In the above picture, I near positively rod to a neutral sphere.
Then I ground the right side (negative side).
Why electrons enter to sphere from ground?
Best Answer
When positively charged rod is brought near sphere all electrons come on side near the rod. other side then becomes positively charged. when the sphere is connected to ground electrons flow from ground to sphere to neutralize the positive charge.
The earth is at a much larger electrostatic-potential, than the sphere is. So, positive charges flow from the earth to the sphere, the number (I speak of number because charge is quantized) of which is equal to the number of electrons that accumulate in the farther end of the sphere. So, due to balance of charges at the farther end, the sphere loses its negative charge on the farther-end, and the positive charges get distributed uniformly all over the sphere. This explanation is based on the conventional flow of current.
Actual scenario is that, not positive charges, but rather the free-electrons in the metal of which the sphere is composed, are the real charge-carriers. These electrons, because of the existing electrostatic potential-gradient, existing between the earth and the sphere, flow through the conducting-wire, from the sphere, to the earth. Then the positive-charges are distributed uniformly, all over the sphere.
The negative charges will want to get as far away from each other as possible. Since our bodies are fairly good conductors, this is fairly easy for the negative charges to move towards ground to achieve this. The repulsion of the negative charges on the rod is just not enough to completely contain all of the negative charge in the electroscope. Negative charges in the tip are much closer to each other than they are to the rod, so this repulsion will win initially to create an overall net positive charge on the electroscope.
If this is unconvincing you could go this route instead. To get a net positive charge on the electroscope we do not need to remove much negative charge from the electroscope. Even though the diagram shows positive charge at the tip before we touch it, this does not mean it is devoid of negative charge. It just means there is a net positive charge there. So when we touch the tip there will be some negative charges where the negative rod will actually aid in pushing these charges to ground through the finger. This is enough to give the electroscope a net positive charge since some negative charge has left it.
Best Answer
When positively charged rod is brought near sphere all electrons come on side near the rod. other side then becomes positively charged. when the sphere is connected to ground electrons flow from ground to sphere to neutralize the positive charge.