IIT JEE , AIEEE Forum - Physics , Electricity

sulekha_hi

Can you please tell while defining for drift velocity why do we say it as the final average velocity of the electrons from lower potential to higher potential under the influence of elctric field? Is it so because it is the electron which moves from lower potential to higher potential. If it is so what will be the direction for any positive charge carrier.

pkg1960

a positive charge carrier always moves from a region of higher potential to a lower potential, while a -vely charged particle moves in the opposite direction, since in a metallic conductor the charge carriers are electrons thus the definition has been based on them.....

Prakriteesh

In electricity when we define potential, potential energy etc. we generally define it for a positive charge. Keeping in mind this point, let us take two points A and B, such that A is at higher potential while B is at lower potential. This means that if a positive charge moves towards A, it will face repulsion. So, in the same case, a negative charge will face attraction which means that A is actually the lower potential point for a negative charge. Thus the point which serves as higher potential for a +ve charge serves as lower potential for a negative charge, and vice versa. So, the negative charge which actually moves from higher to lower potential (defined for a negative charge), conventionally moves from lower to higher potential.

mehul_jaganiya

here in an conductor the binding energy will be soo less tht even at room temp. also the electron will leave the nucleas and will move freely in the conductor, where as the nucleus are held together by nuclear forces and they r the only +ve charge in the conductor so if they will start moving thn we will find tht whole matter is moving as if the nuclear forces r weakened and the conductor has melted............

so as far as i think +ve charges dnt move so only -ve charges contribute to drift velocity...........

edison

The drift velocity is the average velocity that a particle, such as an electron, attains due to an electric field. Since particles can accelerate arbitrarily close to the speed of light in the absence of other forces, the term "drift velocity" can only really apply to carriers in materials, and not to particles in a vacuum. Particles in solids, for example, actually collide or scatter with the crystal lattice (or phonons), which slows them down. Drift velocity is non-uniform as it involves electric field as an externally accelerating agent.

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

$J_{drift} = \rho \cdot \nu_{avg}$ where ? is charge density in units ${\rm C \cdot cm^{-3}}$ , and ?avg is the average velocity of the carriers

$\nu_{avg} = \mu \cdot E$ where ? is the mobility of the carriers (in ${\rm cm^2 \cdot V^{-1} \cdot s^{-1}}$ ) and E is the electric field (in ${\rm V \cdot cm^{-1}}$ ).

Derivation

To find an equation for drift velocity, one can begin with the very definition of current:

$I = \frac{\Delta Q}{\Delta t}$

where

?Q is the small amount of charge that passes through an area in a small unit of time, ?t.

One can relate ?Q to the motion of charged particles in a wire by:

$? Q$	$= \left( \mathrm{number \ of \ charged \ particles} \right) \times \left( \mathrm{charge \ per \ particle} \right)$
	$= \left( n A \Delta x \right) q$

where

n is the number of charge carriers per unit volume

A is the cross sectional area

?x is a small length along the wire

q is the charge of the charge carriers

Now, normally particles move randomly, but under the influence of an electric field in the wire, the charge carriers gain an average velocity in a specific direction. This is what's called drift velocity, v_d. And since ?x = v_d ?t, we can plug it into the above equation.

$\Delta Q = \left( n A v_d \Delta t \right) q$

Putting that back into the original equation and re-arranging to solve for the drift velocity:

$v_d = \frac{I}{n q A}$

edison

Electron Mobility

In physics, electron mobility (or simply, mobility), is a quantity relating the drift velocity of electrons to the applied electric field across a material, according to the formula:

v d = E

where

$, v_d$ is the drift velocity

$, E$ is the applied electric field

$, mu$ is the mobility

In semiconductors, mobility can also apply to holes as well as electrons.

Conceptual overview

In a solid, electrons (and in the case of semiconductors, holes) will move around randomly in the absence of an applied electric field. Therefore, if one averages the movement over time there will be no overall motion of charge carriers in any particular direction. However upon applying an electric field, electrons will be accelerated in an opposite direction to the electric field. The summation of the time between acceleration of electrons due to electric field and deceleration of electrons due to collisions and lattice scattering events (caused by phonons, crystal defects, impurities, etc.) over the mean free path between scattering events results in the electrons having an average drift velocity. This net electron motion must be orders of magnitude less than the normally occurring random motion, otherwise the mobility equation is not valid (i.e., typical drift speeds in copper being of the order of 10^-4 m·s^?1 compared to the speed of random electron motion of 10⁵ m·s^?1).

In a semiconductor the two charge carriers, electron and holes, will typically have different drift velocities for the same electric field.

In a plasma there is analogous behavior with ions and free electrons.

In a vacuum, electrons will accelerate non-stop in an electric field according to Newton's second law of motion. This is known as "ballistic transport". Thus electron mobility is undefined for electronic movement in a vacuum.

In a solid, if the electrons must move only a very short distance (distance comparable with the Brownian motion), quasi-ballistic transport is possible.

In SI units, mobility is normally measured in m²/(V·s). Since mobility is usually a strong function of material impurities and temperature, and is determined empirically, mobility values are typically presented in table or chart form. Mobility is also different for electrons and holes in a semiconductor.

An approximation of the mobility function can be written as a combination of influences from lattice vibrations (phonons) and from impurities by the Matthiessen's Rule:

$mu = rac{1}{ rac{1}{mu_{ m lattice}}+ rac{1}{mu_{ m impurities}}}$ .

Mr.IITIAN007

Hey man , drift velocity, conceptually , is the average velocity of the flow of electrons when current passes through any straight or circular or any other shaped conductors.Now the direction of flow of conventional current ( not electronic ) is opposite to that of flow of electron s. But the drift velocity is dependent entirely on the direction of flow of electron not of the current.Since , electrons flow from lower potential to higher potential in influence of electric field , so is the direction of drift velocity.Similarly, for a positive charge carrier i.e a conductor carrying a positron , the direction of positron is from higher to lower potential ,so is the direction of drift velocity.

sulekha_hi

thank u everybody all answers were very satisfying.

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