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Old 04-16-2009, 11:03 PM   #1
jackson_d
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Default Magnetism and DIY Stirplate

Hey there- I'm still working on getting Anthony's stirplate to work properly. I used the 12V power supply and it keeps throwing the stirbar when i turn the knob up about a third. I ordered a bunch more magnets, still having trouble but getting closer. I wonder, if I stack a bunch of magnets on top of each other, does that create a stronger magnetic field than if i put a bunch of them side by side? Height vs width I guess?

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Old 04-16-2009, 11:17 PM   #2
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To the best of my understanding stacking magnets will not produce a greater attractive force. The magnetic circuit will be closed once you place a negative end with a positive (or vica versa) and the field will be mostly closed (I say mostly because we are talking physics haha). Placing the magnets horizontally allows a wider field to be available for the stirbar to grab on to.

What kind of magnets are you using? Rare Earth? If not, get some for sure.

When all else fails: try it. If you can stack the magnets that means there is a lot of space between the top of the plate and your fan/magnet. Try moving the fan closer to the plate or stack them so that it is closer. Anyways, best of luck!

-RC

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Old 04-16-2009, 11:24 PM   #3
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How long is your stir bar? Try a longer one.

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Old 04-17-2009, 12:51 AM   #4
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Quote:
Originally Posted by jackson_d View Post
I wonder, if I stack a bunch of magnets on top of each other, does that create a stronger magnetic field than if i put a bunch of them side by side? Height vs width I guess?
From KJmagnetics.com:

Does stacking magnets together make them stronger?

Yes, two or more magnets stacked together will behave exactly like a single magnet of the combined size. For example, if you stacked two of our D82 disc magnets to form a 1/2" x 1/4" combined size, the two magnets would have the same strength and behave identically to our D84 discs, which are 1/2" diameter x 1/4" thick.

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Old 04-17-2009, 12:59 AM   #5
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the strength of a magnet varies inversely cubed with distance -> move battery a little closer and it is much stronger

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Old 04-17-2009, 01:01 AM   #6
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For my stir plate, if I use a bar that is more than an 1 1/2 then it gets thrown.

I think its the length that you are having issue with.

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Old 04-17-2009, 03:18 AM   #7
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Quote:
To the best of my understanding stacking magnets will not produce a greater attractive force.
You gain very little.
Stacking two magnets with 10 lb pull force will not give a pull force of 20 lb .
To get 20 lb force only putting the magnets side by side will do the trick.

Quote:
Placing the magnets horizontally allows a wider field to be available for the stirbar to grab on to.
Will still not increase the pull force.

Quote:
beerocd wrote:
Does stacking magnets together make them stronger?

Yes, two or more magnets stacked together will behave exactly like a single magnet of the combined size.
Based on your post, home brewers will assume staking magnets will be the way to go without reading the data sheet.
Looking closer you will see a poor gain of less than 1 lb pound.
I am sure you had all the best intention.

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Old 04-17-2009, 04:59 AM   #8
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IIRC, stacking two magnets will, in fact, double their pull force. IOW, they will act as a single magnet with twice the power.

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Old 04-17-2009, 05:15 AM   #9
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Quote:
IIRC, stacking two magnets will, in fact, double their pull force. IOW, they will act as a single magnet with twice the power.
Based on your answer about stacking magnets what result will we get by putting the magnets side by side?

If we look at the data sheet for D82 = 16.80 lb, as per KJmagnetics.com:
stacking two D82 will act like one D84 which has has a pull force of 17.22 lb, not double.


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Old 04-17-2009, 05:31 AM   #10
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Not necessarily as you are are comparing two completely different magnets. I would need more detailed specifications for eadh before I would leap to a conclusion as you seem to have done. Placing magnets side by side will give you the same net force, just spread out instead of concentrated. Do you think that two magnets attracting one another directly have twice the pull force vs. a single magnet attracted to an ordinary iron/steel surface?

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