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Homework Help: Electrophysiology Quick Homework Problems

  1. Apr 28, 2017 #1
    1. The problem statement, all variables and given/known data
    Hi guys,
    I've been trying to solve these two questions for the last hour and can't seem to find the solution, any help would be very much appreciated. :smile:

    1. An RC circuit is given a step voltage (charging) of V at t = 0 seconds. What is the value of voltage at t = 2RC?

    2. If the conductance of a single ion channel is 1pS and its capacitance per cm2 is 1mF and the time constant for a human neuron is 1ms:
    (a) What is the resistance per cm2 of a neuronal membrane?

    (b) How many ion channels on average must there be in one µm2 of neuronal membranes?

    2. Relevant equations
    T = RC
    Tau = Resistor x Capacitor

    3. The attempt at a solution
    1. Tried attempting this question for a solid hour and not sure where to begin.
    (a) 1 x 10^-3ms = R x (1 x 10^-6F)
    R = 1000W per cm2
    (b) I know to divide the total current by the single current but I can't seem to find the solution to identify the total current.
  2. jcsd
  3. Apr 28, 2017 #2


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    Staff: Mentor

    Welcome to the PF.

    On question -1-, what is the equation for v(t) for an RC circuit that starts charging at t=0? You are correct that the time constant is RC, but how is that used in the actual equation?
  4. Apr 28, 2017 #3
    Hi and thank you for the reply,
    I believe the equation is V(t) = V(1-e^-t/RC). If you require additional information please ask, and thank you for the help.
  5. Apr 29, 2017 #4


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    Staff: Mentor

    Assuming the equation is correct, any particular reason to not just plug t=2RC into?
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