Question

why does this method of charging work ? please watch between 07:30 and 08:30
https://youtu.be/heot2R99YNk?list=PLUdYlQf0_sSsfcNOPSNPQKHDhSjTJATPu&t=439

Answer 1

It is called charging by induction.
Here is an animation which might help?
Ask again if you have any further questions.

http://www.physicsclassroom.com/mmedia/estatics/epn.cfm

Answer 2

Ahh thanks @Farcher
In that video, professor walter lewin has placed the conductor right on top of the charged insulator. But in your link, they are building the charge by induction without touching ?

Answer 3

There are a few methods of charging that come to mind, it's a bit ambiguous of what's going on here, well at least for me. Electroscope is a nice method of induction where a charged object is held near a neutral conductor. The other methods which seems he looks like using is called charging by contact (lol, you guessed it!) when two objects touch the total charge is shared between the objects (the sharing of charge is equal if objects are identical). And lastly charging by friction (also called abduction) I think we all know this one. I sort of explained this in the simplest sense. A quick image to give an idea notice how charges are rearranged within the conductor (this is induction in a nutshell).

Sorry if this isn't a good explanation, I haven't slept yet xD