Curious device

[Mordred]

4 hours ago, Prajna said:

 I want to know what, in the circumstances I described, causes the increase in the potential energy in the steel sheet that is attracted to a magnet positioned above it. Just that. Answer that. Simply and clearly and without diversion, obfuscation or irrelevance. Can you do that?

Stress and strain Both involve energy. If the sheet is attracted to the magnet that induces both.  Of course that also involves Newtons laws of inertia. You have an equal and opposite force as well to consider. 

Hookes law would be applicable here the law isn't restricted to springs. As long as the sheet returns to its original shape regardless of how miniscule the change in shape it's considered an elastic condition.

For the EM side of it however the EM field has a stress energy momentum tensor that would apply. Particularly as Hookes law is only a first order approximation. Using the tensor will get your second order.

For the mechanical side the Cauchy stress tensor would apply.

https://en.m.wikipedia.org/wiki/Cauchy_stress_tensor

 

Edited April 17 by Mordred

[Mordred]

Just a side note the better literature covering the Cauchy stress tensor is any decent textbook on continuum mechanics particularly for Engineers. However you would need decent math skills as well as a decent understanding on classical physics. (No need for any quantum physics or relativity) to get through said textbook.

[Prajna]

10 hours ago, swansont said:

Do you understand that “fixed” is only an approximation? There is no such thing as a perfectly rigid structure. Which means that the mechanical structure flexes. There are mechanical forces, and they act through a displacement, i.e. they do work.

 

Because that’s involved in magnetic attraction.

You have to understand a bit of physics to appreciate the answer. If you don’t, then the answers might look like obfuscation or irrelevance. But you have an obligation here, because demanding an answer that involves a couple of semesters worth of physics, without having that knowledge, isn’t reasonable.

Ah, so there's no simple answer to my question, only answers that demand a couple of semesters studying physics and you guys are exercising your right of pedantry and hubris to dodge the question, or, as a layman might put it, you don't know the answer. No problem. So much for "Trust the science". @exchemist, thanks for the civil and helpful replies. The rest of you, meh.

[swansont]

2 hours ago, Prajna said:

Ah, so there's no simple answer to my question, only answers that demand a couple of semesters studying physics and you guys are exercising your right of pedantry and hubris to dodge the question, or, as a layman might put it, you don't know the answer. No problem. So much for "Trust the science". @exchemist, thanks for the civil and helpful replies. The rest of you, meh.

Dodge? You’ve been told that the work is done by external, mechanical means.

All dipole magnets act like a current loop (permanent magnets, too) and the force on a current is IL X B. That’s a cross product - the force is perpendicular to the current and external field. Work is a dot product of the force and displacement. The work is in the common direction of the force and displacement. Since the force is always perpendicular, this dot product is zero. There is no work done.

No energy comes at the expense of the magnetic field. It doesn’t matter if the magnetic field is from a permanent magnet. It doesn’t change if you try coming up with some clever configuration. The work is always mechanical or electrical. The magnet isn’t depleted, which must happen if the magnet is doing the work, and this would happen pretty quickly if that’s what was happening. 

[exchemist]

14 hours ago, Prajna said:

Ah, I hadn't seen this, perhaps it was edited in after I read your post. Thanks for the correction on Ed's name, I should really have looked it up. Yes, fascinating guy. He did some seemingly miraculous things and free-energy-cranks have been arguing over his book(s) for years, suggesting they were written in cipher, maybe they were. I rather thought that if anyone would be considered a crank then Ed would be right up there. Nice to be disabused regarding that.

Your curious fact is indeed curious, or certainly interesting. Whether I have enough interest to wade deep into relativity and QM to see it is another matter. If you can point me to general info about it that doesn't involve glazing over with esoteric formulae then I would certainly like to read it.

I explained exactly that the magnet is fixed. Maybe it's been there for years. Did I suggest, insinuate or even hint that it's floating? Eddy currents? What does that have to do with my question? I want to know what, in the circumstances I described, causes the increase in the potential energy in the steel sheet that is attracted to a magnet positioned above it. Just that. Answer that. Simply and clearly and without diversion, obfuscation or irrelevance. Can you do that?

