what is thermal velocity and average velocity of free electrons in conductors.what is the average velocity of free electrons in the presence of electric field ? The drift speed of free electrons in metallic conductors is only few metres per second . Then how is the current established almost at the instant when circuit is closed.

Dear Student ,
Thermal velocity :
When a conductor is not connected to a battery, then free electrons move due to their temperature which is known as the thermal velocity. Value of thermal velocity is very high of order 105 m/s but due to random motion of electrons in all possible directions their average thermal velocity turns out to be zero. Hence,  thermal velocity doesn't contribute to any net current.

Average velocity is the drift velocity of the free electrons in a conductor .  The electric field spread out the circuit with speed of light and at every point local electron is induced by the field .Thus here the current attains a steady value almost instantaneously .

Regards

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