Modelling the Charging of a Capacitor

Discussion in 'Homework Help' started by Sci, Feb 25, 2005.

  1. Sci

    Thread Starter New Member

    Jan 21, 2005
    I consulted some text books, which gave me what I attached as a screenshot, for dv/dt of a capacitor from the time it is just to be about charged. To cut a long story short, I'm suppose to model it using the analytical equation and Euler's method. However the voltage DE I got from the text book itsn't working out. It says:

    (dv/dt) = (v - Vs) / RC,
    where v is the voltage across the cap, and Vs is the applied voltage. Using this equation makes me end up with a neg, or just wrong voltage as opposed to the corrent analytical equation of

    v = Vs (1 - e^(-t/RC) )

    What's the proper DE?
  2. hgmjr

    Retired Moderator

    Jan 28, 2005
    Is it possible that you overlooked the negative sign in the expression shown in your screenshot.

    It reads dv/dt = - (v-Vs)/RC. This is equal to dv/dt = (Vs-v)/RC.

    Hope this help?

    Good Luck.