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Natural Science Forum / Physics / General Physics / July 2008Magnetically Coupled Resonance
I was reading about magnetically coupled resonance recently, and a question came to me. How is MCR any different than a radio transmitter and receiver? On Jul 27, 6:03 am, copiouse...@gmail.com wrote: > I was reading about magnetically coupled resonance recently, and a > question came to me. How is MCR any different than a radio transmitter > and receiver? One has just a megnetic field whilst radio waves have an electric field too. K On Jul 26, 4:07 pm, kronec...@yahoo.co.uk wrote: > On Jul 27, 6:03 am, copiouse...@gmail.com wrote: > [quoted text clipped - 6 lines] > > K But if you have an oscillating magnetic field, you have a radio transmitter. If you have an oscillating electric field, you have a radio transmitter. Rapidly changing either type of field creates electromagnetic waves. The typical transmitter uses a rapidly changing electric field, and MCR uses a magnetic field, but the result is the same, is it not. In other words, how is one way better than the other for transferring energy through space? On Jul 26, 11:08 pm, copiouse...@gmail.com wrote: > On Jul 26, 4:07 pm, kronec...@yahoo.co.uk wrote: > [quoted text clipped - 18 lines] > In other words, how is one way better than the other for transferring > energy through space? Actually, since you appear to be acquainted with Maxwell, you know that time variant electric or magnetic fields are idential, since they both produce what (Dhuh) are known as electromagnetic waves. Most people learn this in their first of second year in college, unless they are majoring in the liberal arts. I'm still waiting for you to post your explanation of what "MCR" is, and the basis for your confusion. My guess is that you actually mean MRI of NMR, and are wondering about why that big magnetic field is needed. Right? Harry C. On Jul 27, 10:07 am, "hhc...@yahoo.com" <hhc...@yahoo.com> wrote: > On Jul 26, 11:08 pm, copiouse...@gmail.com wrote: > [quoted text clipped - 33 lines] > > Harry C. MCR = Magnetically Coupled Resonance. I was reading about MIT's experiment powering a light bulb remotely by MCR. And how this could some day mean "wireless electricity". The first thing that occurred to me is that this is nothing new, but wasn't sure. It seems to me that instead of using a rapidly varying an electric field to produce EM radiation, they are rapidly varying a magnetic field. Like you said, they are identical, so what's the new thing about MCR? On Jul 27, 11:42 am, copiouse...@gmail.com wrote: > On Jul 27, 10:07 am, "hhc...@yahoo.com" <hhc...@yahoo.com> wrote: > [quoted text clipped - 7 lines] > > > > > question came to me. How is MCR any different than a radio transmitter > > > > > and receiver? I think in MCR the antennae is larger than the house or city that uses the power. > > > > One has just a megnetic field whilst radio waves have an electric > > > > field too. Radio waves per se exist only under the far field condition. The Electric and magnetic fields obey the laws of optics only at distances far from the antennae. Close to and inside the antennae, the electric and magnetic fields are distinguishable. Although called waves, the term "wave" is technically correct only at a few qwavelengths from the antennae with the electric current. > > > But if you have an oscillating magnetic field, you have a radio > > > transmitter. If you have an oscillating electric field, you have a > > > radio transmitter. Rapidly changing either type of field creates > > > electromagnetic waves. Not under the near field condition. Inside a radio antennae, the electric and magnetic fields are distinguishable. The reason is that the electric current that generates them is close by the fields. > MCR = Magnetically Coupled Resonance. I was reading about MIT's > experiment powering a light bulb remotely by MCR. And how this could > some day mean "wireless electricity". The first thing that occurred to > me is that this is nothing new, but wasn't sure. I think that Tesla did a lot of experiments in this direction, somewhere at the turn of the twentieth century. Poor Tesla was way ahead of his time, and was shamelessly exploited. I suspect MIT is "rediscovering" his work. > It seems to me that > instead of using a rapidly varying an electric field to produce EM > radiation, they are rapidly varying a magnetic field. Like you said, > they are identical, so what's the new thing about MCR? They are not identical in the near field approximation. Although I haven't read up on the MIT experiments, I can make an educated guess based on your description. What the MIT people may be doing is using HUGE coils to transmit the electricity. When I mean huge, I mean the user of the electricity is near the center of the coil. Under those conditions, the near field approximation is valid. So the idea may be like this: Take a city that now uses electric wires to directly carry power to the home. Tear out most of the wiring on all the homes. Use some of the torn out wiring to make a huge circular coil, much larger than the city. Place the city at dead center to this coil. Transmit an oscillating electric current