# How strong would a planetary magnetic field need to be in order to lighten the weight of ferromagnetic metals? [closed]

I am writing a fantasy/sci-fi where humans crash-land on a somewhat earthlike planet (continental distribution, great masses of water, similar element's abundance), however, I would like for one of the planet's "perks" to be that ferromagnetic metals would weight less.

Please notice I am not talking about general gravity, as other materials would be close to a 1:1 weight in comparison to Earth's gravitational pull (maybe I can sacrifice 10 - 15% gravitational pull and make it 0.85:1 if that helps, I'd prefer to avoid it nevertheless, as I'd have human bone structure and overall muscle composition to remain the same in time). We are talking about a magnetic field "push" capable of halving gravitational "pull" for certain materials sensible to the former.

I've read through many similar questions and answers on the site, but almost all of them are asking for stronger "pulling" field effects and applications, which is exactly the opposite of what I want.

Assumptions are the following:

• 1kg of Ferromagnetic material would weight half of that.
• Planetary diameter is similar to that of the earth. Rotational speed can be +/- 20%.
• The "source" of the "reversed" magnetic field is at the core of the planet. It's origin, nature or composition are not important.
• Despite being crash-landed, therefore spacefaring tech, society (the survivors) would have devolved into late middle-ages / renaissance tech levels, with some boosts regarding engineering due to "light" metals. So... no electronics.
• Given the strength of the field, a fine ferromagnetic dust (or cloud) would hover over certain parts of the planet's atmosphere. And Ferromagnetic material deposits would be really near to the surface of the planet.

Question:

• How strong should this magnetic "push" have to be?

If other "factors" of the "similar to earth" equation can be tweaked in order to make it more plausible, feel free to make a suggestion.

All of this can be "handwaved" and just left to suspension of disbelief, but I'd like to base it at least on "some" actual science and know how far can I pull (or push) it's limits.

I am sorry if I make some mistakes with terminology and/or units, my physics knowledge peaked at high-school. Thank you very much for your time and I would greatly appreciate any help.

AlexP:

Iron is a ferromagnetic material. Objects made of ferromagnetic materials are always attracted by magnets; one cannot make an "anti-magnet" which would repel ferromagnetic objects (...)

Thank you for the clarification, it got me thinking. Given that this is sci-fi (and the words "one cannot make" don't fully apply) didn't you just come up with an interesting candidate for an idea? an "anti-magnet"... I would like to explore that further.

John Dallman:

(...) what is the intended effect on your story or plot of ferrous metal being "lighter"? If you can explain that, we may be able to find a more plausible way to accomplish it.

Sure. Basically I want a world/universe without magic (understanding magic as "fantasy" magic i.e. "fire, arcane, ice, healing" and so on, and not as very advanced technology which could be perceived as magic by less advanced cultures), therefore I thought of magnetism as one possible candidate for a force that could bring me the desired results.

Plot-wise, civilization is composed of the descendants of a settlement ship that crash-landed a couple thousand years in the past. Little cities have emerged using the scraps from the hunks of the huge ship. As Iron is naturally abundant (and a "light" material for them), people would use it a lot in combination with "ship-metals" and build with it. The thing is, this anomaly is caused by "something" in the center of the planet, and somewhere along the story, by agency of someone, it stops, and every "light" metal suddenly weights 2 or 3 times more.

I guess, if it is too much to handle "naturally", I can do with the "something" being some artifact/building of unknown origin, I guess everything is possible with alien tech.

• Iron is a ferromagnetic material. Objects made of ferromagnetic materials are always attracted by magnets; one cannot make an "anti-magnet" which would repel ferromagnetic objects. Materials which are repelled by magnetic fields are called diamagnetic. Water is an example of a substance which is diamagnetic. Unfortunately, the effect is minuscule. A magnetic field which would exert a sensible force on a bottle of water would exert a very very much larger force of a piece of iron. Sep 10, 2022 at 15:06
• As per AlexP's comment, you cannot accomplish this with real magnetism. How strong a disbelief-suspender are you expecting your readers to wear? This is, at best, comic-book science, and could easily fail that level if presented carelessly. Sep 10, 2022 at 15:22
• To put it another way, what is the intended effect on your story or plot of ferrous metal being "lighter"? If you can explain that, we may be able to find a more plausible way to accomplish it. Sep 10, 2022 at 15:23
• "all of them are asking for stronger "pulling" field effects and applications" There's a reason for that. When was the last time you saw an ordinary piece of iron or steel be repelled by a magnet? Sep 10, 2022 at 23:59
• @sphennings, fair enough. Edited the original post to reflect the one relevant question. Thank you. Sep 11, 2022 at 17:55

Ok, take this answer with a grain, maybe a truck full of salt because electromagnetism is not my area of expertise, but I researched something similar so maybe that could help.

Basically, ferromagnetic metals are metals "magnets pull". Iron, nickel, cobalt. Magnetic field inside them gets amplified and they align themselves to the field. They don't repelled by magnets. A magnet can only "push" another magnet and only the same poles will push.

So, what you need is how strong planet's magnetic field needs to be in order to magnets with correct alignment to be lighter. To be repelled significantly.

Force applied by magnetic field usually "aligns" a magnet. It an push, or pull. And, aligning effect, depends on the magnetic field shape, can't effectively counter the gravity. Horizontal magnetic field vs vertical gravity. But there is an... interesting idea I have to change this.

Lets say you have a large, disk shaped magnet, and lets assume there is a layer of magnet on the planet underground, or make the magma as liquid magnet with higher density part as one pole, low density one as other pole.

So, each ferromagnetic metal joined to this... magnetic magma, would contribute to your giant one-pole magnet planet, and extracting some of that magma from volcanoes or lava, waiting it to cool down and extracting ferromagnetic metals, can even be a form of mining.

I am honestly not sure if this "spherical magnet" would work. But it kinda makes sense. Another way is to create "magnet lines" with fixed pole magnets. Basically gigantic bent magnet sticks underground. Maybe liquid magnet like that magna. So your "floating" or "lighter" magnets needs to follow certain lines on the planet, but if you make lines thin and material thick enough, your magnet disks can basically "surf" on two or more lines.

This way, the entire planet will effectively work as a "one pole" magnetic field, and magnets in the air can even "levitate" with their correct pole looking down. With this model, all you need is a magnetic field strength relative to the planet's gravity. Higher magnetic field? Floating magnets! Lower magnetic field? Juust lighter magnets. Gravitation force formula should be:

Force(gravity) = (Gravity constant)*((object mass) * (planet mass) )/ (planet's radius)ˆ2

Gravity constant: 6.674*10ˆ(-11)

So, magnetic field strength =? Force(gravity).

Equal? Lower? You decide.

But remember, when you flip the magnets, same strength will now pull you down. But you can forcefully magnetize objects to counter that! Basically, electricity can be used to amplify or reduce the magnetic strength. You can abuse this to even make "controllable weight" materials.

There are intersting side effects with this. Many different meteors containing metals will now be pulled to your planet. And the orbit system can be a bit more interesting. Certain moons with only one side looking, or their rotation may be janky due to the forces applying to them.