What is Drift velocity: Definition and 69 Discussions

In physics a drift velocity is the average velocity attained by charged particles, such as electrons, in a material due to an electric field. In general, an electron in a conductor will propagate randomly at the Fermi velocity, resulting in an average velocity of zero. Applying an electric field adds to this random motion a small net flow in one direction; this is the drift.
Drift velocity is proportional to current. In a resistive material it is also proportional to the magnitude of an external electric field. Thus Ohm's law can be explained in terms of drift velocity. The law's most elementary expression is:


{\displaystyle u=\mu E,}
where u is drift velocity, μ is the material's electron mobility, and E is the electric field. In the MKS system these quantities' units are m/s, m2/(V·s), and V/m, respectively.
When a potential difference is applied across a conductor, free electrons gain velocity in the direction opposite to the electric field between successive collisions (and lose velocity when traveling in the direction of the field), thus acquiring a velocity component in that direction in addition to its random thermal velocity. As a result, there is a definite small drift velocity of electrons, which is superimposed on the random motion of free electrons. Due to this drift velocity, there is a net flow of electrons opposite to the direction of the field.

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  1. M

    AC Drift Velocity: Understand Reactive Power & Movement of Electrons

    Hi All I hope this topic makes sense for this forum and somebody can help me to understand. This is what I understand (please allow numerical approximations): 1. Electrons move in a conductor at a speed anywhere near the speed of light. 2. Electrons gain actual velocity (drift) only when...
  2. jisbon

    Calculating electron drift velocity

    Just wanted to check in my workings to see if they are correct (seemed to be too short to me?) Since electrical conductivity is 820 (ohm.m) which is = n*e*(mobility) Mobility =0.17083? And I can simply get drift velocity by multiplying mobility with an electric field (600V/m)? Cheers
  3. N

    I Drift velocity in P-N Junctions

    When a P-N junction is in reverse bias, the drift velocity of the system increases, so why is there no current flow? Is the drift velocity not connected to the current? Thanks in advance.
  4. P

    I Deriving the E Cross B Drift Velocity: A Puzzling Explanation

    I am looking at this brief derivation of the E cross B drift for a plasma, starting at the top of page 3: http://www.physics.sfsu.edu/~lea/courses/grad/motion.PDF. I am a bit confused as to the justification that the drift velocity is perpendicular to the magnetic field, as stated at the top of...
  5. F

    Find the drift velocity

    Homework Statement A 5.00-A current runs through a 12 gauge copper wire (diameter 2.05mm) and through a light bulb. Copper has 8.5 x 1028 free electrons per cubic meter. c) At what see does a typical electron pass by any given point in the wire? Homework Equations J = nqvd The Attempt at a...
  6. R

    Charge carrier drift velocity of wire

    Homework Statement Homework EquationsThe Attempt at a Solution Current is I = nqvA so drift velocity V is: V = I/nqA Drift velocity for x is: Vx = I/nqA Drift velocity for y is: Vy = 2I/nqA So the ratio of Vx : Vy should be 2:1 since Vy is equal to 2 lots of Vx?? (but correct answer is B)
  7. G

    Why an electron's drift velocity is so slow?

    Electrons are moving VERY fast. However, they don't have a high drift velocity in a circuit. Why? Is it because every time they advance a little bit they collide with an atom? Or is it because the electric field in a circuit is not strong enough so the electrons don't get pushed enough?
  8. stephen8686

    E-Field Through Copper Wire given drift velocity

    Homework Statement If the magnitude of the drift velocity of free electrons in a copper wire is 7.84×10-4 m/s, what is the electric field in the conductor? (Also gives chart that states that the resistivity of Cu is 1.7×10-8 Ωm) Homework Equations [/B] vd=(qEτ)/me (where τ is avg. time...
  9. C

    Electrons emitted per second

    Homework Statement The beam of electrons in a cathode ray tube contains electrons traveling at 8.4x106 ms-1 and the current throgh the tube is 2.8 micro A. a) how many electrons are emitted per second from the cathode of the tube b) what is the number of electrons per unit length of the beam...
  10. moenste

    Find drift velocity of the current carriers in the copper strip

    Homework Statement ABCD is a plane rectangular strip of conducting material of uniform thickness, with a steady current flowing uniformly from AD to BC. The potential difference between E and F, the mid-points respectively of AB and CD, is zero, but when a magnetic field is set up at right...
  11. Dan15

    I How does negative power link to amplification?

