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Homework Help: Calcul q

  1. Oct 11, 2008 #1
    the voltage drop across a certain device and the current through it in the direction of the voltage rop are shown in figure below..Determine:

    the charge q though the device at the end of each 1 s interval from t=0 to t=6s.

    The instantaneous power during the aforementioned intervals.

    the total energy consumed by the device.

    i have a figure where it shows the graph of v(y axis) and time(x axis)

    and another one showing the graph of i(y axis) and time(x axis)

    I am not able to solve this soo please help me..

    i know that i must show that i have tried to solve this..i do not want the spolution..juste a guide

    thanks really for the help
  2. jcsd
  3. Oct 11, 2008 #2


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    Homework Helper

    Welcome to PF!

    Hi qwerty321! Welcome to PF! :smile:

    (always use capitals for V and I :wink:)

    You must learn your electric units …

    current = charge per time, so charge = … ?

    voltage = energy per charge, so energy = … ?

    power = current times voltage …

    and integrate over t. :smile:
  4. Oct 11, 2008 #3


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    The charge is the integral of the current,
    The instantaneous power is the product of voltage and current in the instant considered.
    The energy is the integral of the power.
  5. Oct 11, 2008 #4
    ok i know that..but how can i integrate a graph?
  6. Oct 11, 2008 #5


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    An integral is just the area under a graph (okay, the area between the graph and the x-axis, positive or negative depending on which side it happens to be on). Antiderivatives just happen to be an easier(?) way of obtaining this number than plotting the function and counting graphing paper squares. Or attempting to fit data to a curve, and then finding the antiderivative of that.
  7. Oct 11, 2008 #6


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    Since you have only discrete values in your graph, you can use the trapezoidal rule:
    Let f(t) the value of the function at time t and f(t+T) the value at time t + T.
    So, you have a trapeze with bases f(t) and f(t+T) and height T. The area of the trapeze is:
    [tex]A = \frac{f(t) + f(t+T)}{2T}[/tex]
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