Re: [Phys-l] electrostatic field (gradient?) force

[John Denker <jsd@av8n.com>]

On 12/19/2007 10:46 AM, Anthony Lapinski wrote:

>  Water is a polar molecule, and a charged balloon will easily bend a
> stream of water. Hexane is not a polar molecule, and chemistry demo books
> say the stream should NOT bend. But if you put hexane in a buret, a
> charged balloon DOES bend the stream. A charged balloon should attract any
> neutral object, right? Something seems wrong here...

Water has a huge dielectric constant (because it is polar).

A) Anything with a dielectric constant will be subject to a
force if you put it in an electric field _gradient_.

If the dielectric constant is small (and or the field gradient
is small) the force will be small, perhaps unobservably small.

B) Depending on the geometry of your burette setup, it is quite
possible to _induce_ a surface charge.  This doesn't require a
field gradient;  a plain old field suffices to induce a surface
charge.

If a droplet breaks away from the surface, it will take the
surface charge with it.  Any such stream of droplets will
be subject to a force if an electric field is applied.  Again
this depends on the field, not on the field gradient.

Some guy named Kelvin had something to say about this:

http://lecturedemo.ph.unimelb.edu.au/Electrostatics/El-7-Kelvin-Water-Dropper-Induction-Device