in the classic school experiment where students rub a ruler with cloth and bits of tissue paper are attracted to it, why are the bits of tissue attracted to it when they havn't been charged?

its the properties of the cloth that give the ruler a charge when it is rubbed.

im not sure if the cloth charges it negatively or positively (depending on the details of the experiment) but either way, opposite charges attract; if the ruler is positively charged by the cloth (it is stripped of electrons) or negatively charged (given an excess of electrons) then it will attract a neutral piece of paper (like a magnet attracted to a neutral refrigerator door). this is possible because negative charges can move freely in most materials. if a positively charged rod is placed near a neutral object, the negative charges in the neutral object will be repelled from the positive rod, and in turn the positive charge built up on the other side will be attracted to the rod. (and the same goes for the opposite orientation of charges).

 

if the tissue paper was charged then there could be an even stronger attraction if the charges are still opposite. if the charges were equal (both negatively charged) then the bits of paper would be repelled by the ruler.

oh ok thanks