I was out yesterday (visiting Rochester, on the Medway, a very interesting town with a Norman castle, a c.12th cathedral and a rather fine old high street with a lot of history) so have only just seen this. 

A permanent magnet has energy in its magnetic field. This energy was imparted when the magnet was first magnetised, aligning the magnetic dipoles of the atoms. A permanent magnet is thus in a metastable, higher energy, state, compared to one that has become demagnetised. What happens when a piece of paramagnetic or ferromagnetic material comes under the influence of this field is a bit complicated but I think in energy terms it is something like the following:-

The magnetic dipoles in that material are induced by the field to align with it. This costs energy, relative to the previous field-free, non-aligned state and the energy required comes from the field of the permanent magnet. So there has been a potential energy transfer from the permanent magnet to the material that is being attracted to it. The potential energy of the system can be further lowered by allowing the two objects to move together. It is the stored energy in the field of the permanent magnet that is responsible. (This is made clear when you consider the work you have to do to pull the two objects apart.)

But any repeated process involving separating and moving together permanent magnets simply moves energy into and out of the field. Energy can only be extracted from it once, in the phase in which they move together. After this there is no free lunch.   

20 hours ago, KJW said:

By "brighter", I mean "higher intensity". It should be noted that black body radiation is not just a frequency distribution but also an intensity, and that any radiation that deviates from black body radiation, either by frequency distribution or by intensity, is radiation that can do work. In the case of the perpetual motion machine, the ability to focus black body radiation from an object into a smaller space increases the intensity of the image so that it is no longer black body radiation and can do work.

If one wishes to capture the entire radiation output of a small spherical object as an image, one can place the object at the focus of an internally reflective prolate spheroid. The image forms at the other focus. But note that symmetry demands that the image is the same size as the object, and hence the same intensity. On the other hand, if one uses a magnifying glass to focus sunlight onto a piece of paper, the image is much smaller than the sun, but then one is capturing only a tiny proportion of the total output of the sun.

 

 

Yes I suppose that makes sense. Does it make sense, I wonder, to speak of the radiation distribution having an entropy? What you seem to suggest is that the black body distribution has the maximum entropy of any radiation distribution.

Edited April 17 by exchemist

[Prajna]

11 minutes ago, exchemist said:

I was out yesterday (visiting Rochester, on the Medway, a very interesting town with a Norman castle, a c.12th cathedral and a rather fine old high street with a lot of history) so have only just seen this. 

A permanent magnet has energy in its magnetic field. This energy was imparted when the magnet was first magnetised, aligning the magnetic dipoles of the atoms. A permanent magnet is thus in a metastable, higher energy, state, compared to one that has become demagnetised. What happens when a piece of paramagnetic or ferromagnetic material comes under the influence of this field is a bit complicated but I think in energy terms it is something like the following:-

The magnetic dipoles in that material are induced by the field to align with it. This costs energy, relative to the previous field-free, non-aligned state and the energy required comes from the field of the permanent magnet. So there has been a potential energy transfer from the permanent magnet to the material that is being attracted to it. The potential energy of the system can be further lowered by allowing the two objects to move together. It is the stored energy in the field of the permanent magnet that is responsible. (This is made clear when you consider the work you have to do to pull the two objects apart.)

But any repeated process involving separating and moving together permanent magnets simply moves energy into and out of the field. Energy can only be extracted from it once, in the phase in which they move together. After this there is no free lunch.   

This is a much more reasonable response, @exchemist, thank you. At least you understand what I'm talking about. So what is happening is that there is a movement of energy into and out of the magnetic field, much like storing energy in an inductor, I guess. That's what I was referring to as 'work', perhaps inaccurately. We might consider that the steel sheet is 'falling upwards' towards the magnet in its magnetic field, that when the steel sheet is on the table and the magnet is fixed 30mm above it there is a potential energy imposed by the magnetic field and when the steel is attracted up to the magnet then that magnetically induced potential energy is converted to kinetic energy until the steel sticks to the magnet. Now the magnetic potential energy has been converted to gravitational potential energy. Then if the magnet is an electromagnet and we cut the current to it the steel falls to the table, a conversion of gravitational potential energy to kinetic energy until the steel rests on the table again. Have I understood correctly?

