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Finding Voltage from capacitance, current and time

  1. Nov 11, 2015 #1
    1. The problem statement, all variables and given/known data
    The question asks
    the current through a 2uF capacitor is show. sketch the capacitor voltage Vc between t = 0ms give that the voltage at t=0ms is 0V. Your graph must be correctly scaled with numerical values.

    There is a graph included showing a current rise for 0-2 milliseconds. 0 current for 2-4 milliseconds negative current for 3 milliseconds.

    3. The attempt at a solution
    I understand this question is asking for the difference between graphing Ic and Vc. I have the graph drawn and understand the difference between Ic and Vc in a charging and discharging circuit.

    Now, I must convert my current values into voltage values and then add it to my graph. This is where I am stuck.

    I know that:

    Vc=Vo(E^-t/rc) and ic =-Vo/R(E^-t/rc) are used when a capacitor is charging and discharging.
    both of those have voltages in the formula, so I can't use them.

    I know Q=CV
    I also know coulomb is 1s/A
    can I change that to be A= Δ Q/Δ T ??
    and rearrange it to Q= Δ A x Δ t ??

    I can't figure out how to get the current to voltage!
  2. jcsd
  3. Nov 11, 2015 #2


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    Staff: Mentor

    If you are given the current into or out of a capacitor you know the amount of charge moving into or out of the capacitor (since current is defined to be charge/time). Look up the relationship between charge and voltage for a capacitor. If you know the capacitance and charge at any instant then you can determine the voltage on the capacitor.
  4. Nov 12, 2015 #3


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    Science Advisor
    Homework Helper
    Gold Member

    You are on the right lines by starting with Q=CV. Then differentiate both sides wrt time.
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