Limits of an eddy current force fields

In my story I need a nigh impenetrable force field as an ability for a character. I have done some research that the force field should be a magnetic eddy current. Eddy currents can block conductors moving parallel to them via the law of induction. Essentially any conductor moving towards a magnetic eddy current will be stopped and heated. This character has the ability to convert objects into conductors via line of sight. This would allow them to block anything moving towards them.

So knowing that explosions are plasma/conductors and shrapnel is metal a conductor can they be blocked via eddy currents? There’s more info on eddy currents here, “By Lenz's law, an eddy current creates a magnetic field that opposes the change in the magnetic field that created it, and thus eddy currents react back on the source of the magnetic field. For example, a nearby conductive surface will exert a drag force on a moving magnet that opposes its motion, due to eddy currents induced in the surface by the moving magnetic field. This effect is employed in eddy current brakes which are used to stop rotating power tools quickly when they are turned off”

So assuming this field has as an unlimited source of electricity. What would the limits of a magnetic eddy current force field be? Could it block explosions, bullets, and melee weapons?

• Your own sword and your own bullets are also conductors moving in the same magnetic field as the incoming swords or bullets. If the magnetic field is strong enough to stop the incoming swords or bullets, it is also strong enough to stop your own sword or bullets... And if you say you will use non-conducting swords or bullets, well then, why is the enemy so stupid as not to use the same non-conducting swords or bullets? Commented Jan 4 at 17:55
• You're trying to find a science-as-we-know-it-today explanation for a completely fictional and unrealistic concept. Please set your expectations accordingly.
– JBH
Commented Jan 4 at 18:01
• Just clarified. The character has an ability to convert insulators to conductors so eddy currents should be able to stop insulators as well now Commented Jan 4 at 20:10
• Then why does he needs eddy currents? If he can convert insulators to conductors he can kill/maim any opponent in range of his ability. Commented Jan 9 at 12:00

No

@User6760 linked a great video to @L.Dutch's answer. A person might look at that video and conclude, "look! A basic force field!" But they'd be entirely wrong.

Important Issue #1: The unusually pure copper plate is floating in liquid nitrogen, making it a superconductor. In other words, not every conducting material can be used to do this and when you do have a conducting material that can do this, it must be in a state that allows for superconductivity.

Important Issue #2: The other object involved is a magnet with an unusually high magnetic-flux-to-mass ratio. Had a traditional ferrous magnet with the same magnetic flux as the NdFeB magnet used in the video been dropped on that copper sheet it would have hit hard enough to leave a dent... then I suspect it would have blown itself apart as the liquid nitrogen caused rapid expansion of the ferrous material. Might be wrong about that... but it'd be a cool experiment.

So, what's happening? As the magnet approaches the copper, the magnetic field carried with it causes really small eddy currents within the copper plate, which create in turn an opposing magnetic field that temporarily opposes the motion of the magnet.

Which means you theoretically could give a boost to a rocket by giving it a powerful magnet and launching it from a superconducting plate... except that the weight of the magnet and/or the energy to power it would be more profitably used anywhere else.

Looks like a useful force field to me!

Nope.

In your scenario, your character is creating the magnetic field. In other words, your character is playing the role of the magnet. Infinite electricity! Unbelievable, but OK. That means your character is a walking magnetar destroying everything based on electricity as he/she passes and nobody can communicate by anything other than smoke signals or a semaphore, but let's ignore that.

What it means is that the bullet is playing the role of the superconducting sheet of copper.

• Problem #1: That video's experiment works with a sheet of copper. Not a sphere of copper or a length of copper wire (no matter how pure). Eddy currents aren't dissimilar to eddy currents you see in rivers. They need space to form and that space must be perpendicular to the magnet inducing the current. A wire doesn't have enough width since it's mostly length. A sphere hasn't enough length or width since so much of it is depth and the bigger the sphere the more the currents move into the sphere and away from the perpendicular surface needed to repel the incoming magnet. That's a long way of saying that unless you're trying to stop a wall of copper rushing at you, this won't work.

• Problem #2: The word superconducting can't be stressed enough here. Bullets are made out of a lot of things and, yes, nearly all conduct electricity to one degree or another... but the magnetic field required to induce enough eddy current in a flying non-superconducting bullet with next to no perpendicular cross-section such that it would stop the bullet would also cause a breathtaking amount of damage to pretty much everything within a substantial distance. Why, you ask? I'll get to that in Problem #4, but first...

