Scotch tape, was Re: Curvature of Space

[John Gastineau <gastineau@mindspring.com>]

At 03:46 PM 2/20/98 -0400, Ludwik wrote:
> If the explanation given by John G (electified scotch tape being attracted
> to neutral objects) is correct then the force should be larger when the
> tape is thicker. Can this be verified?

My post and Leigh's arrived more or less simultaneously; Leigh has now
made it clear that he was assuming a uniform electric field, while
nearly everyone else was not making that assumption. That, of course,
makes all the difference. The attraction of charged objects to neutral
objects depends on the field being nonuniform. 

To address Ludwik's question, the tape is the charged object, so I'm
not sure what its thickness has to do with anything. The polarization
is in the other objects, such as your finger, the desk, or a book. Does
the force depend on the quantity of matter? It should. But once you get
beyond a certain size it'd depend more on the polarizability of the
matter, since the inverse-square law severely limits the practical
range of the force. 

Does everyone know how to make a standard charged tape? (It really is
standard, as you charge to nearly the same charge density each time.)
Cheap, generic tape works better than real Scotch tape. Scotch tape
seems to have a coating to minimize charging to make it easier to handle. 

Take a 20-cm strip of tape, fold the last cm over to make a handle, and
stick it to your desk. Smooth it down well. 

Take a second 20-cm strip, make a handle, and stick it on top of the
first strip. Smooth it down securely with your fingers. 

Quickly pull the upper tape off of the base tape. 

Now start experimenting. Depending on the humidity, you'll have from
ten to hundreds of seconds of useful charge time. 

Is the tape attracted to your finger? the desk? everything? You'll
quickly see that there IS a net force between neutral objects and a
charged tape. (To test, take neutral tapes that don't attract each
other. Is a charged tape attracted to the neutral tape?)

Make two tapes. are they attracted to each other? 

There's a nice discussion of all this in Chabay and Sherwood's text. 

JEG



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