Date: Mon, 13 Dec 2010 11:14:45

Author: Doug Johnson

Subject: Re: Simple Rail Gun

Post:

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This reminds me of the deep groove ball bearing (like...>
http://www1.mscdirect.com/CGI/NNSRIT?PMPXNO=3D3441754&PMT4NO=3D0 ), that when
you apply a large current in the center race to the outside race, that the
bearing assembly starts rotating. I bet it is the same effect as with the
rolling rod. Never heard a good explanation of why that works either.

...Doug J.


On 12/13/10 10:38 AM, "david kardelis" wrote:

> The last time I took this out for the video I posted, it always moved the=
same
> direction regardless of polarity.
>=20
> I vaguely remember doing this in class and talking about reversing the
> polarity and explaining that the rod should roll the other way. I then
> reversed the leads and thought that the rod did go the other way. But it =
is a
> vague recollection and when I try it now it only goes one way.
>=20
> I will try it again, the current setup is on the tabletop and the tables =
here
> have steel bases. I used to do it with the rods extending off the end of =
the
> table. I will try that again and see what happens
>=20
> dave
>=20
> On Mon, Dec 13, 2010 at 9:59 AM, Paul Nord wrote:
>> Urs,
>>=20
>> So it could be some variety of shaded pole motor?
>> New understanding physics for ... - Google Books
>> > ;dq=3Dshaded+pole+motor+demonstrator+physics&source=3Dbl&ots=3DqOqF15s=
G4m&a
>> mp;sig=3DniU_pbC_PxycXtWtNXpDEL0UUzw&hl=3Den&ei=3DY08GTcjsFdTNnAetj8nl=
DQ&am
>> p;sa=3DX&oi=3Dbook_result>
>> I never understood that! =A0:)
>>=20
>> It's likely that there is something going on with the intermittent elect=
rical
>> contact between the rods.
>>=20
>> Paul
>>=20
>>=20
>> On Dec 13, 2010, at 1:16 AM, Urs Lauterburg wrote:
>>=20
>>> Paul,
>>>=20
>>> I assume that what really counts is the direction it will start rolling
>>> without any implied external mechanical force. If it does not start rol=
ling
>>> then the external B-field is simply not strong enough. Once you have th=
e 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 m=
ove
>>> the other way when he changes the current's polarity. If his rod does i=
ndeed
>>> move in the other direction then it should be the interaction of the
>>> current's B-field with the one of the earth.
>>>=20
>>> Rolling rod motors are kind of mysterious. In fact I have an arrangemen=
t
>>> 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: I=
t
>>> would rotate in the direction you initially kick it. I always wondered =
about
>>> the reason of this behavior.
>>>=20
>>> I find the things which cannot be simply explained intriguing.
>>>=20
>>> Regards from over here
>>>=20
>>> Urs
>>>=20
>>> Urs Lauterburg
>>> Physics demonstrator
>>> Physikalisches Institut
>>> University of Bern
>>> Switzerland
>>>> Thanks Urs. =A0That would surely be more dramatic.
>>>>=20
>>>> But I still don't understand why Dave Kardelis' setup can roll both wa=
ys?
>>>> =A0I 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 cro=
ss
>>>> bar. =A0But that should only move in one direction. =A0My setup moves in t=
he
>>>> direction I start it rolling in.
>>>>=20
>>>> Paul
>>>>=20
>>>>=20
>>>> On Dec 12, 2010, at 3:24 PM, Urs Lauterburg wrote:
>>>>=20
>>>>> Paul,
>>>>> If you make the whole arrangement smaller to fit within a B-field pro=
duced
>>>>> by a horseshoe magnet you get much better results. In fact that's our
>>>>> traditional arrangement to demonstrate the Lorentz force on charges t=
hat
>>>>> move along a rolling rod on a rail. You can show the effect of changi=
ng
>>>>> both of the polarities of the current as well as of the B-field by
>>>>> flipping the magnet.
>>>>>=20
>>>>> Regards form over here
>>>>>=20
>>>>> Urs
>>>>>=20
>>>>> Urs Lauterburg
>>>>> Physics demonstrator
>>>>> Physikalisches Institut
>>>>> University of Bern
>>>>> Switzerland
>>>>>=20
>>>>>> Ok Dave,
>>>>>>=20
>>>>>> I just tried it.
>>>>>>=20
>>>>>> My little battery charger trips off after a few seconds. =A0And it doe=
sn't
>>>>>> have enough power to get the roller started by itself.
>>>>>>=20
>>>>>> Here's the curious feature... =A0The rod will continue rolling if give=
n a
>>>>>> small nudge. =A0And it might even accelerate just a bit. =A0It doesn't k=
eep
>>>>>> rolling if the power is off. =A0But the thing about that is... it work=
s it
>>>>>> both directions. =A0And it works in both directions regardless of the
>>>>>> polarity of the power.
>>>>>>=20
>>>>>> Does this bother anyone else? =A0Or can I choose to use either a left-=
hand
>>>>>> rule or a right-hand rule depending on my mood?
>>>>>>=20
>>>>>> Paul
>>>>>>=20
>>>>>>=20
>>>>>> On Nov 17, 2010, at 11:51 AM, david kardelis wrote:
>>>>>>=20
>>>>>>> Adam asked about my rail gun.
>>>>>>>=20
>>>>>>> Simple setup, 2 3/4" aluminum rods on the table about 1/2 meter apa=
rt,
>>>>>>> Third aluminum rod laid on top. B field comes from the earth.
>>>>>>>=20
>>>>>>> The power supply is a emergency jumper battery , one lead hooked to=
each
>>>>>>> rod on the table, see the thread on power supplies.
>>>>>>>=20
>>>>>>> Flip the switch and the rod rolls.
>>>>>>>=20
>>>>>>> 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
>>>>>>>=20
>>>>>>> Here is a video on Photobucket I took quickly this morning
>>>>>>>=20
>>>>>>>=20
>>>>>>> http://s366.photobucket.com/albums/oo108/dkardelis/Rail%20Gun/?acti=
on=3Dvi
>>>>>>> ewĄt=3D003.mp4
>>>>>>> >>>>>> iew¤t=3D003.mp4>
>>>>>>>=20
>>>>>>>=20
>>>>>>>=20
>>>>>>> I tried using conduit this morning, to see if=A0=A0could get a longer=A0r=
un,
>>>>>>> and a lighter "bullet".=A0 The conduit just welded itself together wh=
en I
>>>>>>> flipped the switch,
>>>>>>>=20
>>>>>>> dave
>>>=20
>>>=20
>>=20
>=20
>=20



