# The Naked Scientists Forum

### Author Topic: Can I make a magnetic stirrer with magnets from hard discs?  (Read 5610 times)

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##### Can I make a magnetic stirrer with magnets from hard discs?
« on: 19/12/2012 15:43:24 »
The question is about the disk with the magnets, i am using magnets from Hard discks which are rare earth NdFeB like those , and i have 6 of them so i can make them quite strong but which arrangement will be better newbielink:http://postimage.org/image/hge456hb1/ [nonactive]
« Last Edit: 22/05/2013 23:53:23 by chris »

#### CliffordK

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##### Re: Magnetic stirrer
« Reply #1 on: 20/12/2012 22:51:12 »
I assume you are attaching this to some kind of a turntable, with a magnetic stirring rod in a glass container above.

I would think the strongest orientation would be to align the North and South of two magnets.

N-N
S-S

or, lined up in series
N-S N-S

Followed by a single magnet.
N
S

The weakest orientation would likely be having N and South in opposite directions
N-S
S-N

While true North/South may not make much difference, you can quickly determine what attracts and repels.

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##### Re: Magnetic stirrer
« Reply #2 on: 31/12/2012 12:49:32 »
Why i cant make that stir bar to spin i tried all sorts of combinations with those magnets but it keeps jumping, moving and vibrating in the glass, everything else but not spinning  >:(

#### RD

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##### Re: Magnetic stirrer
« Reply #3 on: 31/12/2012 19:11:54 »
Why i cant make that stir bar to spin i tried all sorts of combinations with those magnets but it keeps jumping, moving and vibrating in the glass, everything else but not spinning  >:(

With multiple magnets you've created something similar to a magnetic pendulum which has a chaotic motion ...
t=3m13s

one magnet on the centre of the rotor would be more stable.
« Last Edit: 31/12/2012 19:14:52 by RD »

#### CliffordK

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##### Re: Magnetic stirrer
« Reply #4 on: 31/12/2012 21:19:49 »
Are you using a variable speed motor to run your magnetic stirrer?  DC?

If you are using multiple magnets, make sure all the poles are oriented in the same direction, as above.

My quick experiments with a couple of magnets separated by a CD case indicated that it seemed to be most stable if the N/S was oriented flat, parallel to the surface where the stirrer is located.

How are the HDD magnets magnetized?  If the N/S is oriented through the thin face, they may be difficult to orient them parallel to your stirring apparatus.  Perhaps you could merely stack all the magnets, then orient it parallel to the apparatus.

#### CliffordK

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##### Re: Magnetic stirrer
« Reply #5 on: 31/12/2012 23:13:56 »
Ok, here is a description of the magnet polarity for your hard drive magnets.

Where white solid is the top, and yellow dashed is the bottom.
As I had feared, the polarization is through the short distance from flat to flat.

Worse than that, they are essentially two magnets glued together in the middle.

My experiments with magnets was that they tended to follow each other when oriented flat rather than than vertically with the poles pointing the separation plate.

You may, in fact, be better off cutting your magnets in half so that you will get homogeneous N & S poles.

Then stack all 12 magnets up, and try running them parallel to your plate.

Also, see notes on the internet about separating the magnets from the backing plates.

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##### Re: Magnetic stirrer
« Reply #6 on: 01/01/2013 00:25:55 »
Yes they are like on the picture, pretty mixed poles and if i cut them i think they will rearrange the poles, so maybe it will be better just to by a proper ones
2 of these magnets with vertical oriented poles will be best to use for the job right ?

#### evan_au

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##### Re: Magnetic stirrer
« Reply #7 on: 01/01/2013 00:40:56 »
CliffordK, how did you get that ghostly blue image of the magnetic field?

#### CliffordK

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##### Re: Magnetic stirrer
« Reply #8 on: 01/01/2013 01:20:44 »
CliffordK, how did you get that ghostly blue image of the magnetic field?
By following the link in the post!!!!
Called "Magnetic field viewing film".

I would try it with ONE bar magnet, magnetized through the ends.
Somewhat like these.
http://www.ebay.com/itm/Neodymium-Magnets-N50-1-4x1-4x1-NdFeB-Rare-Earth-Magnet-End-Polarity-Lot-10-/310549143084

Ahh, found those as singles.
http://www.magnet4sale.com/Neodymium-Magnets-N50-0.25-x0.25-X1-NdFeB-Rare-Earth-Magnets.html

Or
This one.
http://www.magnet4less.com/product_info.php?cPath=1_5&products_id=494

Although, the last one one seems pretty big.

The question is whether one would end up with the stir bar spinning with the magnet, or just stuck to the end.  Thus, I might choose a shorter magnet, perhaps an inch or so (1st and 2nd links).

Obviously you will need to connect it to some kind of a slow, or variable speed motor.
« Last Edit: 01/01/2013 01:27:05 by CliffordK »

#### shivshaktibrass

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##### Re
« Reply #9 on: 22/05/2013 11:59:07 »
A magnetic stirrer or magnetic mixer is a laboratory device that employs a rotating magnetic field to cause a stir bar immersed in a liquid to spin very quickly, thus stirring it. The rotating field may be created either by a rotating magnet or a set of stationary electromagnets, placed beneath the vessel with the liquid.

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##### Re
« Reply #9 on: 22/05/2013 11:59:07 »