through this super large coil. Use relatively low frequency of oscillation, which ensures that the near field condition holds. This super huge coil is the transmitter. Now, each home has kept a small amount of wire to make a receiving coil. Orient that receiving coil with the magnetic field of the transmitting coil. Now the magnetic field will induce a current in the receiving coil. Note there are thousands of receiving coils, and only one transmitting coil. Also note that the power is not being transmitted by a true electromagnetic wave. The power is being transmitted through an oscillating magnetic field, with no measurable electric field. The difference is the huge coil. Because the receiving coils are near the center of the huge super coil, the system satisfies the near field condition. I am not sure of the advantage of this over the present system. In any case, the electric current has to be transmitted though a large amount of wires. My best guess is that there is less wiring required for the super coil than for the myriad of subsystems needs to send electricity to each and every home. On Jul 27, 11:42 am, copiouse...@gmail.com wrote: > On Jul 27, 10:07 am, "hhc...@yahoo.com" <hhc...@yahoo.com> wrote: > [quoted text clipped - 45 lines] > > - Show quoted text - OK, I belive that I understand respect your explanation which seems to explain the concept of how my electric toothbrush gets charged when I sit it on its stand at night. Realize that this is simply an air cored transformer and has nothing to do with resonance priciples. A transformer is simply a tranformer. Its winding are simply coupled magnetically. I'm not quite sure of what article you read that was citing an MIT experiment, but let me share with you the fact that MIT students are not incapable of a little leg pulling. There are many reasons why electrical power is not transmitted to wireless consumers. Not the least of these reason is that you cannot control the electromagnetic radiation pattern, only focus it to a small extent. Another reason is that you would possibly 'fry the brains' of anyone who happend to be in the transmission path. Rather nasty lawsuits could result. In his 'declining' years, Tesla proposed such an energy transmission scheme. I am not sure if he was insane at that time, or had never learned the basics of electromagnetic wave radiation. Harry C. On Jul 28, 12:41 am, "hhc...@yahoo.com" <hhc...@yahoo.com> wrote: > On Jul 27, 11:42 am, copiouse...@gmail.com wrote: > [quoted text clipped - 58 lines] > experiment, but let me share with you the fact that MIT students are > not incapable of a little leg pulling. I didn't look up any article on the wireless transmission of energy. I was making an educated guess based on copiouse's incomplete description. He did start out stating he didn't understand what magnetic coupling was. I was addressing that point of confusion, rather than the specific point of how to transmit power without wires. The problem may be one of semantics. Under near field conditions, the transmission of electromagnetic energy isn't properly described by electromagnetic waves. For example: If an observer is you are very close to a radio antennae, within less than half a wavelength of the radio wave sent out by the antennae, the flow of energy can not be described by the wave equation. Therefore, true radio waves only form at a distance from that antennae. Therefore, the transmission of energy close to the antennae shouldn't be referred to as "radio wave transmission." I think that is this semantic point that confuses him. He seems to think that all electromagnetic energy is carried by "electromagnetic waves." I want primarily to dispel this misconception before discussion starts as to how serious the proposal is. > There are many reasons why electrical power is not transmitted to > wireless consumers. Not the least of these reason is that you cannot > control the electromagnetic radiation pattern, only focus it to a > small extent. Ahh, but the energy doesn't have to be carried by "electromagnetic radiation" if by radiation you mean waves. In fact, now that I think about it, electromagnetic waves are a lot harder to control than the electromagnetic field near a current or charge. The magnetic energy generated by a solenoid coil is concentrated inside the magnetic field within the solenoid coil. The electromagnetic energy in a capacitor stays in the electric field of the capacitor. The radio waves from an antennae travel outward in all directions. That is why there is an inverse square law in intensity. The radio waves don't stay in place. It makes sense to me that someone may try to utilize ways of transmitting power that don't depend on true "waves." So I suspect there may be some truth to his story. If I get this point across, maybe he can research the idea better. >Another reason is that you would possibly 'fry the > brains' of anyone who happend to be in the transmission path. If one tried to transmit power with radio waves, this would probably be true. The reason is that radio waves carry half their energy in an electric field, and half their energy in a magnetic field. The electric field interacts with conductors (like our blood) much