Question

In halliday resnick I read a line in current and resistance that

"Although an electric current is a steam of moving charges , not all moving charges constitute an electric current. " I want to ask that how do we get to know that these are moving charges which will produce electric current and others will not.

Answer 1

It is said so to indicate the fact that even when a conductor is kept as such, there will be motion of free electrons(free electrons itself are called charges). But in this case electrons are free to move in any direction(random motion) so there not be any net transport of charge in any direction.So, there is no net current flow.

So how do we know if the moving charges produce a current?
Moving charges produce a current if they starts moving in same direction.So,there'll be a net transport of charge from one point of conductor to another.This is called current. For this there should be a potential difference between two ends of a conductor.
So we can say moving charges result in a current only if there is a potential difference (which cause them to move in a specific direction).