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Can magnetic fields be focused to distant points?

  1. Apr 11, 2010 #1
    I would think that the strength of two different magnets can create a focusing effect where they overlap most strongly, however I'm wondering why we can't extend these fields to distant focal points in the same fashion as a laser (or maybe more where many lasers converge on a point, creating a "hotspot")

    I did a google search for "magnetic laser" (which would of course be a misnomer) and turned up nothing.

    I recently learned that we can even focus sound in a similar way (by a different process) to the focused power effect of a laser. A series of weights transmit force through a metamaterial which focuses the sound into a "hot spot", so although magnetism may be a force rather than an "object" like a photon, can it not also be focused either onto a spot, or even made into a beam where the field lines actually come straight out of the device towards a distant point?

    Magnetic field lines normally cause things to follow a loop like pattern (See plasma on the sun, iron filings on paper with a magnet beneath), but can they create a laser like effect?

    Obviously, the magnetic field cannot be fired as if it were a projectile (though it can fire magnetic projectiles as seen in certain hypervelocity guns), but such a device could transmit the effect to a distant point. A magnetic pointer (as opposed to a laser pointer) could be pointed at a distant piece of metal and either attract or repel it. We could even use the negative and positive fields together to levitate something in the air at a distant point.

    Now, I wonder... Is this device outright impossible according to physics? If so, why? If not, then why haven't we made devices like these yet? Is the technology required vastly beyond what we have today?
  2. jcsd
  3. Apr 11, 2010 #2


    Staff: Mentor

    It is interesting that you mention a laser. Half of the energy carried by a laser is, in fact, carried in a magnetic field.
  4. Apr 11, 2010 #3
    Really? I know that light is electromagnetic radiation, but I'm not sure of what that really means when relating it to the effects we see in actual magnets. I was just thinking of magnetic field lines being formed as a long distance "beam" to manipulate distant magnetic objects. Lasers don't have that kind of application (although you can manipulate VERY small objects by using lasers as "tweezers").
  5. Jun 21, 2010 #4

    I subscribe to this forum, because last week I got that idea and wanted to discuss about it. In fact, I was looking for a way to reduce the exponent n=2 in Coulomb's law: F= q1q2 / (e*r^n)... in order to increase the length of the magnetic force between two magnets. So, I thought maybe if we could concentrate the radiation into an axis like a laser do for light.

    I wonder if we could build a mirror for an electromagnetic field in order to concentrate the field?

    Assuming that we could do that kind of instrument technically. I was thinking of the application related with space propulsion. Having a magnetic south field beam pointing to an object with a north permanent magnets... I guess we could have an object lifting and idealy reaching a distance/speed far enought to be put in orbit around the earth.

    I also thought, what will happen if we put that instrument attach to a box with a wall on the opposite side of the box full of magnets (north pole). If we place that box in a toy boat and place that boat in a receipient of water... When we turn on the instrument... would the boat will move? - Normally in a close system all forces has a counter force... so, I guess no.

    Otherwise.. would be nice for a propulsion system which stay inside an object and has no need for friction or to release mass in the opposite direction in order to move.
  6. Jun 21, 2010 #5


    Staff: Mentor

    Hi fcycles, welcome to PF.

    Coulomb's law has nothing to do with the magnetic force, it is only valid for electrostatics where there is no magnetic field. Also, the exponent is not something that can be altered, it is essentially a result of the fact that space is 3 dimensional so the surface of a sphere goes up as r^2.

    You can certainly build a mirror to concentrate an electromagnetic field, that is called a parabolic mirror and is used in headlights, spotlights, and solar collectors. It won't do what you are thinking of however. In principle, it is possible to gain momentum by shining light off the back, however light carries very little momentum, so you cannot gain very much.
  7. Jun 21, 2010 #6
    Hi DaleSpam,

    Thanks for your reply. Sorry for mixing up with term in physic. What I mean was related with permanent magnets like Neodynium magnets. When I thought about a mirror... was in order to get field force going out all around north and south pole... to have them going straight ahead. Hoping to increase the force at a given distance between 2 magnets...

    But, I admit that I see no way with my knowledge in physic to do that kind of things. :P So, I thought there is 2 choices... a) I read some books about it to refresh my knowledge in physic and get deeper or b) try to see if someone already thought about it.

    When I read Researcher X question.. I thought.. oh, that's the same question as mine I believe... :)
  8. Jun 22, 2010 #7
    Are you familiar with a solenoid? This is simply a device which is used to project a magnetic field in a certain direction, like you are saying. Of course you can put a bunch of solenoids together to focus fields, or you can send the magnetic field further out by increasing the current running in the solenoid. However, a solenoid is essentially just a collection of dipoles, and the field will fall off like 1/r^3 at long distances, meaning that it is very difficult (requires a great deal of current) to make it extend very far.

    Also, the main problem with your idea is that magnetic field lines must close on themselves. So if you want a field to reach across a very long distance, it not only has to cross that distance the first time, but then it must cross the distance again on the way back. That's why you tend to see loops of plasma from the sun, instead of filaments, and that's what poses the restriction on the extension of the loop. With electromagnetic waves or lasers, you don't have this limitation since they are self-propagating, and they keep going outwards without having to connect back.
    Last edited: Jun 22, 2010
  9. Jun 22, 2010 #8


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    Science Advisor

    Magnetic field lines arrange themselves in a way that satisfies two criteria.

    1) Lines shall not cross. (Maxwell's Eqns)
    2) Integral over H*B is minimized. (Minimum energy)

    There is also a condition on curl (vorticity) which will govern how many field lines you have.

    What this tells you is that field lines will bunch up only within certain objects. Magnets for example. In empty space, the field lines will try to spread out as much as possible to minimize energy.

    Without placing something at the target, or very near it, you cannot make field lines converge there.
  10. Jun 22, 2010 #9
    What about focusing the B field with strong gravitational fields , This might take a black hole .
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