# Maxiumum magnetic field strength

1. Oct 19, 2005

### Mk

The maximum theoretical field strength for a neutron star, and therefore for any known phenomenon is 1013 T. Why is there a maximum?

Last edited: Oct 19, 2005
2. Oct 19, 2005

### Toptomcat

Unqualified guess follows.
Because, big and energetic as they are, neutron stars have a finite amount of energy too?

3. Oct 19, 2005

### Danger

I would guess that it's because there's a maximum possible for a normal star of particular mass. When it collapses, the field is concentrated, but there's also a maximum mass which will halt at being a neutron star. A larger one will continue to black hole status.

4. Oct 20, 2005

### rbj

using the cgs version of Maxwell's equations ($\mathbf{E}$ and $\mathbf{B}$ field are dimensionally identical and $\mathbf{v}$ is normalized by $c$ in Lorentz force eq.):
$$\nabla \cdot \mathbf{E} = 4 \pi \rho \ [/itex] [tex] \nabla \cdot \left( \frac{1}{2} \mathbf{B} \right) = 0 \ [/itex] [tex] \nabla \times \mathbf{E} = -\frac{1}{c} \frac{\partial \left( \frac{1}{2} \mathbf{B} \right)} {\partial t} \ [/itex] [tex] \nabla \times \left( \frac{1}{2} \mathbf{B} \right) = \frac{1}{c} \left( -4 \pi G \mathbf{J} + \frac{\partial \mathbf{E}} {\partial t} \right) = \frac{1}{c} \left( 4 \pi \rho \mathbf{v}_{\rho} + \frac{\partial \mathbf{E}} {\partial t} \right) \$$
i guess the magnitude of the $\mathbf{B}$ is limited by the magnitude of $\rho$ or $\mathbf{v}$ in the neutron star. $|\mathbf{v}|$ is limited by $c$ and i suppose there is a limit to $\rho$ in the neutron star.
(just guessing.)

5. Oct 20, 2005

### Tide

There is a limit because nothing can exceed the speed of light.

6. Oct 20, 2005

### Danger

rbj, you make my head hurt.
Tide, could you explain that please?

7. Oct 20, 2005

### Tide

Magnetic fields arise from the flow of current or the intrinsic magnetic moment of a spinning particle (which is essentially a current). The speed of light limits the flow of current and charge is invariant under Einstein-Lorentz transformation. Assuming finite charge density, magnetic fields due to electrical current are therefore bounded by the speed of light.

8. Oct 20, 2005

### Danger

Well... ya learn somethin' new every day. Thanks, Tide.

9. Oct 20, 2005

### rbj

geez, i cannot get the "Edit" button to work on the previous post. can some admin make that previous post go away? i can't even delete it. (sheesh! make a mistake with tex and it kills you.) r b-j
using the cgs version of Maxwell's equations ($\mathbf{E}$ and $\mathbf{B}$ field are dimensionally identical and $\mathbf{v}$ is normalized by $c$ in Lorentz force eq.):
$$\nabla \cdot \mathbf{E} = 4 \pi \rho \$$
$$\nabla \cdot \left( \mathbf{B} \right) = 0 \$$
$$\nabla \times \mathbf{E} = -\frac{1}{c} \frac{\partial \left( \mathbf{B} \right)} {\partial t} \$$
$$\nabla \times \left( \mathbf{B} \right) = \frac{1}{c} \left( 4 \pi \mathbf{J} + \frac{\partial \mathbf{E}} {\partial t} \right) = \frac{1}{c} \left( 4 \pi \rho \mathbf{v}_{\rho} + \frac{\partial \mathbf{E}} {\partial t} \right) \$$
i guess the magnitude of the $\mathbf{B}$ is limited by the magnitude of $\rho$ or $\mathbf{v}$ in the neutron star. $|\mathbf{v}|$ is limited by $c$ and i suppose there is a limit to $\rho$ in the neutron star.
(just guessing.)

10. Oct 20, 2005

### Mk

Ah yes, thank you very much.

11. Oct 20, 2005

### MooMansun

So, what is happening in a magnet?

How does it maintain a current strong enough to generate a repulsive force?

12. Oct 21, 2005

### Tide

That's the cumulative field from the magnetic moment of atoms which align themselves in bunches (domains). I'm not sure what you mean by "maintain" since the magnetic moment is a permanent property of the atom.

The domains are held in place by the lattice but they (the domains) aren't necessarily permanent since they can be altered, e.g., by heating the magnet, mechanical shock or when the magnet does work.

13. Oct 21, 2005

### MooMansun

In this context I meant generate.

I was just reading an article about a problem with a machine at 3M. It was producing a static field in excess of 200Mv and at a certain time of the day, an invisible wall could be felt.

I was wondering if the same/similar principle was involved.

http://www.amasci.com/weird/unusual/e-wall.html1

14. Oct 21, 2005

### -Job-

On wikipedia they say that the magnetic force F is given by:
F = qv X B
Where q is the electric charge, v is the velocity of the electric charge q and B is the magnetic flux density.
Is there a different equation that relates the pull of the magnetic field with the mass of the magnetic body? Suppose i have 2 magnetized bodies A and B where B is twice the size of A, then is B's magnetic field twice as large as A's or is there a different non-linear relationship between the two.

Another question, with these neutron stars having huge magnetic fields, and seeing as a magnetic force is so much stronger than a gravitational force for objects of similar mass, why does light escape such a magnetic field? And what would such a powerfull magnetic field be ablt to pull in? I assume all objects have a slightly inbalanced magnetic distribution, or is it only some metals that have the ability to have poles? Would a neutron star's magnetic field be able to pull on a piece of wood?

15. Oct 21, 2005

### Danger

Aha! Back to something that I know a little bit about. The magnetic field of a neutron star (or black hole, or whatever) is the same strength as that of the original star (allowing for some mass loss during collapse). It's just a lot more concentrated, the same as the mass. It accelerates charged particles to very high velocity (hence the 'pulsar' radio signals and X-ray emissions), but has no attractive effect on normal matter. Light escapes just as it does from any other magnetic source, although it will be distorted by the gravity.

That's a fascinating quote, Moo. I don't have time to check the link right now, but am looking forward to it later.

16. Oct 21, 2005

### MooMansun

Cheers. I found a lot of weird effects that have been studied and it would be great to obliterate the urban myths that have developed around them.

There is another link I have about people who demonstrate a magnetic like ability. Metal objects stick to them. I think, if the reports are accurate, that they are generating a localised cumulative field from the magnetic moment of atoms (thanks Tide . My question would be how?

Perhaps, if anyone else feels it is good idea, a section could be created to store the real scientific information behind these events. It would serve as the primary location to obtain accurate information and assist greatly in reducing the junk science across the web. In time, it would be referenced across the Internet extensively.

Due to the subject content, it would also prove to be one of the more fascinating aspects of the web.

I'll let all of you get back to the topic at hand. Thanks.