# Will the magnetic flux density of 2 magnets of different geometry but the same volume be the same

Discussion in 'Mathematics and Physics' started by neferkamichael, Sep 17, 2013.

1. ### neferkamichaelNew Member

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Dear Sirs,

This image is of 2 permanent magnets of the same volume, 54 cubic inches. On numerous websites that sell magnets, I can use their flux density calculators to calculate the magnitude of the magnetic flux density of the rectangular magnets but none have a calculator for an arc magnet, the magnitude of the flux density of the rectangular magnet at .001 inches from the rectangular magnet is Br = 4010 Gauss or .401 Tesla. My question is, given that the volume is the same for the 2 magnets will the magnitude of the magnetic flux density of the arc magnet be the same as that of the rectangular magnet. Also, where might I find equations to do magnetic flux density calculations of arc magnets. Any help would be greatly appreciated. Thanks. Sorry about the title screw up

2. ### PommieWell-Known MemberMost Helpful Member

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Think about drawing flux lines on the two magnets and keep in mind that the number of lines (total flux) will be the same for both magnets. Now look at the spacing of the lines at various points, the more lines the higher the flux density. It should now be obvious that the two are not the same. The flux density at any point is the total flux divided by the area it's passing through.

Edit, a simple explanation of flux and flux density, http://www.schoolphysics.co.uk/age16-19/Electricity and magnetism/Electromagnetism/text/Flux_and_flux_density/index.html.
Edit2, seems links don't work how they used to.

Mike.

3. ### MrAlWell-Known MemberMost Helpful Member

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Hi,

Orientation and polarity have a lot to do with it too. Two magnets exactly the same but oriented differently will show a different flux density in a given area a certain distance from the magnets. Usually the orientation is such that we want to get the maximum force in a certain place at a certain distance from the magnet.

Given the same amount of homogenus materials the magnet with the most area (being symmetric along one axis as shown) closest to the target will have the most influence on the target. So for two exactly identical equilateral triangular magnets one oriented with one point toward the target and the other with one side toward the target, the one with the side toward the target will exert the most force on the target simply because there is more material closer to the target.

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5. ### first4magnetsNew Member

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Hi neferkamichael,

It is a very valid question. As Pommie has pointed out, the total flux density is a measurement of the number of field lines in any given cm2 of pole area. When plotting lines of flux from a magnet of any shape, the closer the lines are together, the stronger the field will be.

The flux density depends on the grade of magnet and the relationship between the diameter and the magnetic length (L/d ratio).

Magnets with larger pole areas relative to smaller lengths, have the lowest flux densities.

Magnets with smaller pole areas relative to longer magnetic lengths, have much higher flux densities.

It must also be consider that the flux density must be multiplied by the pole area to give the total magnetism, as flux density is only half of the equation when considering overall magnetic force.

We have put together a spreadsheet which may help with calculations relating to cylindrical and rectangular magnets. Unfortunately the forum is not letting me attach it. If anyone knows how to attach it then I will happily post it.

6. ### neferkamichaelNew Member

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Pommie thank you for responding and I know what magnetic flux and magnetic flux density is. I want to know how to calculate the Br magnitude of the arc magnet, the number that you plug into Faraday's equation to calculate the EMF produced. If anybody can direct me to a website or individual that can provide me with equations and information I would be eternally grateful. Thanks

7. ### shortbus=Well-Known Member

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Maybe this will help? http://www.magnetsales.com/design/DesignG.htm

8. ### neferkamichaelNew Member

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Thanks again Pommie, I used magnetsales.com flux density calculator to get the Br magnitude of the rectangular magnet and have Emailed them the same question I'm asking here and they never responded.