As an aside, I used to live in Chatham, Chattenden and Maidstone when I was at RSME and serving in the Royal Engineers. I spent a good deal of time at the Historic Dockyard in Chatham, where I drove steam cranes on the docks at the weekends. So I'm quite familiar with the area. That place is very interesting too. If you get a chance to visit the Officer's Mess at the dockyard you can see the vaulted ceiling that was built by ship's carpenters and is really the upside-down hull of a ship. 

[Mordred]

3 hours ago, Prajna said:

Ah, so there's no simple answer to my question, only answers that demand a couple of semesters studying physics and you guys are exercising your right of pedantry and hubris to dodge the question, or, as a layman might put it, you don't know the answer. No problem. So much for "Trust the science". @exchemist, thanks for the civil and helpful replies. The rest of you, meh.

There is no dodge, I provided three related formulas that are easily looked up. Hookes law, the Em stress tensor, and the Cauchy stress tensor.

The EM stress tensor will involve the cross and dot products mentioned by Swansont via the Maxwell equations.  Maybe our mistake is assuming your aware the magnetic field has a 90 degree polarity shift from the electric field.

Perhaps that will help understand the cross product terms Swansont mentioned.

15 hours ago, swansont said:

Is the magnet hovering in space? I don’t think that’s happening.

(Further, what happens in the metal? You get an eddy current. The force doing work there is electric, not magnetic.)

The work - the energy input - is not the magnet. It’s something else. Magnets don’t get energy depleted by being used. They are not the source of the change in energy of the system.

The best way to understand what Swansont has here is to look at the Maxwell equations. An introductory electrodynamics textbook will cover this. 

By the way it's also a common exam question As to what performs the work. The magnet or the E field.

Edited April 17 by Mordred

[Prajna]

6 minutes ago, Mordred said:

There is no dodge, I provided three related formulas that are easily looked up. Hookes law, the Em stress tensor, and the Cauchy stress tensor.

The EM stress tensor will involve the cross and dot products mentioned by Swansont via the Maxwell equations.  Maybe our mistake is assuming your aware the magnetic field has a 90 degree polarity shift from the electric field.

Perhaps that will help understand the cross product terms Swansont mentioned.

Thanks @Mordred, I'm aware of the polarity shift, referred to as the Right Hand Rule, iirc, if you wrap your hand around a wire with your index finger pointing along the wire, that indicates the current direction, then we have the magnetic field and electric field perpendicular to the current, indicated by the middle finger and thumb. If that's the same thing you're talking about. But anyway, in the above example I am only discussing the change in energy of a steel sheet as it is attracted by a magnet situated above it. I don't think that in general terms we need to consider polarity shifts. I think I am getting closer to a reasonable understanding in the discussion with @exchemist.

[Mordred]

Yes the RH rule is precisely what applies but so does the stress/ strain relations I mentioned. Hence referring you to continuum mechanics.  Your sheet will not have uniform strain as it's mounted on one side so to get an accurate calculation will involve more than Hookes law.

[Prajna]

5 minutes ago, Mordred said:

Yes the RH rule is precisely what applies but so does the stress/ strain relations I mentioned. Hence referring you to continuum mechanics.  Your sheet will not have uniform strain as it's mounted on one side so to get an accurate calculation will involve more than Hookes law.

Ah, I think you misunderstand what I was referring to. I was describing an experiment where there is a table top, 30mm above the table is a magnet fixed to a support. Small sheets of steel are on the table. Pushing a steel sheet across the table top until it is situated under the magnet, the magnet will attract the steel sheet upwards til it sticks to the magnet. In this experiment I describe what is happening is that the magnetic field attracts the sheet so that it rises against gravity and, in the process, the gravitational potential energy in the sheet has increased. I'm not getting bogged down in deformation (which may, indeed, occur to some extent in this situation but is not a major factor in what is happening), simply I am looking at the exchange of energy and how that can be described. @swansont insists it can't be described as the magnet 'doing work' and that magnets 'don't do work'. Ok, then how do we describe it and if a magnet appears to be causing something to move via some force over some distance, if that is distinct from other forms ofwork then how and why?

[Mordred]

What I stated still applies if the sheet will move without deformation it's still detailed under the Cauchy stress tensor. As far as the RH rule apply it separately to the E field and B field. The results may surprise you. The B field is perpendicular to the E field.