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This reminds me of the deep groove ball bearing (like...> http://www1.m=
scdirect.com/CGI/NNSRIT?PMPXNO=3D3441754&PMT4NO=3D0 ), that when you app=
ly a large current in the center race to the outside race, that the bearing =
assembly starts rotating. I bet it is the same effect as with the roll=
ing rod. Never heard a good explanation of why that works either.

...Doug J.


On 12/13/10 10:38 AM, "david kardelis" =
; wrote:

The last time I took this out for the video I posted, i=
t always moved the same direction regardless of polarity.

I vaguely remember doing this in class and talking about reversing the pola=
rity and explaining that the rod should roll the other way. I then reversed =
the leads and thought that the rod did go the other way. But it is a vague r=
ecollection and when I try it now it only goes one way.

I will try it again, the current setup is on the tabletop and the tables he=
re have steel bases. I used to do it with the rods extending off the end of =
the table. I will try that again and see what happens

dave

On Mon, Dec 13, 2010 at 9:59 AM, Paul Nord wrot=
e:
Urs,

So it could be some variety of shaded pole motor?
New understanding physics for ... - Google Books 96&lpg=3DPT296&dq=3Dshaded+pole+motor+demonstrator+physics&a=
mp;source=3Dbl&ots=3DqOqF15sG4m&sig=3DniU_pbC_PxycXtWtNXpDEL0UUzw&=
amp;amp;hl=3Den&ei=3DY08GTcjsFdTNnAetj8nlDQ&sa=3DX&oi=3Dbook=
_result>
I never understood that! =A0:)

It's likely that there is something going on with the intermittent electric=
al contact between the rods.

Paul


On Dec 13, 2010, at 1:16 AM, Urs Lauterburg wrote:

Paul,

I assume that what really counts is the direction it will start rolling wit=
hout any implied external mechanical force. If it does not start rolling the=
n the external B-field is simply not strong enough. Once you have the rod ro=
lling then I would guess that some rather subtile and nonlinear eddy current=
s 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 curren=
t's B-field with the one of the earth.

Rolling rod motors are kind of mysterious. In fact I have an arrangement th=
at only consists of a rather thick and short metal cylinder with both ends i=
nserted in the inner diameters of two ball bearings. Now, if I apply a rathe=
r large current from one outer ring of a ball bearing to the other then I ob=
serve 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 th=
e direction you initially kick it. I always wondered about the reason of thi=
s 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. =A0That would surely be more dramatic.

But I still don't understand why Dave Kardelis' setup can roll both ways? =A0=
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. =A0But =
that should only move in one direction. =A0My setup moves in the direction I s=
tart 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 tradit=
ional arrangement to demonstrate the Lorentz force on charges that move alon=
g a rolling rod on a rail. You can show the effect of changing both of the p=
olarities 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. =A0And it doesn't ha=
ve enough power to get the roller started by itself.

Here's the curious feature... =A0The rod will continue rolling if given a sma=
ll nudge. =A0And it might even accelerate just a bit. =A0It doesn't keep rolling=
if the power is off. =A0But the thing about that is... it works it both direc=
tions. =A0And it works in both directions regardless of the polarity of the po=
wer.

Does this bother anyone else? =A0Or can I choose to use either a left-hand ru=
le 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 ro=
d 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 p=
oints

Here is a video on Photobucket I took quickly this morning


http://s366.photobucket.com/albums/oo108/dkardelis=
/Rail%20Gun/?action=3Dview¤t=3D003.mp4










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