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Author Topic: What force do repelling, matched pairs of magnets apply?  (Read 6331 times)

labview1958

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What force do repelling, matched pairs of magnets apply?
« on: 20/03/2006 12:47:44 »
Let's say I have two pairs of identical magnets. The first pair is 1 Tesla. The second pair is 2 Tesla. If I have like poles facing each other as below, would the total force on the balance be the same?

1T2T

1T2T
Balance

0r

1T2T

2T1T
Balance
« Last Edit: 08/08/2009 22:21:26 by chris »

daveshorts

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• Physics, Experiments
Re: What force do repelling, matched pairs of magnets apply?
« Reply #1 on: 20/03/2006 16:45:08 »
If we assume that the balance is not at all magnetic, then the magnets are not interacting with anything other than each other by any means other than gravity.

This means that possibly apart from relativistic effects they should weigh the same. Because E=mc^2 then changes in potential energy will affect the mass of the system, so slight cahnges in the magnetic potential energy will alter the mass minutelym.

The first case is fighting more replusion, so has more potential energy so could be slightly heavier.

labview1958

• Sr. Member
• Posts: 104
Re: What force do repelling, matched pairs of magnets apply?
« Reply #2 on: 21/03/2006 01:01:54 »
My understanding that the force should be exactly the same. This is because of the diagonal contribution of the repulsion force apart from the main vertical contribution. Do you agree?

daveshorts

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Re: What force do repelling, matched pairs of magnets apply?
« Reply #3 on: 21/03/2006 16:31:28 »
Ok the top set of magnets are not on the balance?

It all depends on the force law, if you model a magnet as 2 magnetic monopoles tied together they will experience an inverse square force.

If this is the case the repulsion from 2 magnets close together is much larger than when they are further apart. So you will get (at least) 4 terms in the force.

1Tv1T  1Tv2T  2Tv1T and 2Tv2T

we can multiply out the 2's

1(TvT)  2(TvT) 2(TvT) and 4(TvT)

where TvT is the repulsion of 2 1T magnets.

or

1,2,2, and 4 (TvT)

If the magnets are set diagonally the repulsion will be less than when they are opposite one another - call this a factor d which will be less than 1.

so we can have the first case where like magnets are opposite one another

(1 + 2d +2d + 4) TvT = 5+4d

or th other case:

(1d+  2 + 2 + 4d )TvT = 5d+4

as d is less than one, putting them opposite one anther will produce more repulsion.

« Last Edit: 21/03/2006 16:33:22 by daveshorts »

labview1958

• Sr. Member
• Posts: 104
Re: What force do repelling, matched pairs of magnets apply?
« Reply #4 on: 22/03/2006 12:40:02 »
"If the magnets are set diagonally the repulsion will be less than when they are opposite one another - call this a factor d which will be less than 1."

This is really wonderful. Is there a way to estimate the value of d?

daveshorts

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Re: What force do repelling, matched pairs of magnets apply?
« Reply #5 on: 24/03/2006 11:07:26 »
Yes but it will depend on the exact geometry of the magnets. If we model the magnets as monopoles it will just be the ratio of the two distances squared. eg
d1^2
----
d2^2

where d1 is the distance between them when they are straight, and d2 is the distance when they are diagonal.

real magnets are not a narrow line, so it would involve lots of fun integrals....

labview1958

• Sr. Member
• Posts: 104
Re: What force do repelling, matched pairs of magnets apply?
« Reply #6 on: 08/08/2009 02:24:00 »
If two magnets are in repulsion, they move away. Could it be possible that the two magnets causes space to bulge outwards thus causing the two magnets to slide away?

lyner

• Guest
Re: What force do repelling, matched pairs of magnets apply?
« Reply #7 on: 08/08/2009 07:09:12 »
A note of caution here. The unit T relates to Magnetic flux density. This tells you the force per m on a wire carrying 1A. It doesn't describe the 'strength' of a permanent magnet (afaik) and nor does it even start to tell you the work done in bringing magnets together (potential energy). I should be wary of any results that rely on the assumptions made in this thread.,
The situation is much moe complex, I think.

labview1958

• Sr. Member
• Posts: 104
What force do repelling, matched pairs of magnets apply?
« Reply #8 on: 12/08/2009 15:56:47 »
I am trying to unite the magnetic force with the gravitational force. My pet theory is both forces bend space.

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What force do repelling, matched pairs of magnets apply?
« Reply #8 on: 12/08/2009 15:56:47 »