Why does wood move when charged with the same rod as an aluminium can?

[mnmman]

So I just started Physics 2 Electricity and Magnetism and I'm already scared for my life. The professor first did a demonstration where he charged up a Teflon rod on some sort of cloth and moved an aluminium can with the rod. I understand how that happens. The can is a metal and conductor so the electrons are relatively free to move around, so all the electrons go to one side of the can and the other side is positively charged so the can moved because of this polarization effect. What I did not understand was the next demonstration where he charged the same rod and moved a piece of wood (which was balanced on a pivot). I did not expect that because I thought wood was an insulator and thus the electrons are not free to move around and no polarization occurs. Can somebody explain why this happens?

 

[sophiecentaur]

What you saw was due to electrostatic induction. Despite the wood being an insulator, the molecules can still distort without any actual flow of electrons. So there will be a net surplus of electrons on the wood near a positively charged rod (polarisation) so this imbalance of charge is enough to produce a force. It's how uncharged pieces of paper and dust are attracted to a charged comb or ruler.
You need to bear in mind that there are an awful lot of electrons, on and near the surface of the insulator and when they are all displaced by a tiny distance (size of an atom) so that you can get appreciable polarisation with no flow of electrons through a substance. The dielectric between the plates of a Capacitor becomes polarised by a voltage applied across the plates - same thing happening and that increases the capacitance over what you would get with just air in between.

 

[mnmman]

OK so what your saying is the electrons still do move, but only within the atom or molecule?

 

[sophiecentaur]

OK so what your saying is the electrons still do move, but only within the atom or molecule?

To be more accurate, the charge distribution around the atom / molecule becomes distorted (polarised). In a bound state, you can't really talk in terms of electrons 'moving'.