stronger than a magnetic field. Thus, a high power radio wave would fry the people it passes through. The electric field would generate an electric current in the blood, heating the blood, and making it boil. However, suppose the energy was carried in a strong magnetic field without an associated electric field. Consider the giant coil example that I proposed. This would have a strong magnetic field, no significant electric field. The people would not fry. > Rather > nasty lawsuits could result. If this giant coil is ever built, stay away from any iron nails left in the city. The magnetic field may interact with ferromagnetic materials, heating them or even pulling them out of the walls. > In his 'declining' years, Tesla proposed such an energy transmission > scheme. I am not sure if he was insane at that time, or had never > learned the basics of electromagnetic wave radiation. I suspect the latter. Tesla was not a theorist. He was a hands- on experimentalist. To analyze his experiments, he often used theories that were valid only under limiting conditions, that coincided with his experiments. I think many of his experiments and devices required "near field conditions." For example, his Tesla coil isn't really a radio wave transmitter. It actually transmits electricity over a short distance. Ever notice, when playing with a Tesla coil, how the ionized gases follow your finger? That is a capacitive effect. This is a near field effect, not a far field effect. On the other hand, maybe that was sufficient. One doesn't need to always use the most general theory to get something done. Maybe in his later years he learned more about radio waves. Or maybe in his later years he came up with a novel method that used near field conditions to transmit electric power. Maybe it would work. I don't think we have the right to call him insane, right or wrong. Tesla didn't like Einstein's theory of relativity. He thought it wasn't really science. That shows how poor a theorist he was. However, Tesla was a was a real good experimentalist. He may have been better than Edison. I think that Einstein and Tesla would have been a real winning combination, if they decided to communicate with each other. I don't think Tesla was insane. He had real problems getting along with people, but he did not hallucinate or delude himself like certain people on this forum. [snip] >I didn't look up any article on the wireless transmission of >energy. I was making an educated guess based on copiouse's incomplete [quoted text clipped - 5 lines] >electromagnetic energy isn't properly described by electromagnetic >waves. Speaking of semantics, this looks like a semantic error here. Or it could be a different kind of error. I'll let you decide;) I think what you meant to say is something like, "transmission of electromagnetic energy is usually described in terms of the far-field approximation to the solution of Maxwell's equations for a radiating dipole (radiated waves): but since they are far-field, they are not good approximations for near field condtions". This would be true. But we are still forced to concede that it is properly described by "electromagnetic waves", since electromagnetic waves _are_ the photons, which _are_ the exchange particles that generate the force. That is, once you take the viewpoint of QED, which is necessary to fully and accurately describe any interaction of matter and free E-M waves, you find yourself forced to use electromagnetic waves for everything -- usually in ugly multi-dimensional integrals of Fourier analysis;) See, for example, http://en.wikipedia.org/wiki/Photon which says: Begin quote----------------------- in theoretical physics, a photon can be considered as a mediator for ANY type of electromagnetic interactions, including magnetic fields and electrostatic repulsion between like charges. End quote---(emphasis mine)------- > For example: If an observer is you are very close to a radio >antennae, within less than half a wavelength of the radio wave sent >out by the antennae, the flow of energy can not be described by the >wave equation. The wave equation is used for describing the propagation of E-M waves. But you seem to have in mind a very special case of it, the homogeneous wave equation with simple boundary conditions. True, that cannot be used here. Ultimately, the right thing to use would be not the wave equation, but Maxwell's equations: then when these in turn imply you can approximate well with the homogenous wave equation (with simple boundary conditions), you do so. >Therefore, true radio waves only form at a distance >from that antennae. That is, freely propagating radiated waves form only at a distance. > Therefore, the transmission of energy close to the >antennae shouldn't be referred to as "radio wave transmission." I >think that is this semantic point that confuses him. > He seems to think that all electromagnetic energy is carried by >"electromagnetic waves." I want primarily to dispel this misconception >before discussion starts as to how serious the proposal is. Perhaps the right way to do this would be to use the term "freely propagating radiated electromagnetic waves". Yet even this term does not clearly distinguish between the real physical wave you clearly have in mind from the infinitely many waves summed up by Fourier analysis to represent any field distribution as described in: http://en.wikipedia.org/wiki/Electromagnetic_wave_equation See especially the subsection titled "Spectral decomposition". Any solution to Maxwell's equations can be written as an infinite sum or integral of electromagnetic waves. And Maxwell's equations describe all E-M phenomena -- even if they sometimes need help via "second quantization". [snip] On Jul 28, 7:59 pm, Matthew Johnson <matthew_mem...