Basics (1)

Q1. If the earth attracts the stone with force F then stone also attracts the earth with same force F (Newton's 3rd law).

So, the stone is in the earth's grav. field and the earth is in the stone's grav. field. That is why both stone and earth have acceleration.

The grav. field due to stone = F/m_stone
The grav. field due to earth = F/m_earth

So, E_stone = E_earth*m_earthm_stone

Q5. As the sheet is infinite in dimensions so, all points are symmetrical with respect to the sheet. So the electric field at all points will be identical. However, if the sheet was not infinitely big then the electric field nearer would be greater than the electric field quite far away (far away relative to the dimensions of the sheet)

Q6. If there is no other external field, and the charge that is being released should not be the one producing the field. There is only electric field then the charge will follow a line of force.

Q4. Yes if you put ANOTHER charge at the point you are referring to it WILL experience infinite force. BUT the charge that is already present will experience a force that is produced by the other charge. If you consider the lower charge, it will not experience infinite force but will experience finite force equivalent to kQq/r². This is because the lines of force is the resultant field due to both charges.

Q2. Lim q-->0 implies that the charge q should be small enough to get induced charge an it should have small dimensions so that it doesnt get affected by gravitational field.

this is what i think may not be correct

How can a point charge have dimensions ??