Edited April 17 by Mordred

[Prajna]

2 minutes ago, Mordred said:

What I stated still applies if the sheet will move without deformation it's still detailed under the Cauchy stress tensor.

Thanks. I'll look it up.

Ok, "The Cauchy stress tensor is used for stress analysis of material bodies experiencing small deformations: it is a central concept in the linear theory of elasticity. For large deformations, also called finite deformations, other measures of stress are required, such as the Piola–Kirchhoff stress tensor, the Biot stress tensor, and the Kirchhoff stress tensor."

I'm not interested in "small deformations" in this example. I think I have clearly stated what I am considering and "small deformations" is an aside to that conversation. What's happening in terms of changes in potential energy and how can that be simply and clearly described?

[Mordred]

Ok so your lifting the plate in say the center so the only upward force is at the center but you have a downward force uniform throughout the plate.

Obviously there will be differences of the sum of forces from the edges and the center.  Potential energy is energy due to location so obviously there will be differences as there is differences in the sum of forces at different points on the plate.

Put simply which leads to non uniform strain.

Edited April 17 by Mordred

[Prajna]

11 minutes ago, Mordred said:

Ok so your lifting the plate in say the center so the only upward force is at the center but you have a downward force uniform throughout the plate.

Obviously there will be differences of the sum of forces from the edges and the center.  Potential energy is energy due to location so obviously there will be differences as there is differences in the sum of forces at different points on the plate.

Put simply which leads to non uniform strain.

Again, you're getting into marginally relevant details. Rather than go there, why not address in general and simple terms the exchange of potential energy? Anyway, I'm sure the upward force is not a point source at the centre of the sheet but is distributed in some way across the sheet.

[exchemist]

2 hours ago, Prajna said:

This is a much more reasonable response, @exchemist, thank you. At least you understand what I'm talking about. So what is happening is that there is a movement of energy into and out of the magnetic field, much like storing energy in an inductor, I guess. That's what I was referring to as 'work', perhaps inaccurately. We might consider that the steel sheet is 'falling upwards' towards the magnet in its magnetic field, that when the steel sheet is on the table and the magnet is fixed 30mm above it there is a potential energy imposed by the magnetic field and when the steel is attracted up to the magnet then that magnetically induced potential energy is converted to kinetic energy until the steel sticks to the magnet. Now the magnetic potential energy has been converted to gravitational potential energy. Then if the magnet is an electromagnet and we cut the current to it the steel falls to the table, a conversion of gravitational potential energy to kinetic energy until the steel rests on the table again. Have I understood correctly?

As an aside, I used to live in Chatham, Chattenden and Maidstone when I was at RSME and serving in the Royal Engineers. I spent a good deal of time at the Historic Dockyard in Chatham, where I drove steam cranes on the docks at the weekends. So I'm quite familiar with the area. That place is very interesting too. If you get a chance to visit the Officer's Mess at the dockyard you can see the vaulted ceiling that was built by ship's carpenters and is really the upside-down hull of a ship. 

Interesting about Chatham. I was surprised to see from the castle battlements an old (decommissioned?) submarine moored in the river, just downstream of the bridges carrying the railway and road. I might pop down the line from Victoria again some time and take a look. I think it's the next stop after Rochester. 

Back on the topic, yes there will be work done when the magnet and steel object move relative to one another under the influence of the force from the field. W= Fd, remember. But when the magnet is static, held to the beam by its magnetism, no work is being done. I think that is what @swansont meant by saying magnets don't do work, i.e. they don't do work when they are just sitting there, simply by virtue of being magnets, as it were! 

And there is no inexhaustible store of energy in a permanent magnet that you can draw on by incorporating it in a  perpetual motion machine. There is finite (fairly small) energy imparted to it when it is magnetised and you can get a bit of that back, once only, by allowing an object to be drawn towards it. But if you separate them again as part of an operating cycle of some machine, you have to put the same energy back each time. So as I say, no free lunch.  

Edited April 17 by exchemist

[swansont]

3 minutes ago, exchemist said:

I think that is what @swansont meant by saying magnets don't do work, i.e. they don't do work when they are just sitting there, by virtue of being magnets, as it were! 

