I understand that, in order to magnetise something, it has to be inside a magnetic field, but how come that, after millions of years of ferrous metals being inside the earth’s magnetic field, they haven’t basically become permanent magnets?
I understand that, in order to magnetise something, it has to be inside a magnetic field, but how come that, after millions of years of ferrous metals being inside the earth’s magnetic field, they haven’t basically become permanent magnets?
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Some of them are – but the Earth’s magnetic field is really weak and it generally takes a pretty strong field to induce magnetism.
Remember that a non-magnetized ferrous material still has small magnetic points, they’re just all scattered in random directions. Well if that material is a solid, then it takes energy to move those particles around. Usually quite a bit more than the Earth’s field has to offer.
In unmagnetized iron it’s full of individual magnetic areas pointing in random directions. You need a certain strength of magnetic field to realign these randomly pointing magnetic areas to face the same way and make the iron become magnetic, and the earth’s magnetic field isn’t strong enough to do this.
However while the iron is molten these magnetic areas are moving around, and while they cool they can be influenced by the earth’s magnetic field. This has allowed us to track how the earth’s magnetic field has changed by looking at these natural iron formations.
They are. It’s not very strong, in and of itself, especially for small pieces. But large ships actually have to account for the orientation they were facing when they were first built when they calibrate their compasses. This is called ship’s magnetism.
If you take a piece of steel and hit it with a hammer it will magnetize according to the earth gravity field. This is a known problem for human navigation. If the “piece of steel” is a ship and “hitting it with a hammer” is its maintenance, the result is that every ship has its own magnetic field that interferes with its compass. This is called magnetic deviation and must be compensated for.
There are magnets that occur naturally, called lodestones, but they are pretty rare. For them to be formed, you need a specific combination of materials that is easy to magnetize, and a strong magnetic field. The Earth’s magnetic field is just too weak for this. It is theorized that lodestones are formed by the very strong magnetic fields of lightning strikes, which also explains why lodestones are found on the surface and not deep down.
The earths magnetic field is very weak.
Objects generally have a weak ability to hold on to being magnetic (or resist being made magnetic -this lack of resistance allows for temporary magnets to be made more easily)
3.Being inside a magnetic field is necessary, but not sufficient, to turn something ferrous into a magnet. Sometimes, that ‘something else’ does happen, though, producing natural permanent magnets, such as lodestone.
TLDR: The field is just far too weak.
Imagine a world where all houses are painted either red or blue. People generally don’t care about the color of their house and have no preference red or blue, but they like to fit in with their neighbors, so if all your neighbors have a blue house and you are the only one with a red house you will probably repaint it. Now the consequence is that there will be neighborhoods where most of the houses have the same color, except maybe a few because some people don’t mind what their neighbors think and also because repainting your house is expensive, so you only do it if you have a reason to.
Now it is possible that two neighborhoods have different colors. Imagine living at the border of two such neighborhoods, you wouldn’t know how to paint you house, so you might as well just keep the color red or blue, or maybe change it when enough neighbors do so as well.
Lats translate this to ferromagnetic materials now: the colored houses are so called “spins” of electrons at distinct atoms in a lattice of the material at hand. The spins can loosely speaking point up or down, but in ferromagnetic materials they tend to align as their neighbors. If they all point in the same direction, this generates what we know as magnetic field. But as with the houses there are different neighborhoods that border each other and because they exist roughly in equal amounts, globally the material ist not magnetized.
Back to the houses. How can we make more houses the same color now? Easy, just make red paint half the price as blue paint and now when it is time to repaint your hose, people will tend to use red paint. Starting with the undecided people at the edge of neighborhoods, but there can also emerge red “islands” in blue neighborhoods. But it might depend a lot on the actual price difference! If the price is only slightly different most people won’t bother to repaint their house.
In terms of magnets, this is the same as applying a field and strong field would mean “larger price difference in color”. So to answer your question the earth’s magnetic field is just not strong enough. In our picture it would correspond to discount one color of paint only a fraction of a cent probably.
There is one more parameter, which is “how much do people care to match their neighbors house color and how strongly do they care to safe money on paint?”. If they care a lot or really want to save money, they might repaint their house at a lower price difference. This is roughly equivalent to temperature in the magnet. The lower the temperature the easier the magnetization. This goes up to a point, where the colors of houses are just random or for the ferromagnetic material, that the spins are not ordered. In physics we call this a phase transition (and in a lot of senses this is similar to a phase transition of say solid to liquid). But this is also roughly the latest where the analogy breaks. The temperature there the phase transition takes place called the Curie temperature. Just for reference, for iron this temperature is 770°C (1418°F).