Magnetic Forms of Energy

  • #1

Main Question or Discussion Point

I'm just wondering whether magentic fields can used to do work on objects? For example, can an electromagnet create a large enough magnetic field within a magnetic bullet to deflect off it's original path? I know this isn't practical, I'm just wondering whether its possible.
 

Answers and Replies

  • #2
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Assuming the bullet is susceptible to magnetic fields, yes, if you could produce a strong enough field you could deflect the bullet.

The bullet wouldn't even have to be magnetic in itself.

Of course magnetic fields can do work.

Have you ever heard of Maglev? (http://en.wikipedia.org/wiki/Maglev_(transport))
 
  • #3
So if I tightly wrapped a conducting wire around a neodynium core, creating a solenoid, and placed two of these on a table, a metal projectile would be deflected from it's path? Would this have practical applications in war?
 
  • #4
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I suggest you try calculating how strong the magnetic field would need to be in order to 'deflect' a bullet. Especially in the magnitude you would require for a practical military application.

Your setup would only produce a small field and would only deflect a small object (let's say 9mm round) if it was travelling at relatively low speeds (< 1m/s).
 
  • #5
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I suggest you try calculating how strong the magnetic field would need to be in order to 'deflect' a bullet. Especially in the magnitude you would require for a practical military application.

Your setup would only produce a small field and would only deflect a small object (let's say 9mm round) if it was travelling at relatively low speeds (< 1m/s).
i wonder if you could explain how to calculate that?
advanced thanks
 
  • #6
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Of course magnetic fields can do work.
I don't think this is a true statement? On the contrary, it can be easily shown that magnetic forces never do work. We have, for some charge moving an amount [tex]d \mathbf{l}=\mathbf{v} dt[/tex], that the work done by the magnetic force [tex]\mathbf{F}_mag=q\mathbf{v}\times\mathbf{B}[/tex] is

[tex]W=\int \mathbf{F_m} \cdot d \mathbf{l} = \int q\mathbf{v}\times\mathbf{B} \cdot \mathbf{v} dt = 0[/tex]

This is always zero since [tex]\mathbf{v}\times\mathbf{B}[/tex] is always perpendicular to [tex]\mathbf{v}[/tex]. See for example Griffiths 'Introduction to electrodynamics', p.207.

Electric forces however, do work. And this does not contradict the fact that a bullet could be deflected by a magnetic field, but there is no work done.
 
  • #7
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  • #9
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Thank you Studiot, very interesting and perfect for what the OP is looking for.
 
  • #10
Thanks for all your input. I know my experiment depends on a lot of factors, but would a tightly wound solenoid with an iron (or other metal) core significantly deflect a small projectile?
 
  • #11
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Yes, it could if your bullet a contains significant % of magnetizable material.

No, if not.

That is how an electric doorbell works, which is a much better use of this technology IMHO.
 
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  • #12
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Again, it depends on the strength of the magnetic field produced.

To deflect a bullet would take a lot.

Imagine a big scrap yard electro magnet. If you shot a bullet past that, it would deflect the bullet slightly. But, unless the bullet is travelling within a relatively close range to it (<1m) it won't have any effect.

I'd also note that unless the bullet is specially magnetised, it will attract it.

If you are thinking of standing a soldier next to a magnetic field, you would basically need a massive electromagnet as above and have them stood right by it. Not feasible at all.

So far as an experiment, you'd need a slow moving, relatively small projectile to pass very close to your rig (assuming it's quite small / low powered) to see any effects.
 
  • #13
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@jarednjames: You are of course right, what I said is only true for charged particles. For magnetized media magnetic forces can do work. I was to into my current course in electrodynamics, which did not deal with magnetized media.
 

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