Date: Tue Feb 20 17:56:54 2007

Author: Urs Lauterburg

Subject: Re: dipole

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?
> Jerry
>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?"