1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Current flow

  1. Oct 17, 2015 #1

    GBC

    User Avatar

    Sodium ion flows a through a cylindrical ion channel --> diameter = 0.85 um and is 5 um long.
    potential difference b/w the ends of the channel is 225 mV
    drift velocity = 0.015 m/s
    in a period of 1 ms, a total of 15*10^6 ions exit the channel


    a. total charge exiting the channel in 1 ms =?
    b. current in the ion channel=?
    i have been trying to find b.) but I think my values might be wrong... PLEASE HELP ME!

    2. Relevant equations
    Q=qnV
    I=qnAv=Q/change in time
    change in time = length/drift velocity
    n=N/V


    so for a. using the formula for Q:
    q=1.6*10^-19
    N=15*10^6
    V=l*PIr^2= (5 um)(PI)(4.25*10^-7)^2=2.8*10^-18
    plugging in this value to the equation of Q, I get:
    Q=2.4*10^-12 (acc to the solutions on the book, this is right)

    Now, my problem is b....
    in this part, I used I=Q/t
    i know t = length/velocity so i plug in giving me =(5 um)(0.015 m/s)
    so now that i have t, i just use the Q from a to solve for I and I get 7.2*10^-9.
    however, according to the solutions this is wrong. it is suppose to be 2.4*10^-9...
    I have a feeling my Q value for this question is wrong since it's asking for the current IN the ion channel and the Q-value i have is the total charge exiting the channel... so how do I find the Q IN the ion channel??
    please help me..!!
     
  2. jcsd
  3. Oct 17, 2015 #2

    gneill

    User Avatar

    Staff: Mentor

    You were given the time interval over which the stated number of ions exited the channel. It was 1 ms. That's the timeframe that is important for figuring the current.

    The drift velocity and channel dimensions are related to the current density. The time for a given ion to traverse the channel is not so important when there are many, many ions in a cross-section of the channel.
     
  4. Oct 17, 2015 #3

    GBC

    User Avatar

    Oh Thank you so much! I have never taken physics back in high school so this is all new to me. I was sort of confused on what it meant by "period of 1 ms" (I thought it was a typo for m/s... I have never seen millisecond used before haha) so thank you for explaining it! :)
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Current flow
  1. Current Flow (Replies: 1)

  2. Current flow direction (Replies: 1)

Loading...