    So I've been studying electrons behavior in a semiconductor superlattice and I am currently analyzing the negative slope for the drift velocity against electric field. This is where negative differential velocity (NDV) takes place. I understand how NDV leads to negative power, but I can't quite...
  12. S

    Show the drift velocity is ExB/B^2

    A charged particle drifts in uniform, constant magnetic and electric fields. The electric field, E, is perpendicular to the magnetic field, B. Show that the drift velocity is given by vd = (E×B)/B2 Heres where I get to: F=e(E+vxB)=0 as v is uniform.Therefore E+vxB=0. Take vector product of B...
  13. LotusTK

    Factors affecting drift velocity?

    Homework Statement I am having a look at drift velocity and i know essentially what it is, and i am now noting down the factors that effect the drift velocity of the charge carriers as i think its a useful thing to know. So, so far i have that the number of charge carriers will effect the...
  14. D

    Electricity through moving conductive liquid

    We Know that in conductive Solids there are lots of free electrons available & they drift when electric field is applied. Their drift velocity is in the range of mm/sec to cm/sec. Now if we consider any conductive liquid which is flowing & we connect the electrodes in such fashion that the flow...
  15. U

    Drift velocity in semiconductor

    Homework Statement [/B] (a) Explain the terms intrinsic, extrinsic, mobility and effective mass in semiconductors (b) What is a hole and explain its mass and charge (c) Why is mobility of holes often less than mobility of electrons? Find the number density of holes and electrons (d) Find the...
  16. kelvin490

    Average drift velocity of electron in conduct

    I have a question on the derivation of the average drift velocity in a conductor: drift velocity is the average velocity which a free charge moving in a conductor has due to the influence of an electric field applied to the conductor. In a metal, the free charge will be an electron. As they move...
  17. B

    Drift Velocity & Current

    Homework Statement [/B] Can someone confirm if I answered correctly? Thanks Ai) Define the term drift velocity with reference to a current in a circuit. ii) Derive an expression to describe the current passing through a metallic conductor of cross-sectional area A having n charge carriers per...
  18. H

    Drift velocity: Why not 1/2 *t*(eE/m)

    Why is the drift velocity of an electron in a wire defined as: tau*(E*e/m) and not 1/2 *tau* (eE/m) as it is the AVERAGE VELOCITY!? because the above defintion of drift velocity is the velocity that is attained before the collision so rather the maximum velocity of the elctron, isn't it?
  19. I

    Electron Drift Velocity in Semiconductors

    Homework Statement "The low [electron] density of a semiconductor means that the conduction electrons travel much faster than they do in conductors." In order to fully understand this, I feel that I need to know why this occurs. Homework Equations I = nAve Where: I = current n = electron...
  20. manogyana25

    Drift velocity (Electricity)

    Can anyone please explain what is drift velocity. I was absent for that class. I need a detailed explanation and derivation of that. Please help.
  21. E

    What is the drift speed for plasma?

    Homework Statement I need to calculate the drift speed, drift orbital period and current density for 1eV protons and 10eV electrons. The density is 5 cm^-3 The assumption is that it is only a grad-B drift Homework Equations <V> = [6*E/(qBo*L*Re)](0,35 + 0.15sin(a)) The Attempt at a Solution...
  22. cseil

    Drift velocity of electrons

    Homework Statement There's an engine connected to a battery with a copper wire. ρ = 1.69x10^-8 n = 8.49*10^28 electrons/m^2 The diameter of the wire is 5mm. The length is 1m. How long does it take for an electron to go from the battery to the engine if i = 100A? Homework Equations ## v_d =...
  23. V

    Question about Drift Velocity in Hall Effect

    So my textbook is explaining the hall effect and the following equation was derived: ∆VH=vd·B·W But if you look at the diagram, it appears that vd is pointing both ways. So which direction is vd pointing in? Or is this because electrons in the opposite direction is the same thing as holes in...
  24. H

