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A magnetic field strength drops-off quickly as the distance from a magnet increases.

Is there any way to use electromagnetic fields to create a magnetic field at a location. For example, if there are strong electromagnetic fields intersecting at a location from many strong transmitters, could a constructive interference be created which creates a local magnetic field?

The engineering idea is that if you create an array of radio transmitters at a strong strength working together, could there be a interference pattern at a point far from the transmitters (maybe 100 meters away) where the fields produce a magnetic field at that location.

I think this could be done as a EM field is composed of a electric field and a magnetic field, shouldn't there be a way to make the magnetic field strong at a location in space.

A positive consequence of this would be that a magnetic placed at that location 100 meters away could be moved by the field. Also, if the field was alternating, rather than steady, then a electromagnet at that location could be alternating, and the force on the electromagnet could provide motive force.

I guess since I have never heard of this, it cannot be done. But then again, it was once said that levitating a normal magnet could not be done, then someone did it with a spinning magnet which levitates above another magnet, now it is a popular toy. The reason it works is that the magnet is spinning so that it cannot flip over. See spin stabilized magnetic levitation.

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Hi, I would like to clarify, I wish to move a physical object, a small magnet at a distance, not to transmit power to the object. Thanks!

--- Update 2

My engineering goal for this is a small magnetic, spin stabilized, being pushed from a distant location. A small spin stabilized magnet pushed from below by some "projected" magnetic field could potentially overcome the force of gravity. Imagine a thin lightweight spinning disk with a magnet in the center. If a projected magnetic field is pushing it upwards then it would stay stable and could be projected from ground into space.

This might be done with an array of thousands of transmitters on the ground, somewhere in the desert far away from people. All transmitters could focus a small magnetic field at this magnet.

Alternatively, if the magnetic field alternates with a frequency, then the magnet in the levitated device could also be alternating via electromagnetism, it could be powered by an battery or microwave power transmitted to the object. This is more or less an engineering problem if there is some basic science which can allow this to be a potential reality.

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7 Answers 7

Sure, it's possible. That's how sets of cell phone radio towers work. In a typical setup, several of these towers are placed in a line, some distance outside of a town, such that the line is roughly perpendicular to the direction to the town. The placement of the towers is designed so that the signals reinforce each other as much as possible only in the town. That way, most of the cell towers' transmitted power gets directed toward the area where most people are actually using their cell phones, and you don't waste a whole lot of power by transmitting into open wilderness.

With the cell towers, though, the only things that really get pushed around by the EM waves are electrons in people's cell phones. Theoretically, there's no reason the same concept couldn't be used to move something larger, like a magnet, but you'd need vastly more powerful transmitters - perhaps more powerful than anyone knows how to build with current technology (depending on how large of a magnet you're trying to move, and how far away it is). Still, it's just an engineering problem.

Also, a side note: it's generally the electric field that's responsible for moving things around, not the magnetic field. Magnetic fields themselves don't transfer energy, they only change the direction of existing motion. But the two kinds of fields mix when you change your reference frame, so it's kind of a semi-arbitrary distinction anyway.

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I wish to move a small magnet at a large distance from the transmitters. –  Phil Nov 10 '10 at 13:59
@Phil you could use the power of the electric field as described above to create a small electromagnet, no? –  anna v Mar 30 '14 at 4:11

This can certainly be done, as induction chargers work pretty much that way. That said, this technique is not very efficient and it is hard to scale at large distances.

For large distances, one of the most popular ideas is using microwave transmission.

There's a very good article over at wikipedia with a round-up of all these technologies with pros and cons.

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Hi, I would like to clarify, I wish to move a physical object, a small magnet at a distance, not to transmit power to the object. Thanks! –  Phil Nov 10 '10 at 13:58
Ok, microwave transmission is not very relevant then, but induction chargers do work by "shooting" a magnetic field and would move a small magnet. As I said, though, there are practical problems of efficiency and design which make this infeasible (but not impossible). –  Sklivvz Nov 10 '10 at 14:26
Voted up for the links. :) Thanks. –  Robin Maben Nov 27 '10 at 5:42

Nope, this method is not feasible.

You are right that the EM field is "Electro"+"Magnetic", but these contributions are oscillating in time. Every interference they produce will oscillate in time as well, even if the peak magnitude of these oscillation can (theoretically) be high (that is the constructive interference you mentioned).

