A permanent static magnetic field appears on the surface of a planet that is sufficiently large and strong to attract non-magnetized ferromagnetic objects up to 100 miles distant from the source, with a pull force of 10N on a 1kg iron object at that distance (assume a small iron sword if the shape really matters). Besides eventually pulling in all such objects into a great big pile (and smashing anything in their path), what else would happen as side effects?

Ignore for purposes of this question how the field is generated--in-story it is created by magic gone awry, but after creation I'd like the effects to follow from more-or-less sound science. If it matters, assume that a lump of material with off-the-charts coercivity is magnetized by an external source (i.e., this isn't an electromagnet with a constant energy source keeping it going).

Obvious side-effects I can think of include:

  • Interfering with compasses, possibly world-wide, as well as interfering with birds' ability to navigate.
  • Seismic activity caused by pulling on iron ore deposits beneath the surface, perhaps to the extent of major reshaping of the landscape.

What else? My understanding of the science of magnetism somewhat limited, but I'm a little concerned that if you managed to create such a magnetic field, you'd end up with world-destroying side-effects (e.g. objects pulled toward the source striking with enough force to cause fusion). My intention is for the 100 mile radius that is affected by the field to become uninhabitable, but not to destroy the entire planet.

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    $\begingroup$ People getting hit by large masses of iron moving at high speed, disrupting the ionosphere,... $\endgroup$
    – nzaman
    Commented Apr 16, 2018 at 14:42
  • $\begingroup$ Does "up to 100 miles" mean non-magnetic ferrous? Or is that the extreme range for something that is currently magnetic? An example of non-magnetic ferrous would be an iron nail. I'm assuming that these objects would be loose; i.e. the nail is not hammered into a board that is part of a structure that is joined to the ground. My point is that the range for magnetic objects is greater than for non-magnetic objects. So which are you trying to express as a hundred miles? $\endgroup$
    – Brythan
    Commented Apr 16, 2018 at 15:10
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    $\begingroup$ I think you need to take one step first: Establish how strong the magnetic field actually is quantitatively. Then you can talk about what the effects are. Currently you might get a zoo of answers, all of them making other assumptions on how strong it actually is when it's a very simple parameter you could just specify to avoid this problem $\endgroup$
    – Raditz_35
    Commented Apr 16, 2018 at 15:24
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    $\begingroup$ not to be picky, but a pull force is not measured in kg. And a cube law is not exponential. $\endgroup$
    – L.Dutch
    Commented Apr 16, 2018 at 16:06
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    $\begingroup$ A pull force on 1kg on how much ferromagnetic material? A field that can exert a "force" of 1kg (assuming you actually mean 10N) on a battleship is a lot weaker than one that can do so on a single iron filing. $\endgroup$
    – user31336
    Commented Apr 16, 2018 at 18:02

4 Answers 4


An attractive force of 10 Newton (1 kg) at a distance of 100 miles is something huge. A permanent magnet's force increases with the cube of the inverse of the distance, so at 50 miles you'd have 80 N, at 25 miles 640 N (enough to lift the weight of the average woman), at 20 kilometers 5120 N (half a ton).

At a distance a little more than one kilometer you'd experience a pull of two thousand tons (20971.5 kN), about six times the maximum thrust of a Boeing 747's Pratt&Whitney turbofan (and a 747 has only four of those).

The field intensity at that distance is two hundred times greater than a NMR machine, more than enough to have a detectable, macroscopic effect on diamagnetic substances like the water contained in a human body.

A magnetic field about ten times weaker is enough to lift a frog.

