Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Current = nqvA

  1. Nov 21, 2012 #1
    Hi all,

    I was wondering whether someone might help further understand the following equation for current. I mean get a good picture of how each valuable relates to each other and how they come to represent current.

    Current = nqvA

    Thanks. I know what each valuable represents but I can't seem to get a good picture how the equal represents current especially; Where time is taken into account since current is the rate of charge carriers/electrons.

    I hope I have made my self clear.

  2. jcsd
  3. Nov 21, 2012 #2
    Oops. No need I understand it now.

    ...but I would appreciate any replies just to make sure.
  4. Nov 21, 2012 #3


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    Could you please say what the variables in that equation all stand for? Where did you find that equation and what was the context? I have a problem with balancing it, dimensionally if the variables are what I assume they are.
  5. Nov 21, 2012 #4
    n = density of charge carriers (1/m^3)
    q = charge of a charge carrier. (coulomb)
    v = average speed of charge carriers. (m/s)
    A = area of cross-section of wire m^2.

    If you multiply them together you get coulomb/s = Ampere
  6. Nov 21, 2012 #5


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    Just checking we're reading from the same hymn sheet..
    This is the 'well known' relationship in which the dimensions are the same each side (that should be reassuring). The way it's derived is to consider a cylinder with of area A and unit length, moving at average speed v. The number of charges in the cylinder will be n times the volume, which is nA (times one, the unit length) so the charge in it will be nqA (per unit length) and the number of charges passing a point in one second will be nqAv. That's I, the current.
  7. Nov 21, 2012 #6

    I remember it as:

    n or p (holes or electrons) = charge carriers
    q = charge per carrier --±1.6E-19 C/(electron or hole)
    v = velocity
    A = area

    So n*q*v*A or p*q*v*A equals current

    Sometimes J (current density) is used. J = I/A = q*(p or n)*v.

    EDIT: Sorry, I should have written in the first line, "concentration of n or p (holes or electrons) = charge carriers/m^3 .

    Last edited: Nov 21, 2012
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook