What fact does einstein's equation e = mc2 describe?

when insulating materials rub against each other, they may become electrically charged . electrons , which are negatively charged, may be 'rubbed off' one material and on to the other. the material that gains electrons becomes negatively charged. the material that loses electrons is left with a positive charge.

beta is electron from nucleus

something from nucleus has changed, but charge must be same for cons of charge