Understand the Experiment !

first illustration

You were told that the ruler made out of plastic has got a negative charge now. Another way to see that is mentioned here :

Let us imagine, that the ruler here (with its negative charge) is represented by the blue stick in the drawing.

The stick which hangs in the air is able to turn left or right.

This depends on the electric force which can be felt by the red stick (with a positive charge).

As you can see : The red positive stick is attracted by the negative stick, just as your tiny hairs on your nose.

second illustration, other charge

Well here is an illustration witch shows the same case just the other way round.

Here you can see that if you approach with a positive stick, the other stick (witch is also positive) will turn away.

So I hope that now you can understand the reason for the behavior much better than before.

But for a better understanding you can also try to learn something about the electric field, which goes hand in hand with the electric force. (so don't worry about this ! Check it out !)

So now let's return to our main topic.