# Drift velocity and mobility

The definition of drift velocity "the average velocity with which the free electrons in a conductor gets drifted towards the positive end of the conductor under the influence of an electric field applied across the conductor"

Then comes mobility where they say "mobility of free electrons is independent of electric field" Doesn't this contradict the the motion of free electrons in an conventional current which is due to applied electric field in specific situation

You are right in saying that motion of electrons is due to applied field. This motion is drift velocity. From your own statement we have,

## $$v_{d}\ \alpha\ E$$

introducing a constant, $$v_{d}\ =\ μE$$

where μ is the mobility, which, obviously does not depend on E

Mobility is an intrinsic property of a conductor, which gives behaviour of electrons (i.e. speed) when electric field is applied.

• Then what are the factors that affect the mobility – user257032 Apr 8 at 18:26
• And one more thing intrinsic properties of matter do not depend on the relationship with other entities ....ohope I am getting it right – user257032 Apr 8 at 18:27
• It depends on temperature and plenty of other factors. I said intrinsic, in the sense that it remains constant for a particular conductor at a particular temperature – Krishna Apr 8 at 18:35

Qualitatively, think of the overall motion of electrons as having two components.

One component involves the mobility of free electrons. That is the very rapid random motion of the individual electrons in a conductor due to thermal energy, for which there is no net collective motion of the electrons in any particular direction. This random motion is related to the mobility, μ, in Krishna's answer.

The second component occurs when an electric field is introduced. The electrons now collectively move toward the direction of the "positive end of the conductor" as you put it. Or, more formally, the electrons collectively move in a direction opposite to the direction of the electric field in the conductor. By convention, the direction of the electric field is the direction of the force (and movement) that a positive charge would experience if placed in the field.

The collective motion of the electrons is called the drift velocity and is directly related to the current in the conductor. This velocity is very slow compared to the random velocities of the electrons that occur with or without the electric field present.

Doesn't this contradict the the motion of free electrons in an conventional current which is due to applied electric field in specific situation

No it doesn't. Because drift velocity, the motion of the electrons collectively which is very slow, is independent of the mobility, which is the random velocities of the individual electrons. Let me give you rough analogy.

Suppose you are on the shore and able to see a school of fish where the individual fishes are freely and rapidly darting about in no particular direction. That's roughly analogous to the mobility. But then you also see the whole school moving slowly from left to right because of the ocean current moving in that direction. That would be their collective drift velocity.

Hope this helps.

• Ok so can I know whether my following conclusions are right 1) mobility arises in a conductor that is when there is no cell connected meaning no potential difference 2) however drift velocity arises when the conductor is s part of a current carrying circuit – user257032 Apr 8 at 18:34
• @HenaAnvar Your two statements about mobility and drift velocity are correct but they don't conflict. See my revised answer with the school of fish analogy. – Bob D Apr 8 at 18:46
• Ohhh there is more clarity now 😌...thank you sir – user257032 Apr 8 at 19:00