To make an electrical eraser in less than 30 seconds you will need:
An old batterie powered hand fan,
An eraser from a pencil.
To assemble it pull the fan from the body and it will get off. If it doesn't get off just cut it out becouse it's not necessary and push the rubber against the little shaft that will appear until it's secure.
Now you're done! You can now erase your pencil without effort!