In those conditions:

  • you would be unable to approach further. While vastly inferior to the ferromagnetic effect, the diamagnetic repulsion also increases at the same rate, so you'd be going "uphill".
  • nothing even remotely ferromagnetic (and several kinds of sand contain ferromagnetic compounds) could remain still.
  • this includes iron minerals in the crust. The pull on those might not destabilize anything and not cause any seismic effect... but on the other hand, it might. Luckily, any fused material would be above the Curie point and not be too reactive to ferromagnetism, so lava fountains are probably out.
  • any charged particle, included the ions in a living organism, would be subject to a Lorentz force that would make them move in circles. This would disrupt some of the more delicate functions e.g. of the nervous system: a significant minority of people are already capable of feeling the magnetic field of a NMR machine. This field would cause a proportionally greater inconvenience, increasing to pain and probably physical damage the nearer you get to the source.
  • Going still nearer, if the mass of ferromagnetic dirt and junk sprouting from the supermagnet didn't stop you, you'd experience - apart from a horrible death, that is - strange refractive effects from surfaces due to Paschen-Back disruption of impacting photons. This also would play merry Hell with most chemical reactions.
  • Just turning around would become difficult, and the extra energy expended to do so would transform into electricity inside your body (as any conductor in a static magnetic field will do; you're now essentially a dynamo).
  • Moving within such a field would also be a hurdle; and if you were in any kind of conductive vehicle? It would be the same as a magnet moving inside a conductive object. Not so spectacularly because at those distances the field varies slowly with distance, so the flux also varies slowly unless you change orientation, so a speeding bullet wouldn't melt and explode. But you would need to move mostly in straight lines. Large conducting objects would feel this most; near the source, they'd be perceptibly hampered.

VERY rough calculation - iron ball

At a distance of 10 km our one-kilo iron ball is subjected to a force of 80000 N, which translates to 800 G of acceleration. It arrives at 10 km with a speed well in excess of that of sound, but from then on, assuming constant acceleration (it's not - the force is still increasing), we can use the impact velocity formula of $v = \sqrt{2 a s}$ to estimate a minimum speed at arrival - s = 10000 m, a = 8000 $ms^{-2}$, gives v = 12.6 kilometers per second. If it wasn't smashing against the magnet, it would be more than fast enough to escape Earth's attraction. And the real velocity is going to be a lot higher: not as fast as the inverse cube of the distance, but the acceleration will continue increasing every meter of those ten kilometers.

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    $\begingroup$ This is great. Any ideas about effects outside the 100 mile radius? I think I need to dust off my calculus knowledge and try to work out how fast a typical object would be going by the time it reaches the center. ~20,000 kN at 1km would cause a monstrous amount of acceleration on smallish objects, right? Question is whether they hit the center before reaching relativistic speeds... $\endgroup$
    – rsandler
    Commented Apr 16, 2018 at 18:15
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    $\begingroup$ (Not an answer) I want to see the guy from XKCD.com do a write-up of this... with lots of "let's add another zero..." :D $\endgroup$ Commented Apr 16, 2018 at 18:15
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    $\begingroup$ @FunkThompson I would love that--I thought about referencing the relativistic baseball, in explaining the "nevermind how it got there" criteria in my question... $\endgroup$
    – rsandler
    Commented Apr 16, 2018 at 18:30
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    $\begingroup$ I suspect that any conductive object moving rapidly thru such a field would develop huge eddy currents on the way in, and would melt into liquid from ohmic heating. $\endgroup$
    – Willk
    Commented Apr 16, 2018 at 19:53
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    $\begingroup$ @AnonymousAnonymous - actually I think the object would no longer be attracted once it was red hot. There is a thermal point where ferromagnetism disappears. The thing would still be moving and so would still have eddy currents, which would serve to slow it down. As it slows, the eddy currents decrease and it would cool, maybe to the point where it was magnetic again. I wonder if this effect would produce a sort of terminal velocity. $\endgroup$
    – Willk
    Commented Apr 17, 2018 at 17:27

Everyone Dies

A force of 10N on 1kg of iron at 100km implies, due to r3 falloff, a force of 0.004 mN on 1kg of iron at 6,000km. At a first-order approximation, there is approximately 2*1024 kg of iron in the core of the Earth at 6,000km. This means your magnet attracts the core of the earth with a force of about 1019 N (Back-of-the-envelope calculations here, since the core of the Earth is a complex magnetic system.) This is many hundreds of times the force exerted by gravity upon the entire Himalayan mountain range. Presuming this magnet is not larger than most countries and extremely flat, it's also vastly more than any force exerted by the ground upon it, or its buoyancy in molten iron.

