Question

What is drift velocity?

Answer 1

The drift velocity is the flow velocity that a particle, such as an electron, attains due to an electric field. It can also be referred to as axial drift velocity. In general, an electron will propagate randomly in a conductor at the Fermi velocity. An applied electric field will give this random motion a small net flow velocity in one direction.

In a semiconductor, the two main carrier scattering mechanisms are ionized impurity scattering and lattice scattering.

Because current is proportional to drift velocity, which in a resistive material is, in turn, proportional to the magnitude of an external electric field, Ohm's law can be explained in terms of drift velocity.

The most elementary expression of Ohm's law is:

• Thus valence electrons of atom becomes the conduction electrons of the metals
• At room temperature,these conduction electrons moves randomly inside the conductor more or less like a gas molecule
• During motion,these conduction electrons collide with ions(remaining positive charged atom after the valence electrons move away) again and again and there direction of motion changes after each and every collision.
• As a results of these collisions atoms moves in a zig-zag path
• Since in a conductor there are large number of elctrons moving randomly inside the conductor.Hence they have not net motion in any particular direction.Since the number of electrons crossing an imaginary area ΔA from left to right inside the conductor very nearly equals the number of electron crossing the same area element from right to left in a given interval of time leaving flow of electric current through that area nearly equals to zero
• Now when we applied some P.D using a battery across the two ends of the conductor,then an electric field sets up inside the conductor
• After each collision electrons starts fresh in random direction ,again get accelerated and loose their gained Kinetic energy in another collision
• This extra velocity gained by the electrons is lost in subsequent collison and the processes continued till the electron reach positive end of the conductor
• Under the effect of electric field inside the conductor ,free electrons have random thermal velocities due to the room tmeperature and small velocities with which they drift towards the positive end of the conductor.
• if τ is the average time between two successive collisions and E is the strength of applied electric field then force on electron due to applied electric field is
  F=eE
  Where e is the amount of charge on electron
• if m is the mass of electron ,then acceleration produced is given by
  a=eE/m
• Since electron is acclerated for an average time interval τ,additional velocity acquired by the electron is
  v_d=aτ
  or v_d=(eE/m)τ (3)
  This small velocity imposed on the random motion of electrons in a conductor on the application of electric field is known as drift velocity
• This drift velocity is defined as the velocity with which free electrons gets drifted towards the positive end of the conductor under the influence of externally applied electric field