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Electrical Engineering/Correct value for the permeability of the metal core in an electromagnet

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Question
Hello Cleggsan,

I am sorry to bother you with something that is probably very basic, but I am having trouble finding clear information on something, with sources online giving what, in my ignorance, seems like conflicting information.
I am trying to calculate the strength of an electromagnet and I am unsure of how to correctly calculate the permeability of the core material. So far I have tried multiplying the permeability value of the material by the permeability of free space, for which I have used the value 0.0000012566.
For example, iron permeability:200 multiplied by 0.0000012566 = 0.25132
The problem is that the permeability given for iron is listed as vastly different values on different websites - some give the value of 200, while others (http://en.wikipedia.org/wiki/Permeability_(electromagnetism)#Values_for_some_common_materials)
give the value as 200,000.
Can you tell me what I am doing wrong here? Is the value of 200 for Relative permeability, and if so, should I still be multiplying this by the permeability of free space?
Am I calculating the permeability of the core correctly, and if so, how can I determine the correct value to use?

I have a feeling that this may be basic knowledge, but it is knowledge I do not have and I am now confused as to how to go about making the correct calculations.

Itsumo arigato,
Eddie

Answer
No, you are not confused. The permeability of materials is not a very exact science as it is dependent on the properties of material under question.  Great variety exists between soft iron and steel or other forms of metals.  The correct value is either measured or obtained from the manufacturer or supplier of the material.

If you google "permeability of iron and air" you will get a series of hits that are very instructive about permeability in general and specifically about materials.

By the way "200 multiplied by 0.0000012566 = 0.25132" is not correct mathematically, anyway.

I think, based on my personal experience in laboratory measurements from my college days you will find large variations between measured and calculated results caused by wild variations in materials that have unknown or questionable permeability values.

If you go to this URL and down towards the bottom is a table showing popular materials:

http://en.wikipedia.org/wiki/Permeability_%28electromagnetism%29

And, finally, the hysteresis and saturation characteristics of the material in question causes great changes in permeability values.

Carry On.

Masugo

Electrical Engineering

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