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Jeff Marx wrote:
a oxygen tank was accidentally left inside the room and it flew "at 20 - 30 feet/sec" across the room into the machine.
I have witnessed such phenomena with my own eyes.
Example: Not a medical MRI machine, but a research magnet
(used for MRI of small samples, among many other uses),
ten tesla, about 50cm long by 20cm diameter. It would
pull small wrenches out of your hand from several feet
away.
To prevent tragedies, we built a _corral_ out of welded
aluminum, plus plywood and plexiglas sides, socketed into
the concrete floor. We wrote all sorts of warnings on it.
We even invented an "international high-field warning" icon
which was kinda cute IMHO.
One fine day a certain grad student (not the sharpest tool in
the shed) was working on the neighboring apparatus. He needed
to bring a full-size gas cylinder over to his apparatus. He
decided that the corral was in his way, so he unsocketed it
and set it aside. When he brought up the gas cylinder, it
flew through the air -- up and sideways several feet -- and
smashed into our apparatus.
Amazingly enough, our magnet survived. It didn't even quench.
When I showed up the next morning, the gas cylinder was still
there, stuck to our apparatus at an angle. I mean reeeeally
stuck; it wouldn't even wiggle if you pushed on it.
If the aforementioned grad student had been between the magnet
and the cylinder, it would have chopped him in half.
don't see an easy way to estimate what must be the necessary force.
Someone interested in taking a crack at this?
See Feynman volume II chapter 10, especially figure 10-9.
The caption says "the force on a dielectric sheet in a
parallel-plate capacitor can be computed by applying the
principle of energy conservation" ... aka the principle
of virtual work.
The same PVW reasoning applies to a chunk of ferromagnetic
material in a magnetic field. See also chapter 27,
"Field Energy and Field Momentum". The field energy
goes like B^2, and is present everywhere except where
the magnetic material is. If you have a dipole field,
the field falls off like 1/r^3, so the energy density
falls off like 1/r^6, and the force falls off like 1/r^7.
More precisely, the force falls off like (L/r)^7, where L is
some characteristic length scale, in this case the length
of the magnet. So you can see that having a high-field
magnet isn't the whole story; it is also very significant
to have a large-sized magnet.
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