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Registered Member #4659
Joined: Sun Apr 29 2012, 06:14PM
Location:
Posts: 158
so, pieces of iron get pulled to the center of a powered coil because they want to help the coil complete it's "magnetic circuit", right?
also, magnetic field lines flow well through iron (this is related to the whole sucking-into-the-coil thing), allowing the field lines to be "steered" with iron pathways. This is why people put external iron on coilguns, no?
so based on that, i had this idea for a wierd widget. Basically, there's a powered DC coil wrapped around this central iron element, producing field lines that flow through and out the iron element, around the edge of the coil+iron device, and back into the other pole.
Then there's this iron cylinder, which fits snugly around the coil+iron device. When you put the cylinder over the device, the coil's magnetic circuit is completed via the iron element and the cylinder.
So my question is - if i put the coil+iron device about halfway into the cylinder and power it on, would it be drawn all the way into the cylinder, because doing so completes the magnetic circuit?
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
Will the cylinder and device be drawn together? Yes, it's just an inside-out solenoid.
Yanom wrote ...
so, pieces of iron get pulled to the center of a powered coil because they want to help the coil complete it's "magnetic circuit", right?
That's the kindergarten way of putting it. The system will tend to move to reduce its total energy. Fields in air store more energy than fields in iron, so the system will tend to move to minimise the volume of air in the path of the field. While both descriptions of the situation lead to the same general conclusion, the energy one allows you to do exact calculations of the force.
Registered Member #1667
Joined: Sat Aug 30 2008, 09:57PM
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Posts: 374
Dr. Slacks explanation is based on energy considerations (which is an integral representation for the problem and equivalent to a differential description when a potential exists). I would like to put more emphasis on the force because forces can be described even without the existence of potential energy.
That said, consider a magnetized needle on a piece of styrofoam on water. The needle will orient itself along the magnetic field of the earth but it will not drift in any direction because the magnetic field of the earth is very homogenous on the length scale of one meter.
What makes magnets attract one another is the change of the field density over distance (the gradient). For more details see
In a cylindrical coil, the magnetic flux density has a maximum in the center. At the ends, the flux density is reduced by about 50%, so the gradient of the flux density is nonzero almost everywhere inside the coil and a bar magnet with the right orientation or a piece of iron will get drawn to the center of the coil.
Now you can replace the simple bar magnet (as chosen for a model dipole) by a paramagnetic or ferromagnetic metal and you achieve almost the same effect, you just get an additional spatial dependence. When exposing a paramagnetic / ferromagnetic metal to an external magnetic field, the "elementary magnets" inside tend to align parallel to the external field, causing a macroscopic dipole moment.
Registered Member #2529
Joined: Thu Dec 10 2009, 02:43AM
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Posts: 600
There's many equivalent ways to look at this.
One way that's not been mentioned is if you calculate or estimate the Ampere currents around the surface of the iron; you can then estimate the forces between the currents in the coil and those currents.
In some ways this is easier to do; Ampere currents are often sheet-like, rather than volumetric.
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question about magnetic circuits and forces on iron
