How are negative ions created in an experiment with a balloon and wool?

[Goodver]

In a famous experiment with a balloon and wool.

http://phet.colorado.edu/sims/html/...atest/balloons-and-static-electricity_en.html

Initially the balloon and wool pullover have no ions.

I understand how positive ions are formed, that you apply energy to the electron, thus electron overcomes the attracted force created by a proton, but how are the negative ions created?

I assume when we rub the balloon with a wool pullover we apply energy to both materials, thus depending on a material (how tight electrons to the nucleus) electrons are escaped from both materials forming positive ions.

BUT, I don't understand how the negative ions created, why would atom which is in equilibrium get more electrons and why at the end we get 2 charged materials?

 

[UltrafastPED]

For positive ions, an electron or more is stripped off.

So to make negative ions, one or more electrons are added.

These electrons need not be associated with a particular atom; for polar materials there is a weak dipole field available to do the pulling/repelling. For a conductor the extra charges just "float about" or are taken from the "electron sea" of conduction electrons.

Rubber balloons and wool sweaters were chosen for a reason: they have the properties required to hang onto or loose electrons.

There is a good presentation at: http://www.regentsprep.org/Regents/physics/phys03/atribo/default.htm