Consider first the case when no electric field is present. The
electrons will be moving due to thermal motion during which they collide with
the fixed ions. An electron colliding with an ion emerges with the same speed
as before the collision. However, the direction of its velocity after the
collision is completely random. This is called thermal velocity of an electron.
At a given time, there is no preferential direction for the
velocities of the electrons. Thus on the average, the number of electrons
travelling in any direction will be equal to the number of electrons travelling
in the opposite direction. So, there will be no net electric current.
If an electric field is applied, the electrons will be
accelerated due to this field towards +Q. The electrons, as long as they are
moving will constitute an electric current. The current for a very short
duration is called as transient Current.
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