Uniform Rectangular Magnetic Field

[jonpui747]

Hi there,

I'm trying to create a sheet that produces a uniform magnetic field,using either rectangular or cylindrical neodymium magnets. I can't tell you much abut their strength, but they would be the common type purchased from a science/hobby shop. Dimensions for cylinder are 10mm dia x 2mm, and the rectangular would be 0.5mm x 0.4mm x 0.2mm. I'm going to assume that many smaller magnets would be better for creating a uniform field that fewer larger ones, but I'm unsure about the best way to arrange them/which would be better suited.

The sheet will be about the same size as a sheet of paper (A4), and should be able to suspend a steel mass of about 200g. How many magnets will i need, which type is better,and what is the best arrangement to use?

Thanks,

Jon

 

[Enthalpy]

200g isn't much.

Putting all N poles at one face isn't the best to do. It's more efficient to alternate stripes of N and S poles. Stronger attraction, and drops off faster so it's safer.

Magnet sellers use to offer plugs meant to attract steel. You might buy such plugs and put several of them regularly spaced. They already bring both N and S to the useful side.

 

[mfb]

If you alternate the polarity, the field is not uniform.

@jonpui747: Do you have some specific application in mind?
For a uniform field, it would be ideal to fill the whole area with rectangular magnets of the same polarity, but I am quite sure that this would not give the intended result.

 

[jonpui747]

Basically I am trying to make a magnetic sheet, so that I would be able to move steel objects in any location on it, and have them remain there without falling off of it. I would like to make it as uniform as possible, so that the area between the magnets isn't that noticeable.

Also, if i make the sheet itself out of metal, would that reduce the magnets' pull on the objects on the other side of it?

 

[Q-reeus]

Basically I am trying to make a magnetic sheet, so that I would be able to move steel objects in any location on it, and have them remain there without falling off of it. I would like to make it as uniform as possible, so that the area between the magnets isn't that noticeable.

In that case best thing is to alternate polarity between each neighboring magnet. A uniform sheet with same polarity - especially if packed tight with rectangular shaped magnets, would give very poor results as the 'magnetic sheet charge' on one side will nearly cancel that of the other. In fact for a very large sheet, you would achieve essentially no external field at all. Alternating polarity overcomes that problem. If possible, have the magnet's area quite a bit smaller than pieces intended to be hung - that will provide greater uniformity of attachment force.

Also, if i make the sheet itself out of metal, would that reduce the magnets' pull on the objects on the other side of it?

It will help things - providing a magnetic return path between poles attached to such sheet in effect neutralizes these poles, and that has the effect of making each magnet seem to have much greater depth than it actually has. In other words, improves the range and strength of the magnets. The steel sheet thickness would need to be a substantial fraction of magnet's depth though for that to be true.
[Above assumes that said sheet is for backing - not acting between magnets and objects to be hung!]