Post by: scientizscht on 28/07/2018 14:41:09
At which extend we can manipulate magnetic fields?
For example, can I choose a specific shape and size of space and put magnets in the right positions so that the magnetic field in that space will be as I want it to be?
In other words, can we design magnetic fields in a specific space and then produce it? With permanent magnets?
Thanks
Post by: Kryptid on 28/07/2018 15:32:23
Post by: Bored chemist on 28/07/2018 16:44:57
Quantum mechanics lets us have diamagnetic materials.
Post by: Kryptid on 28/07/2018 20:58:07
Earnshaw was misinformed, his view was classical.
Quantum mechanics lets us have diamagnetic materials.
If I'm not mistaken, he was speaking of paramagnetic and ferromagnetic materials.
Post by: evan_au on 29/07/2018 00:05:47
Quote from: scientizscht
At which extent we can manipulate magnetic fields?Not nearly as well as we can manipulate electrical flows.
We have very cheap and effective electrical conductors like copper metal (with very high current density), and very cheap and effective electrical insulators like PVC plastic (with very high breakdown voltage). The ratio in electrical conductivity must be in the billions.
We have fairly expensive magnetic "conductors" like mu-metal (but they saturate very easily). We don't have very good magnetic insulators, much better than air or vacuum. The ratio in permeability is a thousand or so, but only in very weak magnetic fields.
If cost is no object, you can use superconductors to provide magnetic shielding, as the Meissner effect excludes a magnetic field - provided the magnetic field is not too strong, and the frequency of the magnetic field is not too high. And it only works properly in "Type 1" superconductors, which require really cold temperatures.
See: https://en.wikipedia.org/wiki/Meissner_effect
Post by: PmbPhy on 29/07/2018 05:04:10
Two poles must always be present and the fields lines must link these poles.That's not true. E.g. when a magnetic field is created by time altering electric fields or by steady currents there's no need for a pole. Take the magnetic field generated by a wire wrapped around a torus as an example or a straight current carrying wire - or any static field created by currents.
Post by: Kryptid on 29/07/2018 05:25:04
That's not true. E.g. when a magnetic field is created by time altering electric fields or by steady currents there's no need for a pole. Take the magnetic field generated by a wire wrapped around a torus as an example or a straight current carrying wire - or any static field created by currents.
This is new information to me. So what would happen if I held the north pole of a bar magnet up to this toroidal electromagnet you mention? Would it be attracted or repelled? Would the same occur if I held the south pole up to it instead?
Post by: Colin2B on 29/07/2018 08:18:51
This is new information to me. So what would happen if I held the north pole of a bar magnet up to this toroidal electromagnet you mention? Would it be attracted or repelled? Would the same occur if I held the south pole up to it instead?You can check it out by using a compass near a wire carrying dc current, the compass will align itself along the magnetic field lines. I seem to remember doing this at school.
Look up Flemimgs Right hand rule
Post by: alancalverd on 29/07/2018 08:48:19
There are constraints on how you can construct a magnetic field. Two poles must always be present and the fields lines must link these poles. Earnshaw's theorem also forbids the construction of a static magnetic field from permanent magnets that results in the passive levitation of another permanent magnet: https://en.wikipedia.org/wiki/Earnshaw%27s_theorem
Unless you buy it from Amazon, of course (other suppliers are available)
61Ev4kvwv7L._SX679_.jpg (49.21 kB . 679x679 - viewed 5371 times)
Post by: alancalverd on 29/07/2018 09:04:03
We can also use "active shielding" to cancel stray magnetic fields entering the MRI room - handy if you have passing traffic or, in one case I worked on, an underground railway station next door.
But as Evan says, it's a lot harder to manipulate magnetic fields than electric fields. When you squish the field lines at A, they tend to pop out at B. Herding rubber cats is easier.
Post by: Bored chemist on 29/07/2018 09:26:01
This is new information to me. So what would happen if I held the north pole of a bar magnet up to this toroidal electromagnet you mention?
This
https://en.wikipedia.org/wiki/Homopolar_motor#/media/File:Faraday_magnetic_rotation.jpg
I think the strict criterion might be that there must be an even number of poles.
Post by: evan_au on 29/07/2018 09:56:36
Quote from: Kryptid
Two poles must always be present and the fields lines must link these poles.I think the criterion is that the magnetic field lines must always form a closed path*.
- In a bar magnet, the closed path extends from south pole to north pole, and back through the body of the magnet to where they started.
