Date: Tue Feb 20 19:04:05 2007

Author: Jerry DiMarco

Subject: Re: dipole

Post:
I would also like to see what happens with a knitting needle that is
sharp at both ends, and another conductor with small spheres at each
end. Without further experimentation we are just guessing. I might be
able to do something on a smaller scale with one of our variable capacitors
but don't hold your breath...

Jerry

P.S. Our winter has been more like Urs', cold with little snow, then warmer
with more snow. I am envious of you Easterners, haven't seen a big snow in
a long time. No lake effect here...

At 11:56 PM 2/20/2007, you wrote:
>Jerry et al.,
>
>No, I had no time to experiment. I was away for a week to recover from a
>long and tiresome busy winter semester all the way through since October
>of last year. I hoped to find some snow up in the Alps to ride on with my
>snowboard but there was not very much, barely enough to do winter sports
>in some places. It certainly looks as if you got the entire share of snow
>of this year's winter. Sam, are you still going to work with the snow
>shoes on ;-)
>
>Anyway, have you made progress with the interpretation of Sam's
>electrostatic movie? I thought about it and can't quite figure out why the
>stick is attracted to one plate but does not touch it. The suspension does
>allow the stick to touch it right, Sam? This is different from my
>arrangement. My dipole is an insulator (wood) who behaves like an induced
>dipole. When drawn against the denser field it would want to touch the
>sphere and is only held back by the suspending thread. It's only in the
>case the wooden stick comes too close to the attracting electrode in
>combination with humidity that some charge transfer will happen that makes
>the wood immediately ''jump'' away.
>
> From what I understand, Sam seems to have a metal coated isolator. If the
> coating is a conductor that covers the entire surface the rod should
> behave like a conductor which describes the behavior of moving back and
> forward by acquiring the charge of the touched electrode. Sam, have you
> tried to use other material? I mean a pure isolator to compare it to a
> pure conducting stick maybe made of rolled aluminium foil? You may also
> want to try to give the stick a little push when the Wimshurst is running
> at full speed to make it touch one electrode (of course by manipulating
> if with a non-conductor) to see if the oscillations occurs thereafter as
> well. I see many things to play with. Maybe the effect is some complex
> superposition of a conducting and a non-conducting material in an E-field.
>
>I have had weird things happening when performing electrostatic fields
>with our big and beautiful wimshurst machine too. For example I show an
>electrostatic monopole by charging an isolated metal rod which has a bunch
>of long paper strips attached at the upper end. This works very well. Then
>next I have two rods of the same kind charged the same to show the field
>geometry of two equal point charges and this works very well too. I used
>to be able to do the same thereafter with the opposite charged paper strip
>devices which are then attracted to each other but for some odd reason
>once after I cleaned the sticks with alcohol to remove some accumulated
>filth the last scene with the opposite charges does no longer work nicely.
>I have never quiet understood it but since then the ends of the paper
>strips always get tangled at the bottom preventing the strips to take off
>against gravity. It seems as if they get strongly attracted to one of the
>supporting rods before repulsion would take them up. Usually I have to go
>about very slowly by carefully and slowly cranking the wimshurst to get
>the paper strips up into the 3D space and around to show the geometry of
>two opposite point charges.
>
>Anyway, if any of you guys encountered similar oddities I would like to
>know it.
>
>Sam when will you preset the next problem of the month-show? I am waiting
>for it, but I guess first we have to solve the present one ;-)
>
>Regards and all
>
>Urs
>
>Urs Lauterburg
>Physics demonstrator
>Physikalisches Institut
>University of Bern
>Switzerland
>
>
>>Urs,
>>
>> Your demo does sound very similar. That sheds a little light on
>> the subject. Sam has been quiet about this. You never told us your
>> theory, Sam. Have you had any more time to experiment with it?
>>
>>
>>Jerry
>>
>>
>>At 02:10 PM 2/10/2007, you wrote:
>>>Jerry,
>>>
>>>I think this is pretty much what is going on here. Reading your
>>>explanation freed me to think it throughly through myself. We have a
>>>demo to demonstrate the behavior of an induced dipole in an
>>>inhomogeneous E-field. It's a small piece of wood about the size of a
>>>match which is suspended on a fine thread between a flat plate and a
>>>small metallic sphere much the same way Sam has hung his mylar stick.
>>>Applying a pure DC high voltage from a high voltage supply will first
>>>rotate the wood to align the induced dipole along the E-field. Then
>>>increasing the voltage further the stick is attracted towards the sphere
>>>where the E-field is denser.
>>>
>>>However the wood is suspended in a way which does not allow the match to
>>>touch the sphere. Sometimes however when humidity is high enough it can
>>>happen that some charge is transferred to the wood to make it
>>>immediately ''jump'' away.
>>>
>>>Manipulating charge in material is indeed interesting and sometimes
>>>tricky theme.
>>>
>>>Thanks for the nice movie Sam and the towels for my tears. I specially
>>>like the background music ;-)
>>>
>>>Urs
>>>
>>>Urs Lauterburg
>>>Physics demonstrator
>>>Physikalisches Institut
>>>University of Bern
>>>Switzerland
>>>
>>>> Here is what I think is happening. The straw is initially drawn
>>>> to one plate by induction. As it nears, charge begins to leak off the
>>>> straw because the end is like a point. The charge leakage reduces the
>>>> inductive attraction, and the straw stops short of the plate in an
>>>> equilibrium position. When you stop cranking the Wimshurst, the
>>>> charge on the plates start dropping. When it drops below the threshold
>>>> voltage where the plate can draw charge off the straw, the straw is
>>>> drawn closer to the plate, again by inductive attraction, and touches
>>>> it. This could perhaps be tested by cranking the Wimshurst much
>>>> slower to keep the voltage below the threshold level. If that proves
>>>> difficult, maybe a variable HV supply would be better...
>>>>
>>>>Jerry
>>
>>
>><><><><><><><><><><><><><><><><><><><><><><><><><><><><><>
>>
>> Jerry DiMarco
>> Manager of Lecture Demonstrations and Instructional Labs
>> Montana State Univ., Physics Dept.
>> Bozeman, MT
>>
>>Our Motto: "What would your mother do?"
>


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