Force exterted on a ferromagnetic object in a magnetic field

by grumpybuffalo
Tags: ferromagnetism, magnetic field, magnetism, solenoid
grumpybuffalo is offline
May9-12, 03:33 PM
P: 2

I'm building a coilgun and I'm confused. I understand that within a solenoid, the magnetic field is homogeneous. I've also read that a ferromagnetic projectile in a coilgun is sucked to the center of the coil. If the magnetic field is the same everywhere through the coil, what is special about the center of the coil? What is it that determines the force felt by the ferromagnetic projectile?

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Dickfore is offline
May9-12, 03:35 PM
P: 3,015
The magnetic field is not homogeneous in a solenoid with finite length and spacing between the turns.
grumpybuffalo is offline
May9-12, 03:46 PM
P: 2
Hm, so in an ideal solenoid (of infinite length and with no spacing between the coils), a ferromagnetic object would feel no force, correct?

So I probably want to keep my coil length pretty short in order to avoid wasting energy creating a relatively homogeneous (and therefore relatively worthless) magnetic field, huh...

That makes some sense. Thanks! I'm still wondering: what exactly determines the force that is exerted on a ferromagnetic object by a magnetic field? Is there a simple formula that answers that question?

Dickfore is offline
May9-12, 03:51 PM
P: 3,015

Force exterted on a ferromagnetic object in a magnetic field

The formula is the following:
\vec{F} = \int_{V}{d\mathbf{x}' \, \left(\vec{M}(\mathbf{x}') \cdot \nabla' \right) \vec{B}(\mathbf{x}')}
truesearch is offline
May9-12, 04:07 PM
P: 349
Can you quote a source for this equation please?
It would be good to have the terms defined for those needing help and guidance.
Dickfore is offline
May9-12, 06:15 PM
P: 3,015
Also, there may be a torque on a magnetic material given by:
\vec{\tau} = \int{d\mathbf{x}' \, \left( \vec{M}(\mathbf{x}') \times \vec{B}(\mathbf{x}') \right)}

As for the source, of these formulas, these are simply summations of the formulas given here:

but for a continuous distribution of magnetization [itex]\vec{M}(\mathbf{x}')[/itex].
berkeman is online now
May10-12, 12:32 PM
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We do not discuss dangerous activities here on the PF. Thread closed.

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