- I have used toroidal inductors as current transformers, and they are very effective at containing their magnetic field. In this case, the closed path runs in a ring through the toroid.
- In a straight wire carry a DC current, the closed paths are concentric with the wire. See Ampere's Right-Hand rule
https://en.wikipedia.org/wiki/Right-hand_rule#Amp%C3%A8re's_right-hand_grip_rule
My recollection is that magnetic fields have the mathematical property of "curl", which means they form closed loops; and they don't have the mathematical property of "divergence", which means that they don't start or stop at a point*.
See: https://en.wikipedia.org/wiki/Magnetic_field#Maxwell's_equations
*This assumes that there is no such thing as the hypothetical magnetic monopole. There have not been repeatable detection of magnetic monopoles, despite several searches, some of which claimed to detect "hits".
See: https://en.wikipedia.org/wiki/Magnetic_monopole
Post by: evan_au on 29/07/2018 12:54:53
Quote from: Kryptid
So what would happen if I held the north pole of a bar magnet up to this toroidal electromagnet you mention? Would it be attracted or repelled? Would the same occur if I held the south pole up to it instead?For low frequency applications (eg 50/60Hz), you could make the toroid by winding a strip of high-permeability alloy (like Mu-metal) into a reel.
- For high frequency applications (eg 1-100 MHz), you could make the toroid by moulding a ferrite powder.
- You then wind a copper coil around the toroid, spacing the copper windings out evenly around the perimeter.
- Both toroidal core materials are ferromagnetic, so they would attract the north or south pole of a permanent magnet that was placed nearby.
This attraction applies if:
- There is no current flowing through the copper winding. The magnet is attracted to the toroidal core.
- There is AC or DC flowing through the copper winding. This magnetic field from the electromagnet is contained within the toroidal core.
But you would try to keep a permanent magnet and DC currents away from Mu-metal, as it saturates easily, and then loses most of its desirable magnetic properties.
In the current transformer application, you pass the "high current" winding (eg 0-100A at 50Hz) through the center of the toroid; this effectively makes a "1 turn" winding, even if you have to trace it back to the power station.
- There is also a "low-current" winding; if this has 1,000 turns, then it will produce 0-100mA at 50Hz.
- Note that you must never operate a current transformer with the low-current winding "open circuit", as it can produce high voltages that are a risk to personnel, and it will probably destroy itself by arcing over.
See: https://en.wikipedia.org/wiki/Current_transformer#Function
Post by: Kryptid on 29/07/2018 15:09:35
Unless you buy it from Amazon, of course (other suppliers are available)
That isn't passive levitation. I own a similar device and it uses sensors to rapidly vary the strength of electromagnets in order to keep the tiny Earth in place.
Post by: alancalverd on 29/07/2018 16:21:37
Imagine two bar magnets with their north poles opposed. They will be pushed apart. Now constrain them in a vertical tube: levitation!
Not to be confused with the Meissner effect, which is inherently stable over a small range of movement.
Post by: Kryptid on 29/07/2018 17:30:06
Interesting. But the question was about levitation, not controlled levitation!
Imagine two bar magnets with their north poles opposed. They will be pushed apart. Now constrain them in a vertical tube: levitation!
I'm not sure how much more specifically I need to qualify my statements. Earnshaw's theorem applies to idea of static ferromagnets suspended in free space solely due to the force of their own fields, not levitating within physical restraints like a tube. I also don't think the OP asked about levitation specifically. It's just something I brought up.
Post by: wolfekeeper on 03/08/2018 05:00:12
When mu is other than 1, Braunbeck's extension to Earnshaw's theorem shows that mu below 1 allows magnetic levitation and when it's greater than 1, it's further destabilising. Neodymium magnets are slightly above (mu = ~1.2).
Anyway, the answer to the question is, can you manipulate magnets however you want. Yes, pretty much, but the freespace field is subject to:
∇.F = 0
for any magnet
Post by: scientizscht on 24/08/2018 22:35:10
Can you tell me how we can construct an electric field of eg the shape of a cylinder of specific dimensions?
Post by: wolfekeeper on 24/08/2018 22:57:38
Post by: evan_au on 24/08/2018 23:00:35
Quote from: scientizscht
how we can construct an electric field of eg the shape of a cylinder of specific dimensions?An electric field will have no effect on uncharged particles.
- An electric field gradient can attract or repel a charged particle.
