Calculate electromagnetic force on iron object

[Heissenberg]

Hello,

I need to calculate/estimate the force between a solenoid electromagnet and a piece of iron on a certain distance from the magnets airgap, preferable in a simple way under ideal conditions rather than a very exact calculation.

I have found the following formula at a couple of places (for example at: http://www.ehow.com/how_5969962_calculate-force-electromagnet.html):

Force = ((N x I)^2 x k x A) / (2 x g^2)

N = the number of turns in the solenoid
I = the current, in amperes (A), running through the solenoid
A = the cross-sectional area, in meters-squared, of the solenoidal magnet
g = the distance, in meters, between the magnet and the piece of metal
k = 4 x pi x 10^-7 (a constant)

However I am a bit confused, as the formula do not contain any variable defining the metal object size or mass, surely the mass of the metal object as well as the material must have an impact on the force between the object and the magnet? Or what "force" is actually calculated in this formula?

Any ideas?

 

[jtbell]

I'm not familiar with that formula, but I suspect it assumes that the metal object is much larger than the solenoid. If you bring one end of a solenoid with a diameter of 1 cm close to an iron cube, would you expect the force to be significantly different for a 1-m cube versus a 10-m cube?

 

[Heissenberg]

Ah ok, that could make sense I guess. So any tips on how I could calculate the force between a solenoid electromagnet and a iron object that is smaller than the magnet? I don't mind asuming ideal conditions of the iron object and magnet itself it this would simplify things!