What happens when insulating materials are rubbed together?
Rubbing a polythene rod with a dry cloth transfers electrons to the surface atoms of the rod from the cloth. So the polythene rod becomes negatively charged
Some insulators become charged by rubbing them together
Insulating materials that lose electrons when rubbed become positively charged
Rubbing a perspex rod with a dry cloth transfers electrons from the surface atoms of the rod onto the cloth. So the perspex rod becomes positively charged