To my knowledge, a voltage source just drive all free electrons of a conductor in a closed circuit to complete a loop (after reaching positive end.. Again to the negative end ... To repeat the cycle) Is that correct or wrong?? Moreover electric current is the quantity of charge passing through a cross sectional area per unit time.. I mean if current increases, how come the electrons quantity increase in a closed circuit conductor.... Correct me if I was wrong.... Thanks in advance
"how come the electrons quantity increase in a closed circuit conductor"
It is not the quantity of electrons in the conductor that changes, in fact that changes very little. As you said, it is (my emphasis)
"quantity of charge passing through a cross sectional area per unit (of) time"
In the water equivalent (which sort of works, but don't let yourself be carried away by it; it is just an analogy, not a reliable model) consider a water pump, with a hose connecting the output back to the input. Pump and hose are filled with water. The amount of water never changes, but when you crank up the pump, the water will circulate faster: more water passes through each cross-section of the hose per unit of time.
Not necessarily. Voltage Source provides an Electric Field that travels through the conductor once it's switched on. This field when reaches a point it moves the electrons, but electrons don't need to travel all the way from + to -. Energy travels from + to -. Consider AC for example, electrons never leave their place, but voltage and energy travels from the generator thousands of miles away to your home.