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Q & A: Linear Accelerator

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Most recent answer: 10/22/2007
Q:
I am a student and I am constructing a linear magnetic accelerator (gauss rifle). I need to know the electrostatic charge of steel to be able to complete part of it. I was wondering if you had any books, or ideas as to it’s charge. Steel is an alloy, and there are a lot of different types that could be made, but I was wondering if there was a certain type of general steel (not stainless steel) that is mass produced (because I am sure that I have that type). Help would be much appreciated. Thanks,
- John B (age 17)
Urban School of SF
A:
John -

We're puzzled about why you're asking about electrostatic charge.
Like other materials, steel is electrically neutral. Isn't it driven
forward by magnetic forces in this gun? We don't know the design
used, but if some of the magnetic force is on currents flowing
through the steel, then the electrical property you're interested in
is the conductivity- how easy it is for that current to flow. A
couple of magnetic properties are also probably relevant, depending
on the design: the magnetic susceptibility (how much a small field
magnetizes the steel) and the saturation magnetization (how magnetic
it gets in a large field). All these properties vary quite a bit
between different steels, as you guessed- stainless, for example, has
particularly weak magnetism and low conductivity. We don't know if some
type of steel is so much more common than others that you could bet
on it being what you have.

We forwarded this to Kevin, a member of our campus
href="http://www.physics.uiuc.edu/groups/physics_society/">Physics
Society
who has experience with the project that you're referring
to. Here's what he had to say:

---------------------
What does he mean by the 'electrostatic charge of steel'? By nature,
steel's neutral, of course. If he's looking for the maximum charge
that steel can hold before it breaks down or something, there's no
need to worry about that... it'll spark before then, I'd be more
concerned about him getting shocked. If he's thinking about electron
holes in the material, that's for semi-conductors. Steel's got a
conduction band instead, and just holds charge. If he's having
problems with steel melting or something (on the off chance that
that's what he's asking) then he should try working with greater
surface area contacts or smaller currents. On the other hand, if
he's having problems with steel melting, then he shouldn't be doing
it in the first place....

If he isn't working with currents (like a rail gun) but a series of
magnets and using steel projectiles, then he's probably thinking the
dipole moment of the projectile in general when placed in a magnetic
field. That depends mostly on the magnetic field, but if he wants a
textbook, then the book by Purcell has a couple of charts, but I
can't remember exactly if it has steel.... Otherwise, if there are
some material sciences books around, they could probably be found in
those. Hope this helps.

PS, I think he has the wrong terminology... I really don't think he
wants the 'electrostatic charge' for steel... that doesn't make any
sense... you can give it some charge, and then it'll be different...
and in the presence of a B-field, the overall charge doesn't change,
it just polarizes domains....

Ask him what he's doing. If he's working with high currents (like
the rail gun) tell him to take precautions and tell him to stay away
from it... If it's just a series of magnets and steel balls, tell
him that any potential calculations he would do with the information
he's looking for is probably a very bad guideline to base the actual
project on. Encourage testing and adding only one set of magnets at
a time, etc. It's not quite as deadly as current, but depending on
the magnets he's using and how many he uses, he can potentially hurt
someone.
---------------------

All of us here at the Physics Van agree with Kevin that this project
is a very bad idea. There are a lot of things that can go wrong and
it could be very dangerous, even potentially lethal to try this
without a lot of experience and resources.

-Tamara

(published on 10/22/2007)

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