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Homework Help: Quick question of electricity flow

  1. Feb 7, 2005 #1
    Measurements with microelectrodes have shown that a 0.30-nm-diameter potassium ion (K+) channel carries a current of 1.8 pA.

    How many potassium ions pass through if the ion channel opens for 1.0 ms?

    What is the current density in the ion channel?

    I am completely lost for this one. Anyone know of any equations that I can use?

    Current is, its I = dQ/dt, the flow of charge through a point, through time. Like measuring the flow of water through a pipe.
    The area the flow is going through (the inner area of the "pipe") because they give you the diameter. area = pi*r^2, 2*r = diameter.

    I = dQ/dt = n*A*q*dx/dt = n*A*q*Velocity
    so first solve I = n*A*q*Velocity

    then use that with velocity = dx/dt to get dx.
    Then use dQ = (n*A*dx)*q
    to get dQ.

    But I am stuck on how to solve for the variables and how to complete the problem. Can anyone help me out with the answer?
  2. jcsd
  3. Feb 7, 2005 #2


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    Staff Emeritus
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    Needless to say, you're making this more difficult than you need to. What's the definition of an ampere? One ampere is one coulomb of charge moving past a fixed point each second. Each potassium ion has a fixed charge, +e. One coloumb of charge contains 6.25 * 10^18 such ions.

    [tex]1.8 pA = \frac{1.8 \cdot 10^{-12} C}{s} \cdot \frac{6.25 \cdot 10^{18} \textrm{ ions}}{C} \cdot 1 \cdot 10^{-3} s[/tex]

    Perform the multiplication and get your answer in number of ions.

    - Warren
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