Re: [Phys-l] Photocopier toner

[John Denker <jsd@av8n.com>]

On 03/27/2009 08:33 AM, Arlyn DeBruyckere wrote:
> .... When I started grading the reports I was surprised to see many 
> students saying that the toner was also charged. 


Think about where the uncharged toner would go.

It would pick up an induced dipole in proportion to the
electric field _gradient_ and would therefore go to the
_edges_ of the image on the drum.  The result would be a 
weird silhouette.

This is not entirely hypothetical;  there is always some
uncharged toner floating around, and it is a well-known
imperfection of old and/or broken copiers that they tend
to exaggerate edges in the image, making copies look
"sharper" than the original.

=========

All in all, a photocopier uses a seriously complicated
multi-step process.  I don't recommend trying to figure it 
out by guessing.

FWIW note that old-style "analog" photocopiers are about
as obsolete and as quaint as penny-farthing bicycles.
Nowadays what you get is a scanner plus a digital printer.

Note that the physics is significantly different.  Digital
printers -- if they use an electrostatic drum at all -- use 
a negative process (the light beam exposes the drum to make 
a dark place in the final document) unlike the analog process 
which was, obviously, a positive process (light erased the 
drum, making a light place in the final document).