Re: [Phys-l] Prising electrons from the atom

[John Denker <jsd@av8n.com>]

> > If you bang an iron hammer against a copper anvil, you
> > *will* transfer electrons every time, guaranteed (unless
> > you take heroic measures to prevent it).

On 12/18/2008 10:54 AM, Bernard Cleyet wrote:
> Please confirm (or reject) my idea of the mechanism.  The "banging"  
> is not what does it; the banging or rubbing is to bring the two  
> materials into contact. Touching doesn't necessarily do it, because  
> of thin layers of junk (oxides gas, etc.).

That's it.

1) To best understand the explanation, it helps to quantify
the situation that does *not* work:  Take two clean,
optically flat pieces of metal, and bring them together
carefully, face to face.  They won't make contact,
because of the layer of air.  It's easy to calculate
how long it would take to squeeze out the air.  It's
a long, long time, because the flow geometry is so
unfavorable.

You can easily observe that this is happening, because
there will be relatively little sliding friction between 
the two pieces.  It's sorta the poor man's air table.

If you bring together two non-flat items, they will
have asperities that make contact.  The air doesn't
prevent true contact in this case.

If you bring together two items edge-to-face or
edge-to-edge, the air doesn't prevent true contact.

If you bring together two clean, optically flat 
items in vacuum, you'd better watch out.  You may
never be able to get them apart again.

This is probably why people, roughly speaking, associate
static electricity with fur and fabric.  The gaps
between the fibers provide escape routes for the
air.  Non-porous materials still generate "some"
electricity;  they just do it more inefficiently.