- Note that practical electric fields do not exist in one place, and suddenly drop to zero - they tend to have a gradient, or terminate on a conductive surface (eg a metal shield).
The simplest way to produce a cylindrical electric field is to take a metal cylinder, and charge it (positive or negative)
- The electric field will have a roughly cylindrical shape outside the cylinder, decreasing in strength with distance
- The electric field will be zero inside the cylinder
- If you want to shape the electric field more precisely, you can use a plastic dielectric around the cylinder - but charged particles don't tend to move inside a plastic dielectric
- You need to hold it in an insulating vacuum, or the charge will leak away (even in a vacuum, cosmic rays or UV light will cause it to leak away eventually...)
- Or you could connect to to an electrical circuit that keeps it charged, relative to some fairly neutral object like the Earth
For this design, there will be some deviations from a perfect cylindrical field, because if you charge the metal cylinder negatively, all the excess electrons will try to get as far from each other as possible - which means there will be more of them at the farthest extremities of the cylinder.
To design something accurately for a specific purpose (eg for the LHC), it is best to use some simulation software that helps you model electric and magnetic fields, so you can optimise your design.
I am sure there are many such tools around, including free ones; one I have heard of is here (https://www.comsol.com/comsol-multiphysics). I am sure @alancalverd has seen some suitable tools in his line of work.
Post by: wolfekeeper on 25/08/2018 03:14:39
OK so we agree that we cannot manipulate magnetic fields and construct a specific magnetic field, as easy as electric fields.Actually, no we don't agree with that. The electric field has essentially the same constraints as the magnetic field; Earnshaw's theorem applies- in fact it applies to any inverse square law field including gravity. magnetism and electrostatics.
Post by: alancalverd on 25/08/2018 11:33:29
Post by: Bored chemist on 25/08/2018 12:37:56
Earnshaw's theorem applies- in fact it applies to any inverse square law field including gravity. magnetism and electrostatics.Did you see the video clip I posted which drives a coach + horses through Earnshaw's theorem?
Post by: wolfekeeper on 25/08/2018 13:43:14
Using electromagnets you can produce any shape magnetic field you want. You can tie knots in your magnetic field if you so wish. You can't do it in free space though.
Post by: Bored chemist on 25/08/2018 14:10:58
It doesn't invalidate Earnshaw's theorem with Braunbeck's extension,
I have a theorem that states that English monarchs must be male.
There's an exception where it doesn't cover queens.
Is it still a theorem?
On a more interesting note, as you say, it's perfectly possible to tie a knot in a magnetic field
Wind a long flexible coil,(something like this*) tie it in a knot then pass a current through it and you have a field with a knot in it.
https://en.wikipedia.org/wiki/Rogowski_coil
Are you aware of this trick?
https://www.funology.com/betcha-cant-tie-a-knot/
You can't untie a knot in a closed loop without the rope passing through itself.
So, what happens to the knot in the field if, with the current still flowing, you untie the knot in the coil?
Post by: wolfekeeper on 25/08/2018 15:08:26
"I think you'll find it's a bit more complicated than that."It doesn't invalidate Earnshaw's theorem with Braunbeck's extension,
I have a theorem that states that English monarchs must be male.
There's an exception where it doesn't cover queens.
Is it still a theorem?
Post by: chiralSPO on 25/08/2018 17:17:40
You can't untie a knot in a closed loop without the rope passing through itself.
So, what happens to the knot in the field if, with the current still flowing, you untie the knot in the coil?
Unless part of the circuit is a conductive fluid (like a plasma, liquid metal or ionic solution) I don't think there is any way to maintain a complete circuit and flowing current while interconverting a loop that is an even numbered knot to an odd numbered knot.
https://en.wikipedia.org/wiki/Knot_theory#Knot_equivalence
Post by: Bored chemist on 25/08/2018 17:30:19
Post by: chiralSPO on 25/08/2018 17:42:52
Dos that mean the knotted magnetic field of the coil will be transferred to a knot in the wire?
I believe so.
Post by: scientizscht on 31/08/2018 18:50:29
Is it possible to do so for an electromagnetic field?
Is it possible to produce any shape and extremely strong electrostatic field? What kind of materials would be susceptible to its forces? I know for magnetic fields, most metals would work.
Post by: evan_au on 01/09/2018 02:06:08
Quote from: scientizscht
It is possible to generate magnetic field with no energy. Just by using a permanent magnet.Yes. It takes some energy to magnetize the magnet during manufacture, but the field is "free" once you have paid for the magnet.