@newsguy.org> wrote: > In article <d6d5cee7-e1b8-4428-850b-fe3da10d0...@m73g2000hsh.googlegroups.com>, > Darwin123 says... > This would be true. But we are still forced to concede that it is properly > described by "electromagnetic waves", since electromagnetic waves _are_ the > photons, which _are_ the exchange particles that generate the force. Okay, you just made a leap. I was talking in terms of strictly classical electrodynamics. When you bring in photons, you have to bring in quantum mechanics. In fact, you have to bring in a little quantum electrodynamics (QED) theory. Although the same system can be described in QED theory, it is unnecessary to answer his question. Power transmission can be described using plain old Maxwell's equations, with no quantized anything. If you want to go there, however, you have to look at the concepts of QED closely. > That is, once you take the viewpoint of QED, which is necessary to fully and > accurately describe any interaction of matter and free E-M waves, you find > yourself forced to use electromagnetic waves for everything -- usually in ugly > multi-dimensional integrals of Fourier analysis;) Semi-wrong. The Fourier analysis by itself doesn't necessarily refer to traveling waves. The confusion is that traveling waves in classical electromagnetic theory carry half their energy in their electric fields, and half their energy in magnetic fields. Power transmission can't always be described in terms of such waves. For example, classical electrodynamics accurately describes static electric fields and static magnetic fields. Static electric fields and static magnetic fields are not traveling waves. Yet, Maxwell's equations describes how charges interact with them. If you broaden the definition of wave, you can include electric and magnetic fields. However, I don't think it was this type of wave that Copiouse was talking about. He was talking about traveling waves. You can't describe always describe the transmission of power in terms of traveling waves. QED has virtual particles as well as real particles. The real particles correspond to electromagnetic waves. The real particles refer to electromagnetic fields under far field conditions. In other words, real particles correspond to traveling waves. The virtual particles refer to electromagnetic fields under near field conditions. The virtual particles correspond closer to most types of power transmission. Therefore, radio waves would correspond to real particles. Magnetic-field induced-power would correspond to virtual particles, as would electrical-field induced power. Real particles don't disappear, they keep going and going. So transmission by radio waves corresponds to shooting real photons in all directions, most of the power being lost. In my giant coil example, the virtual photons would be staying in the area of the coil disappearing before they left the coil. Therefore, in the large coil proposal that I talked about the power can be said to be transmitted by virtual photons. The magnetic field can be pictured as being caused by a particular type of virtual photon. This is an overly convoluted way to look at it, I admit. However, you were the one who brought in QED. QED includes virtual photons for exactly the type of systems we are talking about. However, I prefer to stay with classical electrodynamics, when I can. In any case, traveling waves are not the answer to every electrodynamics problem whether in classical electrodynamics or QED. So I still think the issue Copi is talking about is a near field way to transmit power over large distances. I think a coil inside a coil approach could do it. Maybe this is what the MIT guys were talking about. > See, for example,http://en.wikipedia.org/wiki/Photonwhich says: > [quoted text clipped - 44 lines] > > [snip] >On Jul 28, 7:59 pm, Matthew Johnson <matthew_mem...@newsguy.org> >wrote: [quoted text clipped - 5 lines] >> photons, which _are_ the exchange particles that generate the force. > Okay, you just made a leap. Yup. >I was talking in terms of strictly >classical electrodynamics. Which may have been the right thing to do. Then again, it is still remotely possible that the OP may have heard something about how the electromagnetic field can be represented as a sea of harmonic oscillators, and have been confused by that. >When you bring in photons, you have to >bring in quantum mechanics. In fact, you have to bring in a little >quantum electrodynamics (QED) theory. Although the same system can be >described in QED theory, it is unnecessary to answer his question. >Power transmission can be described using plain old Maxwell's >equations, with no quantized