I’m saying if you lift something with a magnet, you are supplying the energy, not the magnet. It’s not different, conceptually, from attaching a chain to something and lifting it. The chain is involved, but it’s not supplying the effort to lift the object. The chain doesn’t have a store of energy that does the lifting.

It doesn’t matter how convoluted or clever a scenario you come up with. The energy to do the lifting (which is what work is, in physics - the energy supplied by exerting a force through a displacement) comes from somewhere else. 

 

[exchemist]

2 minutes ago, swansont said:

I’m saying if you lift something with a magnet, you are supplying the energy, not the magnet. It’s not different, conceptually, from attaching a chain to something and lifting it. The chain is involved, but it’s not supplying the effort to lift the object. The chain doesn’t have a store of energy that does the lifting.

It doesn’t matter how convoluted or clever a scenario you come up with. The energy to do the lifting (which is what work is, in physics - the energy supplied by exerting a force through a displacement) comes from somewhere else. 

 

Indeed. However, But when a magnet and something attracted by it move closer together, under the influence of the force of attraction between them, work is done. I am saying this comes from  a reduction in the stored energy in the magnetic field. 

[Mordred]

2 hours ago, Prajna said:

Again, you're getting into marginally relevant details. Rather than go there, why not address in general and simple terms the exchange of potential energy? Anyway, I'm sure the upward force is not a point source at the centre of the sheet but is distributed in some way across the sheet.

Really explain how it's going to be distributed uniformly across the plate if the surface area of the plate is greater than the magnet.  Go ahead give it your best shot.

I honestly don't know why you can't determine your PE relations in regards to applied force I shouldn't have to explain something already shown via Hookes law you won't need the spring constant per se but the elastic and gravitational potential energy is applicable.

Every detail I have provided including the stress tensors are applicable. Regardless of your opinion.

I take it you never lifted a sheet of plate steel using a magnet and seen the outer edges flex downward due to gravity ?

Edited April 17 by Mordred

[Prajna]

1 hour ago, exchemist said:

Interesting about Chatham. I was surprised to see from the castle battlements an old (decommissioned?) submarine moored in the river, just downstream of the bridges carrying the railway and road. I might pop down the line from Victoria again some time and take a look. I think it's the next stop after Rochester. 

Back on the topic, yes there will be work done when the magnet and steel object move relative to one another under the influence of the force from the field. W= Fd, remember. But when the magnet is static, held to the beam by its magnetism, no work is being done. I think that is what @swansont meant by saying magnets don't do work, i.e. they don't do work when they are just sitting there, simply by virtue of being magnets, as it were! 

And there is no inexhaustible store of energy in a permanent magnet that you can draw on by incorporating it in a  perpetual motion machine. There is finite (fairly small) energy imparted to it when it is magnetised and you can get a bit of that back, once only, by allowing an object to be drawn towards it. But if you separate them again as part of an operating cycle of some machine, you have to put the same energy back each time. So as I say, no free lunch.  

Chatham: yes, there was a nuclear submarine port there, I believe. Sailing on the Medway is very interesting too, that sub being one fascination.

Good, we're getting closer now. I think @swansont is still confused in thinking I'm talking about lifting something by attaching it to a magnet and lifting the magnet whereas I'm talking about the magnet being fixed and the object being lifted by the magnetic field. There's some ambiguity somewhere and I am not sure if it is in what I described or in @swansont's understanding of what I describe. No matter. I hope he can agree that in the thought experiment I described the magnet appears to be doing work as in W = fD. Certainly it looks like work is being done. Once we get this thought experiment done and dusted then maybe we can examine my proposed device and see how we correlate what is happening in it with thermodynamic laws. Maybe we can even discuss it in general terms without having to divert into obscure formulae.

Can we say then, in this thought experiment with a fixed magnet attracting to itself, that work is being done by the magnet? Can we say also that the strength of the magnet is not depleted in the process of that? In that example some work must be done to remove the steel sheet from the magnet, if it is a permanent magnet, or the current must be interrupted to let the sheet fall if it is an electromagnet. So we can include that work and so on and develop the experiment further to include the mechanics of removing the sheet again etc. but let's leave that there and move back to the SMT.