    Electron Drift Velocity problem

    Following are the text from an Electrical Engineering Textbook. "it is seen that contrary to the common but mistaken view, 1. the electron drift velocity is rather very slow 2. is independent of the current flowing 3. independent of the area of the conductor" first point can be...
  25. A

    Drift velocity of electron

    I need help to understand the concept of drift velocity. Here is part from my book: Does this velocity refer to the velocity of each electron? And I don't understand the sentence: The acceleration due to the field and the collision with the crystal counteract, leading to a constant velocity...
  26. S

    Drift velocity formula derivation confusion

    In the derivation of drift velocity i have seen two variations and want to know which one's correct. s=ut +\frac{1}{2}at^{2} Assume that the drift velocity of any electron in any conductor is : v_{d}=l/t Due to the electric field the acceleration of electrons in any conductor...
  27. W

    Drift velocity in uniform magnetic field near charged wire

    Problem (a) Derive and expression (using cylindrical coordinates) for the drift velocity of a particle of mass m and charge q in a uniform vertical magnetic field B\hat{z} when the particle is located at a perpindicular distance p from a long, straight vertical wire carrying a line charge...
  28. R

    Drift Velocity in a Vaccum Diode

    Is the standard equation for drift velocity of electrons also applicable to conduction in, say, a vacuum tube? I assume it is, and if so what is the relation between the drift velocities at the interface of vacuum-metal at the anode of a vacuum diode? The current, electronic charge and Area...
  29. G

    Drift velocity in a conductor

    Dear all, I have a question on the derivation of the drift velocity in a conductor, and was wondering if somebody could help me with it: drift velocity is the average velocity which a free charge moving in a conductor has due to the influence of an electric field applied to the conductor. In...
  30. O

    Why is BOTH the electron & hole drift velocity positive?

    Good Morning, The equations under investigation: Electrons: J = enμE + eD(dn/dx) Holes: J = epμE - eD(dp/dx) n or p = concentration of electrons or holes respectively D = diffusion constant μ = mobility The question in mind is as follows: If holes are...
  31. P

    Current density and drift velocity

    Homework Statement 1. A copper wire 2 mm in diameter carries a 4 A current. Determine the current density in the wire, drift velocity of the electrons and the mean time between collisions. Assume that each copper atom contributes one free electron. Hint: you may need some additional parameters...
  32. J

    Drift Velocity ambiguity in relation

    How is it possible that the two relations i.e., v=(eE/m)t and v=(eE/2m)t both holds good? where e is the elementary charge, m is the mass of the elementary particle, E is the electric field, v is the drift velocity and t is the relaxation time.
  33. S

    Deriving Current from Drift Velocity Basics

    I've been sitting at my desk trying to prove this and nothing, already tried googling.. Any help guys? I want to know how to derive it from basic principles. Thanks How I came by it: I'm a high school student and was doing my exam then get asked about drift velocity and it's not even in the...
  34. J

    Drift velocity and charging a capacitor.

    I'm having trouble understanding where the charging time in a capacitor actually comes from. Is it possible to derive the \tau of a capacitive circuit from the drift velocity of the electrons? Are charges literally moving from the conductors onto the metal plates of the capacitor?
  35. D

    [Drift velocity] Factors affecting velocity

    1. Is the charge density (No. of charges/volume) constant for a specific material ? e.g. Copper has a constant charge density no matter how big it is ? 2. Why someone told me that increasing the cross-sectional area of a wire will not affect its drift velocity ? The resistance will be reduced...
  36. N

    Which is the correct Expression for Drift Velocity of Electrons in a conductor ?

    i am really really confused . I know what Drift velocity is but the problem is I have three Sources which tell absolutely different expression for drift velocity and I cat relate . please tell which is right . This website and MIT Profs. Walter Lewin In his Lectures says Vd = eE/m x (Tau)...
  37. M

    Current density, drift velocity and electric field

    Homework Statement A) A current I flows along a straight wire of cross-sectional area A. If the unit vector I denotes the direction of current flow, write down the expression for the current density J (assuming it to be uniform throughout the wire ) and give its SI units. B) (This isn't...
  38. M

