In above picture. When near positive rod to the electroscope, electrons go to top of electroscope. then connect electroscope to ground. Why electrons go from ground to electroscope? in top of electroscope exist electrons and those electrons should repel electrons that is coming up from ground.
Best Answer
Consider the free electrons in the ground wire. The attractive force due to the continued presence of the positive rod is greater than the repulsive force due to the electrons gathered at the top of the electroscope in diagram 2. [This because only a finite number of electrons moved up the stem of the electroscope between diagrams 1 and 2. Electrons stopped moving up the stem when the field in the stem due to the separation of charge in the electroscope was equal and opposite to that due to the positive rod.]
We gain greater insight by considering potentials. When free electrons have stopped moving in the electroscope stem, the leaves, stem and top cap are all at the same potential. This potential is positive because of the proximity of the positively charged rod. Hence electrons flow from ground to the electroscope when the ground connection is made.