• Problem #3: You mention heating up things. Eddy currents are nowhere near that strong... even with an infinitely strong magnetic field. Well... maybe with an infinitely strong magnetic field. But what's the point if your force field's main component is strong enough to affect the nuclear fusion in your sun?

• Problem #4: You're a walking magnet! So far we've only been considering the difficulties of inducing the eddy currents, but the reality is that if any part of the incoming bullet can be magnetized, the force of attraction will overcome the eddy current-induced force of repulsion by exponential orders of magnitude. Which is a verbose way of saying, "congratulations, you just drew every bullet in a thousand-mile radius into you." You've also drawn in every belt buckle, tank, spoon... even the ferrous rocks in the ground around you! It's all be drawn toward you with enough force to turn you into a lovely pink paste.

One last thing... There might be plasma in an explosion, and plasma may be electrically conductive (it actually is... but not in the way you're thinking), but the plasma isn't the primary problem with an explosion. It's the concussive force. Even if you could produce a strong enough magnetic field to repulse atomic oxygen, etc., the result would be your character's painful death from suffocation.

The problem with introducing "realism" into your science fiction is that science couldn't care less about your worldbuilding efforts

If you're honest about what you're doing, then you must admit that what you're actually looking for is realistic-sounding technobabble to rationalize your force field. That's OK! That's actually what we're here to help you do!

What we are NOT here to do is to use real-world science to engineer an entirely fictional piece of technology. Science is a harsh task master. It couldn't care less about your story or your goals. You either conform to science or you don't. Asking "is is possible" or "is it realistic?" doesn't work here because the answer is usually, "science says no. Now what?" Can you use eddy currents to create a force field that can repulse or burn away bullets?

No

Even if your super-powered/magic-weilding character can change incoming bullets (that's pretty fast thinking, there, Tex!) into superconducting materials (they can do that and still need a force field? Why not change them into oxygen and ignore the force field?)... there's not enough cross-section and you're attracting too much unwanted attention from magnetizable stuff.

But if you're looking for a bit of technobabble to help you rationalize your idea so you can move on with your story...

Then you didn't need to ask your question. The concept of eddy currents works great to rationalize a force field and nobody but electrical engineers and physicists would know that what you're talking about isn't realistic at all.

And that's no big deal. Hard science people must constantly suspend their disbelief to enjoy even the most "realistic" stories. Oh, the author/director's/whatever's PR firm may claim the story is the most "realistic" portrayal of something or other since the invention of fire... but it's still fake and we know it.

So what? We're still willing to pay for the streaming service, the book, or the movie tickets to enjoy the show. We love it when people present "what if?" in a pretty package.

• Thanks, @JBH. I was going to do a shorter version of this, but you got it covered. Commented Jan 4 at 22:42

I don't see how an eddy current can block anything:

an eddy current (also called Foucault's current) is a loop of electric current induced within conductors by a changing magnetic field in the conductor according to Faraday's law of induction or by the relative motion of a conductor in a magnetic field. Eddy currents flow in closed loops within conductors, in planes perpendicular to the magnetic field. They can be induced within nearby stationary conductors by a time-varying magnetic field created by an AC electromagnet or transformer, for example, or by relative motion between a magnet and a nearby conductor.

A force field which requires you to wrap yourself in a conductor to be protected is nothing more than a fancy marketing spiel to sell a good old metal armor, with the additional burden of a variable magnetic field generator.

Summarizing, your force field will block as much as the conductor you are using to produce it will be able to withstand.

• i think the OP is referring to the trick shown in this video Commented Jan 4 at 16:30

Forcefields really kind of can't work period based on what we currently know about physics. Diverting some material is one thing but making a wall of force that acts like an impenetrable physical object for anything trying to get through it isn't something that any known mechanism can produce. When force fields are used in sci fi, they are generally just future science magic and little is done to explain the nitty gritty of how they work beyond meaningless technobabble. Given that force fields and interstellar travel tend to go hand in hand, I tend to like to logic spacetime curvature as the way forcefields work in fiction. If you already have some kind of warp bubble tech, encasing yourself in such a bubble would be an extremely effective forcefield. Bullets could be diverted around you or even just fizzle out before they penetrate too deeply into the field, having traveled much further than their apparent distance through that curved spacetime. It's still a technobabble explanation, but I like that it links two commonly associated sci-fi technologies as relying on similar principles, and so gives the feeling of internal consistency and logical technological progress. It still isn't an explanation, but it feels enough like one that you will be able to get into the heads of the characters using and responding to it and make decisions about how specifics work in universe. Any such explanation is going to have this character. Forcefields aren't real and probably can't be. Yours just have to sound realish enough that people will accept it and move on with the story.