anything. Ah, but this 'description' usually relies on glossing over the interaction of the E-M field with matter, relying on phenomological results that are all too often not clearly stated. Remember that radiating dipole I mentioned? You can find such a derivation of radiated/traveling waves in oodles of elementary physics books, or in electrical engineering texts (even at more advanced levels), yet how many of them mention that these dipoles have no physical existence? They are 19th century models with no physical reality. Yet we still use them in 20th century texts! The physical reality is QED. >If you want to go there, >however, you have to look at the concepts of QED closely. [quoted text clipped - 5 lines] > Semi-wrong. The Fourier analysis by itself doesn't necessarily >refer to traveling waves. I didn't say that it did! I said rather that any electromagnetic field (whether stationary or varying, and over any arbitrary volume) can be represented as an infinite sum or Fourier integral (depending on the case) involving infinite waves -- waves that mathematically look a lot like your "traveling waves". However, I did not state it clearly, and you are probably right that we do not need to go there to address the OPs question. >The confusion is that traveling waves in >classical electromagnetic theory carry half their energy in their >electric fields, When traveling in what medium?? What are you assuming for permittivity and permeability? > and half their energy in magnetic fields. Power >transmission can't always be described in terms of such waves. True. Then will it help to delineate the cases in which power transmission _can_ be described in terms of such waves? >For example, classical electrodynamics accurately describes static >electric fields and static magnetic fields. And "quasi-static" too, i.e., when the circuit components are small compared to the highest relevant wavelength, e.g., shortwave frequencies in a shortwave radio transceiver. But not in a VHF transceiver or modern computer, where we have to start using fancier techniques, even microwave techniques, such as S-matrices. [snip] On Jul 29, 8:02 pm, Matthew Johnson <matthew_mem...@newsguy.org> wrote: > In article <95c14cc4-6808-4057-b6b5-d3eb927dc...@t54g2000hsg.googlegroups.com>, > Darwin123 says... [quoted text clipped - 15 lines] > > Which may have been the right thing to do. Then again, it is still >remotely possible that the OP may have heard something about how >the electromagnetic field can be represented as a sea of harmonic oscillators, and have been confused by that. I didn't get that. However, you may be right. I was answering the question according to my interpretation of his question. I hope he makes a comment on our discussion, and tells us if there is anything in the thread that has enlightened him. His question has stimulated an interesting thread. > >When you bring in photons, you have to > >bring in quantum mechanics. In fact, you have to bring in a little [quoted text clipped - 39 lines] > When traveling in what medium?? What are you assuming for permittivity and > permeability? I am assuming that both permittivity and permeability are entirely real quantities, with no imaginary components. The most important examples are the vacuum and the lower atmosphere of earth. Almost material with zero conductivity, which is neither ferromagnetic or ferrimagnetic, would support electromagnetic waves that have the following properties: 1) Half the energy is in the magnetic field and half in the electric field. 2) The electric field, the magnetic field, and the wave vector are all orthogonal to each other. This type of polarization is called transverse. The are planar polarization modes and circular polarization modes, both of which are types of transverse polarization. Of course, there are many materials that don't obey those conditions. There is a big topic of metal optics. Light waves can pass through thin layers of metal, and on the surface of metals. While they are in the metal, the electric field is suppressed more than the magnetic field. There is a longitudinal polarization mode in a metal. > > and half their energy in magnetic fields. Power > >transmission can't always be described in terms of such waves. > > True. Then will it help to delineate the cases in which power transmission _can_ > be described in terms of such waves? Not for the original question. Maybe I am misrepresenting him, but I think I see where he is confused. Maybe I merely remember my own confusion when taking these courses, and have projected on him. Copi knows that radio waves carry power. He mentioned them. Therefore, he doesn't need more examples of how electromagnetic radiation carries power. He refers to electromagnetic waves having BOTH electric fields and magnetic fields. He is 100% right. In fact, traveling waves in air and vacuum carry half their energy in their magnetic fields and half in electric fields. His question can be rephrased: How on earth can anyone transmit power through the air by means of purely magnetic coupling? What happened to the electric field? I was trying to offer a conjecture on how it can be done. > >For example, classical electrodynamics accurately describes static > >electric fields and static magnetic fields. [quoted text clipped - 4 lines] > have to start using fancier techniques, even microwave techniques, such as > S-matrices. Yes, exactly. The near field condition includes electric and magnetic fields that are oscillating very slowly. However, I think the multipole expansion is the easiest way to introduce classical electromagnetic theory. That is how it is done in "Classical Electrodynamics" by J. D. Jackson. The generalization is a little heavy for beginners. It is easier to refer to static electric and static magnetic fields, which many people have direct experience of. > >On Jul 28, 7:59 pm, Matthew Johnson <matthew_mem...