In the SMT the magnets are fixed at the end of rockers (levers really, on an axle at their centre). When a metal finger is rotated into the space between the magnets then the magnets will switch from repelling each other to being attracted to the finger. Obviously some work will be required in order to rotate the finger into the space. That work will be reduced, maybe even to some extent overtaken by the finger being attracted into the field in the gap between the magnets. Obviously the reverse will be true as the finger exits the gap since the field is still attracting the finger back into the gap and this will add to the work required to rotate the finger out of the gap. So these two effects should balance or cancel each other out. This is what I mean by 'symmetrical and why I disregard this effect since overall it neither adds to or reduces the force required to turn the rotor.

There will be some eddy current drag on the finger as it passes through the magnetic gap, since any ferromagnetic material passing though a magnetic field acquires an induced magnetic field opposite to the magnetic field inducing it. How much drag will that be? Probably it will be proportional  to the strength of the magnetic field that gives rise to it, that's logical and I'm happy yo accept that being the case.

The magnets, in attraction to the finger on one side of the rockers and repelling each other on the other side, will cause movement, work in fact, on the rockers, causing them to rotate in a reciprocating manner as each magnetic gap encounters either a finger or a space. The force involved in that movement will depend on the strength of the magnetic fields which will vary dependant on the distance the opposed magnets are from each other - when the magnets are close they will exert more force, either in attraction or repulsion, and the converse; according to the inverse square rule.

It may be that whatever the power of the magnets the eddy current drag will always be equal to the forces generated by the magnets in switching from attraction (to the finger) and repulsion (on the side that has a space rather than a finger.) Or maybe the eddy current can be reduced, perhaps by making the fingers small enough that they only just switch the magnets, perhaps by redesigning them to use some form of lamination such is done in transformers to reduce eddy currents.

I'm not sure if you have noticed but when a magnet is stuck to a sheet of metal the force required to remove it can be much reduced by sliding it transverse to its field. In the SMT the fingers pass transverse to the field rather than in line with it and, like sliding a magnet off a sheet of steel, there may be less work required than if something was moving in line with the field.

Thanks for further constructive comments, @exchemist

34 minutes ago, Mordred said:

Really explain how it's going to be distributed uniformly across the plate if the surface area of the plate is greater than the magnet.  Go ahead give it your best shot.

I honestly don't know why you can't determine your PE relations in regards to applied force I shouldn't have to explain something already shown via Hookes law you won't need the spring constant per se but the elastic and gravitational potential energy is applicable.

Every detail I have provided including the stress tensors are applicable. Regardless of your opinion.

I take it you never lifted a sheet of plate steel using a magnet and seen the outer edges flex downward due to gravity ?

Did I say the area of the plate was greater than the magnet? You're again talking about attaching something to a magnet and then lifting the magnet. I'm not. I'm talking about a SMALL plate of metal being ATTRACTED to a magnet that is in a fixed position. Do your stress tensors and Hookes laws and elastic distortions really contribute to what I'm discussing? Thanks for your interest but I really don't think what you've offered is contributing to illuminating what I was discussing.

[exchemist]

27 minutes ago, Prajna said:

Chatham: yes, there was a nuclear submarine port there, I believe. Sailing on the Medway is very interesting too, that sub being one fascination.

Good, we're getting closer now. I think @swansont is still confused in thinking I'm talking about lifting something by attaching it to a magnet and lifting the magnet whereas I'm talking about the magnet being fixed and the object being lifted by the magnetic field. There's some ambiguity somewhere and I am not sure if it is in what I described or in @swansont's understanding of what I describe. No matter. I hope he can agree that in the thought experiment I described the magnet appears to be doing work as in W = fD. Certainly it looks like work is being done. Once we get this thought experiment done and dusted then maybe we can examine my proposed device and see how we correlate what is happening in it with thermodynamic laws. Maybe we can even discuss it in general terms without having to divert into obscure formulae.

Can we say then, in this thought experiment with a fixed magnet attracting to itself, that work is being done by the magnet? Can we say also that the strength of the magnet is not depleted in the process of that? In that example some work must be done to remove the steel sheet from the magnet, if it is a permanent magnet, or the current must be interrupted to let the sheet fall if it is an electromagnet. So we can include that work and so on and develop the experiment further to include the mechanics of removing the sheet again etc. but let's leave that there and move back to the SMT.

