Electromagnet pull force calculation:

1. Mar 2, 2014

Dash-IQ

I'm trying to predict the possible pull force of some electromagnets from the following equation:

FPull= ( n x I ) 2 μ0$\frac{A}{(2 g ) ^ 2}$

F : Force.
n : Number of turns.
I : Current.
μ0 : permeability of air.
A: Area in m2
g : the gap that is separating the electromagnet and the object.

Maxwell's force formula.
Is this a good way to predict the force of attraction?

Online calculators use this formula as well.

2. Mar 2, 2014

gburkhard

The most straightforward way to predict a force is just to remember that the force is the gradient of the potential :)

3. Mar 5, 2014

4. Mar 5, 2014

gburkhard

I am not 100% sure about the formula you wrote yourself since I didn't actually do the problem out myself, but the methodology in the link you posted is correct. They do take the gradient of the potential to get the force, which is what you can do for any system, and the maxwell's force formula just applies this to this particular case: (you'll note the step they used, "F = dW/dg Equation FRP"). Here they used W to signify energy and g to signify the distance (g is for gap, but they should've used a different variable since g should be the fixed gap distance, not just a position variable). But anyway, yes, so if you take the energy of the total system and then take the position derivative of the energy and evaluate in the gap, you will get the right answer. However, that is for a simplified system where they assume the field is just constant in the gap. If you want the real answer, you're probably best actually going around the coil and integrating the force on differential current elements in the wires just using the regular lorentz force equation.

5. Mar 6, 2014

Dash-IQ

The formula I posted is from the link., not from me.
But, I was highly skeptical of the online calculator until I review that link, and it started to make sense.

I don't usually look at the potential gradient, due to my lack of understanding. But I will re-read the link again to review.

6. Feb 8, 2016

losbellos

I think something is wrong with it. If this would be true you could lift very big weight with few kilowatts and then you could make an impossible machine outputting megawatts. You could pull a piece of metal with great force would result very high torque and if your velocity is high enough you could make power output several ten times more.
So something must be wrong. Using FEMM outputs somewhat similar but still something must be wrong with it.
Must find a practical example to validate.

This is the problem with physics, mentally ill insanes like John Hagelin (religious extremist hindu terrorist, LG nobel prize winner LOL ) or Einstein publicate relativity (its been known by indians maybe thousands of years ago) and the result is usually catastrophic misunderstanding and inability to calculate the reality.

Very sad how the world is now.