Quote
Is it possible to do so for an electromagnetic field?Pedantic Quibble: Magnetism is a subset of electromagnetism, so, strictly speaking, all magnetic fields are electromagnetic fields.
But I assume that you are talking about an electromagnet, where the magnetic field is produced by a current running through a coil of wire?
Yes, this is the way MRI machines work.
They use a superconducting wire which is chilled to very low temperatures, and then a current is started in this wire, forming a powerful magnetic field.
The magnetic field takes some energy to start the current flowing, but the magnetic field will continue as long as the temperature stays low. So in practice, it does take energy to power the cryogenic refrigeration system.
These magnetic fields can be very strong:
Quote
I know for magnetic fields, most metals would work.It depends on what effect you are seeking:
- If you want something which is strongly attracted into the strongest part of the magnetic field, you need a ferromagnetic material, such as iron, cobalt or nickel which have unpaired inner electrons. Most metals will not work! See: https://en.wikipedia.org/wiki/Ferromagnetism#Ferromagnetic_materials
If you are interested in minimising wasted energy in this application, choose a "soft" ferromagnetic material, with a narrow hysteresis loop. See: https://en.wikipedia.org/wiki/Magnetic_hysteresis
- If you want something which is strongly repelled by the magnetic field, you need a permanent magnet, whose north pole will be repelled from the north pole of the other magnetic field. For this application, you need a "hard" magnetic material, or the external magnetic field will demagnetise it.
- If you want something whose motion is retarded when in a strong magnetic field, you should choose metals which are good conductors, like aluminium, silver or copper. In this case, the kinetic energy is turned into heat (ie it is very inefficient).
Post by: wolfekeeper on 01/09/2018 02:28:56
The equivalent with electrostatics is called an electret:
https://en.wikipedia.org/wiki/Electret
and can store a very long lasting electrostatic charge. You can shape electrostatics with a metallic surface, to form an equipotential of arbitrary shape.
Post by: yor_on on 01/09/2018 10:30:18
If i take a loop and then then wind it around itself as in 'A tricky unknot diagram by Morwen Thistlethwaite' https://en.wikipedia.org/wiki/File:Thistlethwaite_unknot.svg
How would that correspond to ' an even numbered knot to an odd numbered knot.' Or is it something entirely different you're discussing here?
=
As in stating that a even numbered 'loop' can't become a odd numbered? How do you count the numbers? As in the former simple loops 'edges sticking out' of its new design, or??
Maybe we can simplify it by asking if you're referring to a knot that can't be returned to a loop/unknot, still, the rest of my questions will stand, slightly modified in the case of it not being a 'unknot', anyway :)
Post by: scientizscht on 01/09/2018 22:55:34
Permanent magnets take energy to create; it's not possible to make them with no energy, but they do generally persist for very long periods.
The equivalent with electrostatics is called an electret:
https://en.wikipedia.org/wiki/Electret
and can store a very long lasting electrostatic charge. You can shape electrostatics with a metallic surface, to form an equipotential of arbitrary shape.
What forces can electrets generate? Similar to strongest permanent magnets?
Also, if get close to another electrostatically charge, what will happen? Will they lose their electrical properties?
Post by: PmbPhy on 02/09/2018 01:04:10
Hello!But there will be material in the place where you want the field, i.e. the magnets themselves. And the field outside the magnet is not the same as the magnetic field inside.
At which extend we can manipulate magnetic fields?
For example, can I choose a specific shape and size of space and put magnets in the right positions so that the magnetic field in that space will be as I want it to be?
In other words, can we design magnetic fields in a specific space and then produce it? With permanent magnets?
Thanks
Simply put, you can create any field allowed by Maxwell's equations and boundary conditions (i.e. continuity). This means that some fields are impossible such as a magnetic field corresponding to a magnetic dipole (unless that part of Maxwell's equations is wrong).
Post by: evan_au on 02/09/2018 06:15:26
Quote from: PmbPhy
impossible... magnetic dipoleDid you mean to say that a magnetic monopole is forbidden by Maxwell's equations?
See: https://en.wikipedia.org/wiki/Magnetic_monopole#Pre-twentieth_century
Post by: PmbPhy on 02/09/2018 13:22:03
One of Maxwell's equations is div B = 0 which means there are no magnetic monopoles. As I said, it may be wrong. Physicists have been searching for it for a while now.Quote from: PmbPhyimpossible... magnetic dipoleDid you mean to say that a magnetic monopole is forbidden by Maxwell's equations?