Assuming this force isn't counteracted by authorial fiat:

Work done is force times distance. Even while the field is still relatively far away from most of the iron in the Earth, we're still getting the equivalent of 1019 J released per meter fallen. Since the magnet is plowing through the mantle, this energy is deposited into the mantle in the form of shockwaves (as the magnet will almost certainly be moving faster than the speed of sound in any material it encounters). After even 1,000km, long before our back-of-the-envelope assumptions stop working, the equivalent of the detonation of 2.5 Petatons of TNT has been deposited into the system.

Conclusion: The magnet drops to the centre of the Earth in short order, releasing the energy equivalent of the Chicxulub impact in some kind of super-earthquake-volcano-thing, causing, even with conservative estimates, the death of the majority of life on the planet and the total destruction of civilisation.

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    $\begingroup$ I think you've greatly overestimated the impact - if you look at Orders of magnitude (energy), you'll see that the Chicxulub impact had about 10x the energy of the 2004 Indian Ocean earthquake. So yeah, devastating local effects, but people on the other side of the planet are likely going to be fine. $\endgroup$
    – Rob Watts
    Commented Apr 16, 2018 at 19:05
  • $\begingroup$ Also, this answer is easily invalidating by the OP deciding that the magic also causes the magnetic field to stay on the surface of the Earth. I'd suggest having a first-line summary saying that if the magnetic field doesn't magically stay on the surface, it's almost certainly going to pull itself to the center of the Earth (or pull the Earth toward it so that it becomes the center of the Earth). $\endgroup$
    – Rob Watts
    Commented Apr 16, 2018 at 19:08
  • $\begingroup$ It appears I was using a less conservative estimate of the Chicxulub impact energy than that wikipedia page. The number I got was 10^25J, so you could sub in "20 times the Chicxulub impact" if you want. It's mass-extinction level energy at any rate. $\endgroup$
    – user31336
    Commented Apr 16, 2018 at 19:09
  • $\begingroup$ I was also worried that energy release by deformation of the core of the earth, since the field is non-uniform, would also be a mass-extinction event, but that's a real calculation involving real data and real integrals, so I left that off the answer, but any magic would also have to probably deal with that. $\endgroup$
    – user31336
    Commented Apr 16, 2018 at 19:14
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    $\begingroup$ @rsandler When forces of that level are involved, everything becomes spheres. The magnet would not remain a rod long without large amounts of handwavium. And if it did remain a rod, the earth (or at least the core) would likely deform to a similar shape. $\endgroup$
    – GOATNine
    Commented Apr 17, 2018 at 17:08

Effects, from first, weak intensity, to last, strong intensity:

  1. The first effect of a strong magnetic field is communications disruption - as our satellites and phones etc. use finely tuned transmitted EM fields this would be disrupted and cause a breakdown in all data communication.

  2. As the field intensifies, sensitive devices relying on magnetic fields would be wiped. (ie magnetic hard drives) and compasses turn.

  3. As it further strengthens, currents would be induced in most integrated circuits, burning out or frying most unprotected electronic devices.

  4. Intensifying further, you may notice more intense Aurora in the sky as spiralling electrons get caught in the field and emit light, depending on field direction.

  5. Currents induced in metal produce heat and sparks, disrupting national power grids.

  6. As charge in the atmosphere is usually different to the ground, there may be increased levels of lightning.

  7. As the currents induced in metal increase as the field increases, they may get so hot they may start to melt, and burn away any material in contact.

-- by the way, you may notice the above is the same as an atomic bomb, which causes a large pulse, as we continue let's see what happens --

  1. As more induced currents in metal melt, ground would essentially turn into lava with enough induced energy.

  2. Super-large magnetic fields have the ability to start stripping electrons from their nuclei, essentially creating charged ions and promoting higher energy reactions. These reactions would propogate through matter and expand the generation of radiation.

