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I know that earth has magnetic fields/forces, but why we don't feel them? So if I hold a magnet and earth's magnetic field is positive and my magnet is from the positive side, then why the magnet does still fall down? Shouldn't be there a pullsive force since they will be of the same polarity?

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  • $\begingroup$ There are organisms who can indeed sense the magnetic field. What do you mean by "earth's magnetic field is positive"? Why shouldn't the magnet fall down even if there is some magnetic force between it and the earth? $\endgroup$
    – ACuriousMind
    Jul 26, 2014 at 23:03
  • $\begingroup$ @ACuriousMind the earth has a polarity that is the same as the magnet polarity, so the magnet shouldn't fall down, right? $\endgroup$
    – Jack Twain
    Jul 26, 2014 at 23:11
  • $\begingroup$ @ACuriousMind try to put two magnets together with the same polarity, do they stick or go way? $\endgroup$
    – Jack Twain
    Jul 26, 2014 at 23:12

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To good approximation, a magnetic dipole in a uniform field feels a torque, but not a force. The length scale for variations in the field produced by the dynamo in the earth's core is comparable to the size of the earth's core: many hundreds of miles. For a magnetic dipole the size of an animal (any animal) the earth's natural field is uniform, and you feel only a torque, not a net force.

This is different from a pair of coin-shaped or bar-shaped magnets, where it's pretty easy to move the magnets from "touching" to "separated by many times their own size." In that case the field is quite non-uniform, and you get an attractive or repulsive force.

I read several years ago about a gadget for magnetic field sensing as a "sixth sense." The gadget was a sequence of vibrating motors that you wore around your waist like a belt. It had some sensor and logic so that the northmost motor vibrated, no matter which way the wearer was facing. Testers reported that after only a few hours they had stopped noticing the vibration but had a much stronger sense of place and direction.

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    $\begingroup$ a compass is simpler than the gadget you describe. true you have to look at it $\endgroup$
    – anna v
    Jul 27, 2014 at 4:18
  • $\begingroup$ Good heavens, all that babbling about torque and I forgot to use the word "compass"! $\endgroup$
    – rob
    Jul 27, 2014 at 10:20
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Here is a compass, a small dipole magnet in the shape of a needle: as the person holding it turns around it moves pointing to the geographic north in the location,

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then the directions are all defined on the face of the compass.

So that is the way a human can see/sense simply the magnetic field of the earth. Semantics on the compass is a bit complicated by tradition, as the dipole pole that points to the north is a pole similar to the south pole.

As Rob says the earth is a huge dipole, and the compass is a small one. There is a direction that the forces arising from the magnetic fields are minimized; there is a balance of forces when the axis of the dipoles are aligned , south next to north ( in a consistent definition of "pole").

The field of the earth is very weak and the field lines go along the south north direction, not up and down, so "fall down" has no meaning except at the north magnetic pole itself, and there the attractive force is too weak to cause a "fall" similar to the "fall" of getting the north and south poles of two strong bar magnets close.

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  • $\begingroup$ marry me anna :D $\endgroup$
    – Jack Twain
    Jul 27, 2014 at 10:30

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