    Calculating drift velocity of electrons in a conductor

    Homework Statement Calculate the drift velocity of the elctrons in a conductor that has a cross-sectional area of 8x10^-6m^2 and carries a current of 8A. Take the concentration of free electrons to be 5x10^28 electrons/m^3 with 1.6x10^-19C charge on each electron. Homework Equations...
  39. D

    Curent densitys relation to drift velocity

    Homework Statement A beam contains 4.0x10^8 doubly charged positive ions per cubic centimeter, all of which are moving north with a speed of 1.2x10^5 m/s. (a) What is the magnitude of the current density ? (b) What is its direction? (c) What additional quantity or quantities are needed...
  40. A

    Question on relationship between drift velocity and resistance

    Is there a relationship (or equation) between a charge carrier's drift velocity through a conducting material and that material's resistance? I'm trying to reconcile the idea that drift speeds are bigger when cross sections are smaller and the idea that resistance is bigger when cross sections...
  41. D

    Drift velocity in a resistor?

    Problem: An Si resistor is at 300Kelvin, and is 100μm long, 1μm thick, 5μm wide. The conductivity is 7.128 Ωcm. The resistance is 28058Ω. A voltage of 5v is applied. What is the drift velocity of the electrons, given that n = 1.01x103? Other constants: μn = 1350 cm2/Vsec μp = 450 cm2/Vsec...
  42. C

    Drift Velocity: missing 1/2?

    If a is the acceleration of an electron in a conductor due to an electric field, and t is the mean time between collisions, than v_d, the average drift velocity, is v_d = at 1. There should be a 1/2 there! If you followed one electron, its average v_d would be at/2 by kinematics. 2. Book...
  43. L

    Drift velocity of electrons

    Homework Statement A potential diffference of V = .497 V is applied across a block of silicon with resistivity 8.75 x 10-4 Ωm. The dimensions of the block are width = a = 2.00 mm, and length L = 14.7 cm. The resistance of the block is 54.1Ω, and the density of charge carries is 1.23x1023 m-3...
  44. A

    Current density and drift velocity

    Homework Statement J = ne(v). (yes. it's a vector equation and I haven't indicated that, but just ignore that. It's not important to my question.) Let's say you have some positive ions traveling downward, and some negative ions traveling upward. The electric field is downward. Let's say...
  45. H

    What Force Causes Drift Velocity Increase in Wires of Different Diameters?

    consider two wires of different diameter. when current enters from that of greater diameter to lesser diameter drift velocity increases. which force causes it to increase?
  46. S

    Electron drift velocity of an electric arc

    Seeing the other thread about "How I view AC" and the talk of drift velocity reminded me of a question I had a number of years back, and never found an answer to ... what is the electron drift velocity of an electric arc? I'd be interested in the wide scope of everything from lightning strikes...
  47. H

    Drift velocity and copper wire

    Homework Statement A copper wire of diameter 2.00mm carries a current of 0.50A.The number of conducting electrons per metre cube of copper is 8.00 x 10^28. a)Calculate the drift velocity of the conducting electrons. b)What is the number of conducting electrons in 10.0cm length of the wire...
  48. H

    Why Does Temperature Affect Drift Velocity in the Drift Velocity Formula?

    1.I would like to know why temperature is increase and the current kept constant from the formulae I=nave v=I/nae =the value I,n,a,e are constant when the temperate increasing. But why,i don really understand how to see though the...
  49. E

    Electric circuit, drift velocity, battery

    Hello, This is a problem that is buzzing me around for quite sometime since I took fundamental physics course. Consider an electric circuit with one battery and one resistor. And ignore the internal resistance of the battery and the resistance of the wires. 1) Does battery supply...
  50. B

    Calculating Drift Velocity in Pure Silicon: Need Help

    I have a small question that can't solve. How long does it take an average electron to drift 1um in pure silicon in an electric field of 10^4 V/m ? drift velocity v_d = I / (n*q*A) but don't know how to calculate can you help me ? thank you.