[wolfekeeper]

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.

[alancalverd]

It's a lot easier to shape an electric field because we have materials (conductors) that are completely impenetrable, poles (electrodes) that are infinitely separable, and negligible temperature dependence of field strength. When controlling an electron beam, as in a cathode ray tube, focussing and rapid changes of  deflection over small angles are generally easier accomplished with electrostatic plates inside the tube than magnetic coils outside, though for really large deflections (more than about 45 degrees) you certainly need a magnet.

[Bored chemist]

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?

[wolfekeeper]

It doesn't invalidate Earnshaw's theorem with Braunbeck's extension, and there's no diagravitic material, nor am I aware of any diaelectric (as opposed to dielectric) material.

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.

[Bored chemist]

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?

[wolfekeeper]

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?

"I think you'll find it's a bit more complicated than that."

[chiralSPO]

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

[Bored chemist]

Dos that mean the knotted magnetic field of the coil will be transferred to a knot in the wire?

[chiralSPO]

Dos that mean the knotted magnetic field of the coil will be transferred to a knot in the wire?

I believe so.

[scientizscht (OP)]

It is possible to generate magnetic field with no energy. Just by using a permanent magnet.

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.

[evan_au]

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.

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:

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).

[wolfekeeper]

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.

[yor_on]

Chiral, would you mind explaining the way you refer to a '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.' ?

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 :)

[scientizscht (OP)]

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?

[PmbPhy]

Hello!

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

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.

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).

[evan_au]

impossible... magnetic dipole

Did 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

[PmbPhy]

impossible... magnetic dipole

Did you mean to say that a magnetic monopole is forbidden by Maxwell's equations?

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.

[wolfekeeper]

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?

No, electrostatic forces are usually weaker than magnetic forces, but they're not negligible and you can build generators and motors that work electrostatically.

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.

[scientizscht (OP)]

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?

[wolfekeeper]

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.