@newsguy.org> > >wrote: > >>In article <d6d5cee7-e1b8-4428-850b-fe3da10d0...@m73g2000hsh.googlegroups.com>, > Remember that radiating dipole I mentioned? You can find such a derivation of > radiated/traveling waves in oodles of elementary physics books, or in electrical > engineering texts (even at more advanced levels), yet how many of them mention > that these dipoles have no physical existence? Yes, these dipoles have physical existence. To study them in detail, one has to look on a smaller distance scale where one has to use quantum physics. However, on size scales larger than the molecule these dipoles are real. Remember, the nineteenth century physicists didn't have a description of molecules on a subatomic level. They didn't even know what force keeps the dipole from imploding. They knew they didn't know what held electrical particles together. However, they did know that on a gross scale of space and time the molecules acted like dipoles, quadropoles, etc. So that analysis is still useful today. I used to work in the field of picosecond spectroscopy. I have measured the lifetime of an induced dipole. Most of the dipole components are well modeled by the molecule physically turning. The molecular dipoles of the molecules really line up. Classical equations are sufficient to model how they snap back to their original equilibrium. There is a very short femtosecond component which is caused by the electrons being moved. Accurate modeling of the femtosecond component requires quantum mechanics. However, phenomenological models based on classical physics have been developed that help model the data. The dipoles in classical electrodynamics, such as described by Jackson, are real. Saying their unreal sounds like something Spaceman would make up. >They are 19th century models with > no physical reality. Depends on what you call "physical reality." They used parameters that at the time were phenomenological. The phenomenological parameters have a pretty clear classical interpretation. However, we can now sometimes make an a priori calculation of these parameters using quantum mechanics. Maybe what you mean is that the a priori model is more "physically real" than the phenomenological parameters. Maybe. I am a hierarchal reductionist. I prefer to say that the phenomenological description is physically real on a molecular scale of time and space. The quantum mechanical model is physically real on a subatomic scale. The different models are "physically real" on different levels of reality. >Yet we still use them in 20th century texts! Yes, I find 20th century texts useful. Actually, I occasionally find 19th century texts useful. Rarely, but it is true. The earlier scientist understood some things less than we do. However, they had to think through some details that are taken for granted today. Look how you and Copi had so much trouble figuring out what "magnetic resonance coupling" means. I'll bet Maxwell would have figured it out 1-2-3! On Jul 26, 11:08 pm, copiouse...@gmail.com wrote: > On Jul 26, 4:07 pm, kronec...@yahoo.co.uk wrote: > > > On Jul 27, 6:03 am, copiouse...@gmail.com wrote: > In other words, how is one way better than the other for transferring > energy through space? Over large distances, they are the same. An electromagnetic wave can travel huge distances from an electric charge or magnet. However, the electromagnetic wave has both electric and magnetic fields associated with it, as measured in all inertial frames. There is no way to separately analyze the electric and magnetic field. Look up "near field approximation" and "far field approximation." These are two limiting cases of Maxwell's equations. Electromagnetic waves per se can only be described in the far field approximation. In the near field approximation, one has to analyze electric and magnetic fields separately. Electric and magnetic fields don't couple with each other at short distances from the electric charges and currents that cause them. However, over short distances there is a difference between electric and magnetic fields. If one is looking at an electric charge in the inertial frame of that electric charge, one finds an electric field associated with that magnetic charge. However, if one is looking at an electric charge in an inertial frame where the electric charge is moving very fast, the electric charge starts to behave like an electric current. So there is a large magnetic field associated with the electric charge. So I think that magnetic coupled resonance is associated with small