Are You a Head Master, Teacher, Parent or Student? we appreciate your help, Please register and add materials to this site or mail to us for all quiries - info@myschoolvision.com
Comment
 
Name
Email Id

Submit

Drift velocity

The drift velocity is the average velocity that a particle, such as an electron, attains due to an electric field. In general, an electron will rattle around in a conductor at the Fermi velocity randomly. An applied electric field will give this random motion a small net 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 is, in turn, proportional to the magnitude of an external electric field, Ohm's law can be explained in terms of drift velocity.

Drift velocity is expressed in the following equations: J_{\it drift} = \sigma \cdot v_{\it avg}, where Jdrift is the current density, σ is charge density in units C/m3, and vavg is the average velocity of the carriers (drift velocity);

  • v_{\it avg} = \mu \cdot E, where μ is the electron mobility in (m/s)/(V/m) and E is the electric field in V/m.

Derivation

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

I = \frac{\Delta Q}{\Delta t}
where
ΔQ is the small amount of charge that passes through an area in a small unit of time, Δt.

One can relate ΔQ to the motion of charged particles in a wire expecting a dependence on the number density of the charge carriers and using dimensional analysis:

ΔQ = \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
Δx 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, vd. And since Δx = vd Δt, 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}
Alternative derivation

Using the definition of current density:

J_{drift} = \sigma \cdot \nu_{drift}

where σ is the density of charge per volume and the fact that

J= \frac{I}{A}

We can simply express:

\sigma = n \cdot q

to get

\frac{I_{drift}}{A} = n \cdot q \cdot  \nu_{drift}

and the same result as above:

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

As a numerical example,for a copper wire of 1 square mm area, carrying a current of 3 amperes, the drift velocity of electrons would be about 0.00028 metres per second (or just about an hour to travel one metre).

Drift velocity

The drift velocity is the average velocity that a particle, such as an electron, attains due to an electric field. In general, an electron will rattle around in a conductor at the Fermi velocity randomly. An applied electric field will give this random motion a small net 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 is, in turn, proportional to the magnitude of an external electric field, Ohm's law can be explained in terms of drift velocity.

Drift velocity is expressed in the following equations: J_{\it drift} = \sigma \cdot v_{\it avg}, where Jdrift is the current density ,σ is charge density in units C/m3, and vavg is the average velocity of the carriers(drift velocity);

  • v_{\it avg} = \mu \cdot E, where μ is the electron mobility in (m/s)/(V/m) and E is the electric field in V/m.

Derivation

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

I = \frac{\Delta Q}{\Delta t}
where
ΔQ is the small amount of charge that passes through an area in a small unit of time, Δt.

One can relate ΔQ to the motion of charged particles in a wire expecting a dependence on the number density of the charge carriers and using dimensional analysis:

ΔQ= \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
Δx 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, vd. And since Δx = vd Δt, 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}
Alternative derivation

Using the definition of current density:

J_{drift} = \sigma \cdot \nu_{drift}

where σ is the density of charge per volume and the fact that

J= \frac{I}{A}

We can simply express:

\sigma = n \cdot q

to get

\frac{I_{drift}}{A} = n \cdot q \cdot \nu_{drift}

and the same result as above:

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

As a numerical example,for a copper wire of 1 square mm area, carrying a current of 3 amperes, the drift velocity of electrons would be about 0.00028 metres per second (or just about an hour to travel one metre).







Why use my school vision?

  • Attendance

    View a child's attendance by various views from weekly, monthly through a summary feature of view detailed information on specific events.

  • Homework tracker

    Allow parents to keep up-to-date with the current and past homework assigned to their child along with past marks and class averages.

  • Progress reports

    Customise the extent to which you wish to keep parents updated with the amount of detail shown in the real-time reports.

  • Projects and
    lessons plans

    Set, receive and mark projects, courses, cover and lessons. Create multipart lessons that can be used for a single lessons or modular courses.

  • News stories

    Allow members of staff to update the school news to help keep parents and visitors of your website up-to-date on the latest goings on.

  • Events

    Display the up and coming events for your school, take bookings, payments and manage attendees.

  • Learning zone

    Introduce pupils of all ages to online learning with regular additions to learning games covering subjects from numeracy, literacy and further afield.

  • About your school

    Manage public details about your school through pre-defined templates such as year groups, classes and members of staff.

  • Joy of reading maths
  • Joy of reading physics
  • Joy of reading chemistry
  • Joy of reading biology
  • Joy of reading maths
  • Joy of reading physics
  • Joy of reading chemistry
  • Joy of reading biology
SV Informatics
Top
  • Follows us our servcies