  3. The more charged ions created the more the environment may be converted into a vortical plasma, a rotating vortex of ions.

  4. With massive field strength, charged particles could move at relativistic speeds, which had been predicted by Einstein and demonstrated in particle accelerators, where their collisions could be high impact enough to form new elements, such as in particle accelerators. The field may also 'trap' charged particles in the same way particle accelerators control the path of a particle, containing the effect into a sphere.

-- Here we start the process of stellar formation --

  1. With the increase in particle collisions, low level ions (such as hydrogen, helium) would merge to form higher level particles (such as helium and carbon) up to the elements of iron and nickel. We are essentially turning the planet into a small young star, with the entrapment mentioned above taking the place of gravity.

  2. As the field increases still further, helium will begin to fuse into Lithium and yet higher more energy intensive reactions. Similar to super dense stars that can produce denser elements.

  3. As the field strength increases further, it is possible that paradoxially the planet will shrink as the radius and particle energy increase, eventually reducing the planet to the size of perhaps a pea.

  4. Eventually, the density of the pea would create a singularity, and form a very unusual artificially created black hole. This is of course using a tremendously strong field. Turn off the field and who knows what happens.

  • $\begingroup$ I added (I think right after you posted this answer) a pull force at 100 miles distance from the source. Given that parameter, where would this field fall on the scale in your answer? $\endgroup$
    – rsandler
    Commented Apr 16, 2018 at 16:00
  • $\begingroup$ The force delivered by a magnetic field on an object is not a very good measure of the strength of the magnetic field. How a magnetic field exerts force on a ferromagnetic object depends on many factors including the object's shape, composition, and velocity relative to the magnetic field. $\endgroup$
    – Plutoro
    Commented Apr 16, 2018 at 16:04
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    $\begingroup$ Most of those effect are only from an oscillating magnetic field, not a permanent one. Specifically, a magnetostatic field cannot increase the kinetic energy of a particle. $\endgroup$
    – LSerni
    Commented Apr 16, 2018 at 16:35
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    $\begingroup$ @flox if the field were oscillating, I wouldn't want to be within several light-seconds of the guy. Preferably in a soft-iron Gilbert cage. $\endgroup$
    – LSerni
    Commented Apr 16, 2018 at 16:45
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    $\begingroup$ This is mostly wrong for static magnetic fields (which is what the question asks for with "A permanent magnetic field") and even for oscillating fields much of it is dubious. But I have no idea how to critique it -- certainly this comment field is too small. ("I have truly marvelous comment, but this margin is too small for me to write it.") $\endgroup$
    – Mark Olson
    Commented Apr 16, 2018 at 16:51

200 miles is about 320 km. If we treat the magnet as a monopole, then the force falls off with the square of the distance. Then the potential energy is the reciprocal of the distance. So at half the distance (160 km), the energy for 1kg will be (10N*320 km) = 3.2 mJ. The specific heat of iron is .45 J/g degree, so we have 3.2*10^6 J over 10^1000 g, or 3.2*10^3 J/g. Thus, this would be enough to heat the iron by 7 thousand degrees.

The smaller the source is, the closer the object would be able to get, and then more energy it would have. At 1 km, it's about 1 gJ, or enough to heat the object by 2 million degrees. Hydrogen fuses at 100 million, so not quite enough (but at 10m, it would be). With no losses, the object would be travelling at around 20 km/s, or about 45 thousand mi/hr. However, if it has a cross-section of 10 cm^2, then that is .001 m^2, so every km, it will be displacing 1 m^3 of air, or about 1kg. So if it were able to bleed all of its energy with the air, the air would be heated to about 20k degrees.

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    $\begingroup$ “If we treat the magnet as a monopole”... you may be aware what that actually means, but you should say so in the answer because readers may not. $\endgroup$ Commented Apr 17, 2018 at 12:49

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