In the SMT the magnets are fixed at the end of rockers (levers really, on an axle at their centre). When a metal finger is rotated into the space between the magnets then the magnets will switch from repelling each other to being attracted to the finger. Obviously some work will be required in order to rotate the finger into the space. That work will be reduced, maybe even to some extent overtaken by the finger being attracted into the field in the gap between the magnets. Obviously the reverse will be true as the finger exits the gap since the field is still attracting the finger back into the gap and this will add to the work required to rotate the finger out of the gap. So these two effects should balance or cancel each other out. This is what I mean by 'symmetrical and why I disregard this effect since overall it neither adds to or reduces the force required to turn the rotor.

There will be some eddy current drag on the finger as it passes through the magnetic gap, since any ferromagnetic material passing though a magnetic field acquires an induced magnetic field opposite to the magnetic field inducing it. How much drag will that be? Probably it will be proportional  to the strength of the magnetic field that gives rise to it, that's logical and I'm happy yo accept that being the case.

The magnets, in attraction to the finger on one side of the rockers and repelling each other on the other side, will cause movement, work in fact, on the rockers, causing them to rotate in a reciprocating manner as each magnetic gap encounters either a finger or a space. The force involved in that movement will depend on the strength of the magnetic fields which will vary dependant on the distance the opposed magnets are from each other - when the magnets are close they will exert more force, either in attraction or repulsion, and the converse; according to the inverse square rule.

It may be that whatever the power of the magnets the eddy current drag will always be equal to the forces generated by the magnets in switching from attraction (to the finger) and repulsion (on the side that has a space rather than a finger.) Or maybe the eddy current can be reduced, perhaps by making the fingers small enough that they only just switch the magnets, perhaps by redesigning them to use some form of lamination such is done in transformers to reduce eddy currents.

I'm not sure if you have noticed but when a magnet is stuck to a sheet of metal the force required to remove it can be much reduced by sliding it transverse to its field. In the SMT the fingers pass transverse to the field rather than in line with it and, like sliding a magnet off a sheet of steel, there may be less work required than if something was moving in line with the field.

Thanks for further constructive comments, @exchemist

Did I say the area of the plate was greater than the magnet? You're again talking about attaching something to a magnet and then lifting the magnet. I'm not. I'm talking about a SMALL plate of metal being ATTRACTED to a magnet that is in a fixed position. Do your stress tensors and Hookes laws and elastic distortions really contribute to what I'm discussing? Thanks for your interest but I really don't think what you've offered is contributing to illuminating what I was discussing.

It's very simple. W=Fd is all you need to keep in mind. If there is no change in d, the distance in the direction of F, no work is done. Move them apart and you do work on them, causing energy to go into the magnetic field (a form of potential energy). Allow them to move closer and the reverse happens: they do work on whatever is holding them apart, with the necessary energy coming from the magnetic field.

As for the magnet sliding, that is a distraction. It does not in fact alter the strength of the magnetic force holding it to the metal sheet. Any motion perpendicular to the force means d does not change, so no work is done. 

(What it may do, though, is reduce the frictional force parallel to the sheet which resists the magnet being slid. Typically the frictional drag between two surface that are sliding past one another is less that the limiting friction just before the force is overcome and sliding commences. This is to do with asperities on the surface interlocking, which does not happen to the same degree when they are in motion. But this is a tribological digression.)

 

[swansont]

52 minutes ago, Prajna said:

Good, we're getting closer now. I think @swansont is still confused in thinking I'm talking about lifting something by attaching it to a magnet and lifting the magnet whereas I'm talking about the magnet being fixed and the object being lifted by the magnetic field.

No. It’s just that there’s no real difference. In one case it’s a person doing the work, in another it’s a structure doing it. The magnetic field is doing the lifting, just as with a chain, but it’s not doing the work.

