Date: Tue Feb 20 19:04:05 2007

Author: Jerry DiMarco

Subject: Re: dipole

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...


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 Lauterburg
>Physics demonstrator
>Physikalisches Institut
>University of Bern
>> 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?
>>At 02:10 PM 2/10/2007, you wrote:
>>>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 Lauterburg
>>>Physics demonstrator
>>>Physikalisches Institut
>>>University of Bern
>>>> 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 DiMarco
>> Manager of Lecture Demonstrations and Instructional Labs
>> Montana State Univ., Physics Dept.
>> Bozeman, MT
>>Our Motto: "What would your mother do?"