Let's say you just take the magnetic field vector of your $i$th beam at the position of your (pointlike) object:

$B_i(t) = B_{0i} \sin(2 \pi \nu_i t + \phi_i)$

If you have a $n$ waves that constructively interfere, which have to have the same frequency ($\nu_i = \nu$), and the the total magnetic field will be

$B(t) = (\sum_{i=1}^n B_i) \sin(2 \pi \nu t) = n B \sin(2 \pi \nu t)$

with the final, very simplified step being true if all the field magnitudes are the same. Thus your field will still have the oscillation at the original frequency (eg. radio frequency (~ tens-hundreds of MHz) or laser beam (hundreds of THz). No macroscopic object would react to this high frequency perturbation due to inertia. And even if they would, the resulting motion wouldn't be a push or a pull, but oscillation on the time scale of $1/\nu$. That's why in general, motion is induced with stationary or slowly moving fields.

... and this haven't even taken into account a lot of practical limitations to this scheme.

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What about an opposing oscillating magnetic field at the same frequency generated by the object which is to be moved? –  Phil Nov 18 '10 at 4:09
I mean - north/north repels, south/south repels. Therefore a push. –  Phil Nov 18 '10 at 4:26
Electromagnetic fields (under these conditions we discussed here) follow the rules of superposition: the field strength is just the sum of the different waves. If your object generates waves that is just one more $B_i$ in the above sum. Also, these waves don't "move" each other, they affect the material: the field changes the motion of a charged particle/piece of object. –  Greg Nov 18 '10 at 6:24
Hi Greg, what I'm trying to do is find a condition under which I can get the object to move under this strong electromagnetic field where most likely there is a strong sum of the different waves, a strong constructive interference. –  Phil Nov 18 '10 at 14:34
So what I was thinking is that this is a magnetic field at a location in space (at the constructive interference point), and the field is alternating + then - very quickly. So, if I had a electromagnet which is alternating + and - quickly in a opposite phase to the field at this point, then couldn't this electromagnet feel a force acting upon it in a linear direction, if the + and - were opposite to the + and - of the field at that point. –  Phil Nov 18 '10 at 14:36

This is possible, it's just very very difficult to do.

People regularly do this with the electric field of light to move dielectric particles (insulators) in the lab, and the technique is known as "optical tweezers":

The reason you don't want to try this with magnetic particles is that the magnetic field of light is much weaker than the electric field, or rather that it doesn't interact very much with most magnetic materials.

Greg's answer above is half correct - The fact that these fields oscillate in time means that the applied force would oscillate as well, however, the field gradient is exploited instead to make optical tweezers work.

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Could powerful lasers pointed up at some object scale up the optical tweezers effect by creating millions of optical tweezers, lift an object upwards? –  Phil Feb 3 '13 at 14:10
billions, trillions, of optical tweezers all over the bottom of a spin stabilized object –  Phil Feb 3 '13 at 14:43
@Phil The lasers could easily used to lift the object by ablation... And it may be not easy to do anything else without ablation... –  Volker Siegel Jul 3 at 5:41

An optical lens concentrates radiating electromagnetic fields and these fields (including the magnetic B-field) may be much higher than at the source, so yes it is possible. For more constant and strong B-fields, it also depends on the type of field you want to generate (cannot disobey Earnshaw theorem, Maxwell equations, etc.). In general today, static and dynamic magnetic fields are usually generated in a more simple fashion with fields at the source higher than at the target. That doesn't mean it is impossible the other way around, and if doable and practical it would be a great breakthrough, since generating magnetic fields in a region higher than in the source, would allow for us to generate very high B-fields in general, fields of strength that would be impossible to generate today because they would destroy the source by electrical breakdown. Such high B-fields could have major applications for nuclear fusion, ion rockets, nuclear magnetic resonance, and particle accelerators. It is not a trivial problem. Possibly a solution could be found considering dynamic non-radiating fields (Schott discovered these) or long wavelength evanescent wave emissions (wireless energy transfer) with some kind of wave interference/cancelation in the source. Go for it.

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projecting a magnetic field under a magnet to force movement and levatison can give forward inertia to produce perpetual Motion

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the magnetic feild is projected from with in the ship and each time its projected it makes the ship move and a smaller magnetic feild makes it stay stationary –  user14045 Oct 14 '12 at 22:41

No because you can't project a magnetic force in a beam to levitate an object because of opposing force +- makes a Field opposed to as a stream of directed energy energy radiates as opposed to form a beam like water forms to a container waves do not behave this way magnetic force is also unstable any other magnet force the smaller one would cause the ship to flip catastrophicly because of attraction opposites attract remember you could use a high amounts of electricity flowing though a single magnet in the upper atmosphere already to maybe ride earths magnetic fields it would be very unpredictable and complicated to get into though right now but yeah no

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