Do you know what “work” is in physics? It has a specific definition, as I explained in an earlier post. It’s not some general idea of effort or force. Work has units of energy; it’s energy transferred because of a force acting through a displacement. Magnetic forces do not do this; they are perpendicular to displacement. If a magnet is held by a structure, that structure will flex under a load; that is the source of the energy (energy stored in the structure’s configuration, and/or a reduction in its potential energy because the structure shifts downward) As I said, there’s no such thing as a perfectly rigid structure. 

 

[Mordred]

1 hour ago, Prajna said:

 

Did I say the area of the plate was  I'm not. I'm talking about a SMALL plate of metal being ATTRACTED to a magnet that is in a fixed position. 

Look at your own image is not the rotor plate larger than the magnets and the placement off center ? 

Now am I correct those magnets will be shifting inward and outward ? So as it shifts outwards as opposed to inwards you will get variations

Edited April 17 by Mordred

[Prajna]

40 minutes ago, exchemist said:

It's very simple. W=Fd is all you need to keep in mind. If there is no change in d, the distance in the direction of F, no work is done. Move them apart and you do work on them, causing energy to go into the magnetic field (a form of potential energy). Allow them to move closer and the reverse happens: they do work on whatever is holding them apart, with the necessary energy coming from the magnetic field.

As for the magnet sliding, that is a distraction. It does not in fact alter the strength of the magnetic force holding it to the metal sheet. Any motion perpendicular to the force means d does not change, so no work is done. 

(What it may do, though, is reduce the frictional force parallel to the sheet which resists the magnet being slid. Typically the frictional drag between two surface that are sliding past one another is less that the limiting friction just before the force is overcome and sliding commences. This is to do with asperities on the surface interlocking, which does not happen to the same degree when they are in motion. But this is a tribological digression.)

 

Ok, the situation in the device though is that the magnets are not moved closer nor separated by the operator, rather the magnets move under the effect of their changing state of repulsion or attraction governed by whether there is a finger or no finger between them. So, in my view, energy is being exchanged between the magnetic fields on one side of the device and the fields on the other. Is this a reasonable description? The operator is not moving the magnets, not even by poking them with a stick, he is merely turning the rotor that determines which side of the device has a finger in the gap and which doesn't. Maybe I should rename my 'fingers' to 'tabs' to avoid the idea that there is any poking of the magnets going on.

I don't see where there is any direct link between the work or energy or effort required to turn the rotor and the work or energy or effort produced by the switch state of the magnets, unless there is some reason to suggest that the eddy current drag is directly related to the output. The drag will certainly depend on the strength of the magnetic field on the side of the device that the finger/tab is passing through and the speed of the finger/tab through that field and, I guess, on the area of the tab that's in the field, perhaps even other things I have yet to consider.

[Mordred]

We cross posted see my last post 

[Prajna]

4 minutes ago, Mordred said:

Look at your own image is not the rotor plate larger than the magnets and the placement off center ? 

Ah, ok, you were talking with regard to the device and I was (at that time) talking with regard to the thought experiment. Cross purposes, sorry for the misunderstanding. Yes, I'm sure that at some point I will need to consider distortion in the fingers/tabs but engineers often just over-engineer things, add s fudge factor, rather than fuss about such details. This particular design is just a proof of concept and the objective was to make it easy for people to grasp the principal of operation. The rotor is designed so that the number and geometry of the tabs can be changed easily in order to test different configurations.

50 minutes ago, swansont said:

No. It’s just that there’s no real difference. In one case it’s a person doing the work, in another it’s a structure doing it. The magnetic field is doing the lifting, just as with a chain, but it’s not doing the work.

Do you know what “work” is in physics? It has a specific definition, as I explained in an earlier post. It’s not some general idea of effort or force. Work has units of energy; it’s energy transferred because of a force acting through a displacement. Magnetic forces do not do this; they are perpendicular to displacement. If a magnet is held by a structure, that structure will flex under a load; that is the source of the energy (energy stored in the structure’s configuration, and/or a reduction in its potential energy because the structure shifts downward) As I said, there’s no such thing as a perfectly rigid structure. 

 

That provokes images of some structure bowing down to hoist a tiny metal sheet up into the air. Sure, to some barely detectable extent a solid structure will distort due to the forces involved in the thought experiment I described but really, is it really fair to say in this situation that the structure is doing all the work and the magnet is doing none? That is a very strange way to describe the situation in my view.