distances. If you have two sets of moving charges very close to each other, the sets couple. Of course both the electric and magnetic fields contribute to the coupling. The question is which field causes the most coupling. If the charges are rotating, the coupling is probably more magnetic than electric. In a transformer, for instance, two coils of wire are practically on top of each other. The laws of optics are not strictly applicable to describing the operation of a transformer because of the proximity of the coils. Each coil contains both positive and negative charges, but the charges in each coil are moving in circles. So the magnetic field couples the two coils. I am sure that microscopically the electric field contributes. However, in the inertial frame of the transformer there are equal amounts of positive an negative electric charges. So the effect of the electric field is negligible. If the two sets of charges are far away from each other, it makes no sense to ask which field is causing the coupling. What causes the coupling is the electromagnetic waves, which contain both electric and magnetic fields. Although one can use Maxwell's equations directly, it is easier to use the laws of optics which are mostly based on the far field approximation. I think this issue of "far field" and "near field" has confused many a novice scientist. Optics are a limiting case of Maxwell's equations in the far field approximation. The simplest expressions for the laws of induction are limiting cases of Maxwell's equations in the near field approximation. The problem is that most introductory problems are best solved in either one limiting case of the other. The continuity of the physics is sometimes missed in these introductory problems. On Jul 26, 2:03 pm, copiouse...@gmail.com wrote: > I was reading about magnetically coupled resonance recently, and a > question came to me. How is MCR any different than a radio transmitter > and receiver? Please clarify exactly what you mean by MCR. Google only recogonize the term as a rock group. Are you perhaps asking about MRI (Magnetic Resonance Imaging) or Nuclear Magnetic Resonance (NMR)? Harry C, On Jul 26, 2:03 pm, copiouse...@gmail.com wrote: > I was reading about magnetically coupled resonance recently, and a > question came to me. How is MCR any different than a radio transmitter > and receiver? You've received a number of relevant responses, but none of them seem actually to be familiar with the work at MIT! I'm vaguely familiar with it, but before I post some links, here are some gleanings, thoughts and personal misconceptions of mine. ;-) (1) "near field" is certainly a phrase of power here, as several have noted (2) several have also noted the possible relation of the magnetic transformer, which is certainly a near field device, but I think that is not the complete story (3) the technology involved is (I think advertised by the inventors) a close relative of that used in "RFID". These are the ID cards (or labels) which, by means of an internal antenna and a chip, manage to transmit back a response to "readers". They are definitely a near field technology. An interesting thing about RFID devices is that they are powered by the incident field: their electronics wakes up long enough to send the response signal -- or at least modify the reflected signal, whichever description is more appropriate. And, I think they involve some role for resonance -- not just any incident frequency properly excites them. I think the MIT work essentially uses larger versions of these, and a near field, largely magnetic, which couples with appropriate devices over the space of a room, say. This technology is going to be very popular with the scared of EM radiation crowd. Only MIT geeks are going to want to work in an environment saturated with enough potential EM power to heat toasters, whatever the engineers assurance that it can't couple to our bodies. Right! Anyway, here are the promised links, to refine and illuminate: http://web.mit.edu/newsoffice/2007/wireless-0607.html http://www.sciencemag.org/cgi/content/full/317/5834/83 That will do for now. I may even read them myself. :-) [snip] >This technology is going to be very popular with the scared of EM >radiation crowd. Until they figure out that it is just as risky! After all: if they could so easily ignore all the evidence that E-M radiation (of the relevant frequencies) does not affect our bodies, they could just as easily ignore the more slender evidence that such magnetic fields do not affect our bodies. Also, the latter effect has been much less studied: only in MRI machines do humans regularly get exposed to such high magnetic fields. But that, of course, is only for a short time. Yet if this method of powering devices becomes widespread, high exposure will become much more common and lengthy. On Jul 29, 11:25 pm, Matthew Johnson <matthew_mem...@newsguy.org> wrote: > In article <982fa9f8-cfe9-4543-90b8-008c63dcb...@w7g2000hsa.googlegroups.com>, > Edward Green says... [quoted text clipped - 5 lines] > > Until they figure out that it is just as risky! Ahem. That was a highly ironical statement. No technical comments? |
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