Magnetic Force Question

jonny5, Sun Nov 25 2012, 10:39PM

This is kinda a dumb question, but in a coilgun how does the coil exert force on the projectile?

I remember learning that the force between two magnetic poles goes something like:

F = (u x M1 x M2)/(4 x pi x d^2)

where u is the permeability, M1 and M2 are the magnitudes of the poles, and d is the distance between them.

I get that if I push current through the coil, I've made an electromagnet. How does the projectile become magnetized?
I know that iron is super permeable, and as long as the projectile is in the proximity of the flux lines created by the coil,
magnetic flux will prefer passing through the iron than the air. Is the projectile magnetized by the flux passing through it?

Also, if I'm cranking hundreds (if not thousands) of amps through the coil, I'm probably saturating the iron. If it's saturated,
that means that the material cannot hold any more flux, and its effective permeability approaches that of air. Does that
mean that, once the projectile is saturated, it can't be magnetized any further?

If that's so, then what I'm trying to figure out is:

1)Before the projectile saturates, is the force between the coil and projectile increasing rapidly as coil current increases,
since the stronger the coil's magnetic field, the stronger the projectile is magnetized, force is proportional to M1 x M2, etc?

2)After the projectile is saturated, more coil current can't magnetize the projectile any further, but increasing the coil current
does increase the coil's magnetic field...so does force between coil/projectile still increase, but only due to the coil's contribution?

Thanks for helping me sort this out.