Post by: wolfekeeper on 02/09/2018 16:06:29
No, electrostatic forces are usually weaker than magnetic forces, but they're not negligible and you can build generators and motors that work electrostatically.Permanent magnets take energy to create; it's not possible to make them with no energy, but they do generally persist for very long periods.
The equivalent with electrostatics is called an electret:
https://en.wikipedia.org/wiki/Electret
and can store a very long lasting electrostatic charge. You can shape electrostatics with a metallic surface, to form an equipotential of arbitrary shape.
What forces can electrets generate? Similar to strongest permanent magnets?
Quote
Also, if get close to another electrostatically charge, what will happen? Will they lose their electrical properties?No, they're fairly stable. I'm sure you can erase the charge in a powerful enough electric field, or by overheating them, but in normal use you won't.
Post by: scientizscht on 02/09/2018 22:12:02
Post by: wolfekeeper on 03/09/2018 02:52:40
Post by: Professor Mega-Mind on 04/09/2018 16:27:04
Post by: Professor Mega-Mind on 05/09/2018 06:01:43
Surprisingly , the answer is yes !
Project a very strong positive electric field onto a dielectric material . The molecules and electrons will shift position , presenting a negative bias towards the field generator . Reverse the field polarity in less than ten picoseconds second . The neg. bias facing the neg. field generator will produce a momentary repulsion before the dipoles realign to accommodate the new field . Repeat this process continuously , and you have a sustained repulsion similar to magnetic repulsion , but involving completely non-magnetic materials . The shifting dipoles would , of course , generate friction heat .
At any rate , sounds like a technology revolution to me . Let's hear it now , Who's the Mega-Mind?
I am ! P.M. ( The Last Dragon )
**Reference Article :
^ quora.com/Is-electrostatic-related-Propellantless-propulsion-possible/answer/Derek-Hendricks-7?ch
Post by: scientizscht on 16/09/2018 01:39:36
So can we create a magnetic, electrostatic or electromagnetic field that exists eg only in a tubular shape of specific diameter and length and nowhere outside these boundaries?
More or less yes, but there are limits; fields change smoothly rather than abruptly, shielding is never perfect, fields outside of materials are subject to Laplace equation etc.
How can we achieve this with permanent magnets?
Post by: Professor Mega-Mind on 16/09/2018 03:29:52
The technological and industrial applications of such devices would be widespread , and revolutionary as well .
Okay , go ahead and make your fortune . Just remember to kick a bit back to old coach ! ........P.M.
Post by: scientizscht on 16/09/2018 17:10:05
Magnetism is but a shadow of the electric force . The electric force is fundamentally much the stronger of the two . Superfast electric switching devices already exist . Increasing their size from micro to macro would do the trick , likewise the necessary capacitors .
The technological and industrial applications of such devices would be widespread , and revolutionary as well .
Okay , go ahead and make your fortune . Just remember to kick a bit back to old coach ! ........P.M.
Can you elaborate please?
Post by: Professor Mega-Mind on 16/09/2018 18:09:17
Imagine a small lizard w/webbed feet . To escape predators he can roll his legs reaal fast . If a body of water is in the way , no problem , he just slaps them feet down hard and fast , and enough upward push is created to hold his body above the water , so predator foiled !
The above analogy is to demonstrate that many small pushes can create a spectacular result . It's just a matter of figuring out the trick ................P.M.
Post by: Bored chemist on 16/09/2018 19:43:51
Yes indeedy ! I run off at the mouth real good !And would you like to answer the question now?
Imagine a small lizard w/webbed feet . To escape predators he can roll his legs reaal fast . If a body of water is in the way , no problem , he just slaps them feet down hard and fast , and enough upward push is created to hold his body above the water , so predator foiled !
The above analogy is to demonstrate that many small pushes can create a spectacular result . It's just a matter of figuring out the trick ................P.M.
Post by: Professor Mega-Mind on 17/09/2018 00:19:56
This be my basic answer , however, you could do what I did in days of yore ; delve into the micro-electronics liturature and supplies parts spec.s . You could , of course track down an electronics engineer or at least graduate student . You could also ask yourself ; does positive repel positive , and does negative repel negative . Continue with ; if most materials are full of dipoles , how can I use that ? If you don't let material limitations restrict your imagination , you might " do like professor do " , and come up with something , yeehaw !
....................P.M.