# The Naked Scientists Forum

### Author Topic: Why do magnets attract each other in one orientation, but not another?  (Read 1221 times)

#### mrsmith2211

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##### Why do magnets attract each other in one orientation, but not another?
« on: 02/01/2016 09:37:38 »
« Last Edit: 02/01/2016 10:34:40 by chris »

#### chris

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##### Re: Why do magnets attract each other in one orientation, but not another?
« Reply #1 on: 02/01/2016 10:40:19 »
The intensity of a magnetic field falls according to an inverse square law: in other words, the force of attraction is proportional to 1/d^2 (one over distance squared); that is, if you double the distance between them, the force will be 1/(2 squared), or one quarter of what it was at the previous separation.

I speculate that the magnets are mounted on a non-susceptible, non-magnetic back-plate, which are effectively acting as spacers, dramatically reducing the field intensity.

[EDIT ADDED: See the link from Evan two posts below, which explains this perfectly.]
« Last Edit: 03/01/2016 10:13:25 by chris »

#### evan_au

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##### Re: Why do magnets attract each other in one orientation, but not another?
« Reply #2 on: 02/01/2016 20:54:27 »
Oops! The video didn't appear until after I wrote a response... Thinking about it, it's possible that the magnets form a Hallbach array, but I'm not clear how this would perform a useful function in a hard disk drive (apart from an emergency data eraser).

It's not clear what type of magnet you are considering, but...
• For a bar magnet, one end is the "South" pole, and the other is the "North" pole (which will point to Earth's North magnetic pole, if you let it turn freely).
• The rule for magnets is that "Opposite poles attract, Identical poles repel". So the North end of one magnet will be attracted to the South end of another magnet. But the North end of one magnet will be repelled from the North end of another magnet*.
• A horseshoe magnet is like a bar magnet bent around in a semi-circle.
• If you are thinking of a pair of fridge magnets, these are magnetized in a series of "stripes" across the back, presenting a stronger magnetic field on the "fridge" side, and very little magnetic field on the advertising side. So two fridge magnets won't attract each other much when the advertising sides are adjacent, but they will grip when the two fridge sides are adjacent.
If you try to slide the fridge sides of fridge magnets past each other, in one direction they will slide smoothly along the stripes, but in the other direction they will jump over the opposing stripes.

*At this point you may wonder why the North pole of your bar magnet is attracted to the North pole of the Earth. This is because the North pole of the Earth is actually the "South" pole of a big bar magnet inside the Earth(!)
« Last Edit: 02/01/2016 21:00:15 by evan_au »

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#### evan_au

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##### Re: Why do magnets attract each other in one orientation, but not another?
« Reply #3 on: 02/01/2016 21:20:19 »
This web page explains that these are the head actuator magnets for a hard disk drive.
http://www.reuk.co.uk/Hard-Disk-Drive-Magnets-For-Wind-Turbines.htm

In the disk drive, these magnets form a cavity which has a north pole at one end, and a south at the other. A computer-controlled electromagnet moves backwards and forwards within the cavity to move the read/write head across the surface of the disk.

It is very important that magnetic fields do not spread beyond the actuator, or it could demagnetize the hard disk, losing the stored data near the rim of the disk. To prevent this, the magnets would be encased in a soft-iron backing plate, which acts as a magnetic shield; this "shorts out" the external magnetic field on the backing plate side, while focusing an intense magnetic field on the interior of the cavity.

#### alysdexia

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##### Re: Why do magnets attract each other in one orientation, but not another?
« Reply #4 on: 17/01/2016 14:09:24 »
inverse cube

The soft iron conducts half of a magnètic circuit with both poles.  I don't think it focuses like a Halbach array or other homopolar arrangement: https://www.quora.com/How-much-power-and-wire-do-I-need-to-create-a-5-tesla-electromagnet/answer/Autymn-Castleton.
« Last Edit: 17/01/2016 14:15:56 by alysdexia »

#### The Naked Scientists Forum

##### Re: Why do magnets attract each other in one orientation, but not another?
« Reply #4 on: 17/01/2016 14:09:24 »