Date: Mon, 13 Dec 2010 08:16:56

Author: Urs Lauterburg

Subject: Re: Simple Rail Gun

Post:

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Paul,

I assume that what really counts is the direction it will start
rolling without any implied external mechanical force. If it does not
start rolling then the external B-field is simply not strong enough.
Once you have the rod rolling then I would guess that some rather
subtile and nonlinear eddy currents could also have a significant
impact.

It would be interesting to know if Dave Kardellis's arrangement would
move the other way when he changes the current's polarity. If his rod
does indeed move in the other direction then it should be the
interaction of the current's B-field with the one of the earth.

Rolling rod motors are kind of mysterious. In fact I have an
arrangement that only consists of a rather thick and short metal
cylinder with both ends inserted in the inner diameters of two ball
bearings. Now, if I apply a rather large current from one outer ring
of a ball bearing to the other then I observe the same behavior as
you do. The metal cylinder and the inner parts of the bearings would
begin to rotate rather fast much like an electric motor would. In
this case it is indeed the way you describe: It would rotate in the
direction you initially kick it. I always wondered about the reason
of this behavior.

I find the things which cannot be simply explained intriguing.

Regards from over here

Urs

Urs Lauterburg
Physics demonstrator
Physikalisches Institut
University of Bern
Switzerland

>Thanks Urs. That would surely be more dramatic.
>
>But I still don't understand why Dave Kardelis' setup can roll both
>ways? I thought this was just the interaction of the field produced
>by the current in the rails with the field produced by the current
>in the cross bar. But that should only move in one direction. My
>setup moves in the direction I start it rolling in.
>
>Paul
>
>
>On Dec 12, 2010, at 3:24 PM, Urs Lauterburg wrote:
>
>>Paul,
>>
>>If you make the whole arrangement smaller to fit within a B-field
>>produced by a horseshoe magnet you get much better results. In fact
>>that's our traditional arrangement to demonstrate the Lorentz force
>>on charges that move along a rolling rod on a rail. You can show
>>the effect of changing both of the polarities of the current as
>>well as of the B-field by flipping the magnet.
>>
>>Regards form over here
>>
>>Urs
>>
>>Urs Lauterburg
>>Physics demonstrator
>>Physikalisches Institut
>>University of Bern
>>Switzerland
>>
>>>Ok Dave,
>>>
>>>
>>>I just tried it.
>>>
>>>
>>>My little battery charger trips off after a few seconds. And it
>>>doesn't have enough power to get the roller started by itself.
>>>
>>>
>>>Here's the curious feature... The rod will continue rolling if
>>>given a small nudge. And it might even accelerate just a bit. It
>>>doesn't keep rolling if the power is off. But the thing about
>>>that is... it works it both directions. And it works in both
>>>directions regardless of the polarity of the power.
>>>
>>>
>>>Does this bother anyone else? Or can I choose to use either a
>>>left-hand rule or a right-hand rule depending on my mood?
>>>
>>>
>>>Paul
>>>
>>>
>>>
>>>On Nov 17, 2010, at 11:51 AM, david kardelis wrote:
>>>
>>>
>>>>Adam asked about my rail gun.
>>>>
>>>>Simple setup, 2 3/4" aluminum rods on the table about 1/2 meter
>>>>apart, Third aluminum rod laid on top. B field comes from the
>>>>earth.
>>>>
>>>>The power supply is a emergency jumper battery , one lead hooked
>>>>to each rod on the table, see the thread on power supplies.
>>>>
>>>>Flip the switch and the rod rolls.
>>>>
>>>>I find I need to sand the aluminum rods before doing this as the
>>>>oxidation prevents good electrical contact. The rods do get
>>>>pitted along the contact points
>>>>
>>>>Here is a video on Photobucket I took quickly this morning
>>>>
>>>>
>>>>http://s366.photobucket.com/albums/oo108/dkardelis/Rail%20Gun/?action=viewĄt=003.mp4
>>>>
>>>>
>>>>
>>>>I tried using conduit this morning, to see if could get a
>>>>longer run, and a lighter "bullet". The conduit just welded
>>>>itself together when I flipped the switch,
>>>>
>>>>dave

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Paul,

I assume that what really counts is the direction it will start
rolling without any implied external mechanical force. If it does not
start rolling then the external B-field is simply not strong enough.
Once you have the rod rolling then I would guess that some rather
subtile and nonlinear eddy currents could also have a significant
impact.

It would be interesting to know if Dave Kardellis's arrangement
would move the other way when he changes the current's polarity. If
his rod does indeed move in the other direction then it should be the
interaction of the current's B-field with the one of the earth.

Rolling rod motors are kind of mysterious. In fact I have an
arrangement that only consists of a rather thick and short metal
cylinder with both ends inserted in the inner diameters of two ball
bearings. Now, if I apply a rather large current from one outer ring
of a ball bearing to the other then I observe the same behavior as you
do. The metal cylinder and the inner parts of the bearings would begin
to rotate rather fast much like an electric motor would. In this case
it is indeed the way you describe: It would rotate in the direction
you initially kick it. I always wondered about the reason of this
behavior.

I find the things which cannot be simply explained intriguing.

Regards from over here

Urs

Urs Lauterburg
Physics demonstrator
Physikalisches Institut
University of Bern
Switzerland

Thanks Urs. That would surely be
more dramatic.

But I still don't understand why Dave
Kardelis' setup can roll both ways? I thought this was just the
interaction of the field produced by the current in the rails with the
field produced by the current in the cross bar. But that should
only move in one direction. My setup moves in the direction I
start it rolling in.

Paul


On Dec 12, 2010, at 3:24 PM, Urs
Lauterburg wrote:

Paul,

If you make the whole arrangement smaller
to fit within a B-field produced by a horseshoe magnet you get much
better results. In fact that's our traditional arrangement to
demonstrate the Lorentz force on charges that move along a rolling rod
on a rail. You can show the effect of changing both of the polarities
of the current as well as of the B-field by flipping the
magnet.

Regards form over here

Urs

Urs Lauterburg
Physics demonstrator
Physikalisches Institut
University of Bern
Switzerland

Ok Dave,


I just tried it.


My little battery charger trips off after
a few seconds. And it doesn't have enough power to get the
roller started by itself.


Here's the curious feature... The
rod will continue rolling if given a small nudge. And it might
even accelerate just a bit. It doesn't keep rolling if the power
is off. But the thing about that is... it works it both
directions. And it works in both directions regardless of the
polarity of the power.


Does this bother anyone else? Or
can I choose to use either a left-hand rule or a right-hand rule
depending on my mood?


Paul



On Nov 17, 2010, at 11:51 AM, david
kardelis wrote:


Adam asked about my rail gun.

Simple setup, 2 3/4" aluminum rods on the table about 1/2 meter
apart, Third aluminum rod laid on top. B field comes from the
earth.

The power supply is a emergency jumper battery , one lead hooked to
each rod on the table, see the thread on power supplies.

Flip the switch and the rod rolls.

I find I need to sand the aluminum rods before doing this as the
oxidation prevents good electrical contact. The rods do get pitted
along the contact points

Here is a video on Photobucket I took quickly this morning


http://s366.photobucket.com/albums/oo108/dkardelis/Rail%20Gun/?action=view¤t=003.mp4



I tried using conduit this morning, to see if could get
a longer run, and a lighter "bullet". The
conduit just welded itself together when I flipped the switch,

dave






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