is because field interacts with atomic dipoles which are quantized and can not adjust it's orientation perfectly to cancel working effect of field. Other reason is field interaction with spin which is, of course, not current loop to which classic nowork result applies. 


#31
Mar1510, 12:14 PM

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#32
Mar1510, 12:19 PM

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#33
Mar1510, 12:20 PM

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I just found this post by Vanadium50 in another thread
http://www.physicsforums.com/showthread.php?t=347539 that does a better job explaining my point than I am doing: 


#34
Mar1510, 01:06 PM

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Here is a challenge to those who say a magnetic field can do no work.
Design a nonmagnetic system that will operate my solenoid door bell. The requirements are that no part of the doorbell may be altered, except the magnetic generator may be removed if desired. I maintain that there is no known force that can replace the magnetic coupling to transfer energy from the battery to the plunger. 


#35
Mar1510, 04:12 PM

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Anyway, you could just design one out of clockwork. Press the plunger against a piezoelectric element which sparks a tiny gap firing the circuit... etc. Of course, you lose the solenoid... ... Oh wait, nothing can be altered except the removal of a system. How is that conducive to DESIGNING a system? 


#36
Mar1510, 04:48 PM

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Since I have disassembled my solenoid in anticipation of a flood of non magnetic couplings and whilst I am waiting for all the geniuses (or genii?) here I am employing a man with a hammer. He told me his name was J Arthur Rank.
Seriously can you replace the magnetic coupling with some other sort of coupling? The point is the solenoid transfers energy, magnetically to a mechanical striker and bell. 


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