I was studying about sharing of charge between two charged conductors when I came across this statement –
When two conductors are joined together through a conducting wire, charge begins to flow from one conductor to another till both have the same potential.
I always thought that charges flow from one point to another until they are able to neutralize the charges at the other point. I felt this was obvious.
But the above statement caused me to reconsider things. Can you tell me why do charges flow till they have same potential? (why not same charge)
Best Answer
What if that other point is already neutral? For example if you connect a neutral (uncharged) metal piece to a negatively charged metal piece. Will no charges move to the neutral piece, since there is no net charge to neutralize here? Of course it will - the charge will spread out as much as it can. Until not the charge, but the potential at any point is the same.
Before connection, there are excess charges (electrons) caught at the charged piece. These are pushing each other as far away as possible - they all repel. They can't move beyond the objects surface, though, because the air is not conducting. But when the other neutral but conducting object is connected, they can move over there. They are pushed over there due to the repulsion of the others - and there is no (or at least a smaller) repulsion from the neutral piece. The push is stronger towards the neutral piece than away from it. And so, the move.
This is potential. That charges are pushed away from a point is what we call a potential (potential energy per charge). When there is a potential difference between two points, it means that the charge is being pushed more towards than away from a point. And so, the charge will move if it can. And it can when there is a connection between the two points.
It will not want to move if there is no potential difference. In the same way, a ball on a shelf will not want to move sideways, since this is a point of equal (gravitational) potential. It will only want to move downwards, towards a decrease in potential.
Indeed, the motion of charges is caused by the amounts of charge present, but we are not talking about charge being neutralized. Rather about electric forces being balanced - in other words, about electric potentials being balanced.