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Homework Help: Simple Charging RC Circuits

  1. Apr 1, 2012 #1
    1. The problem statement, all variables and given/known data
    You need to modify a simple charging RC circuit (power supply, resistor, and capacitor in series) to maintain the same maximum current but lower the the time constant and make it charge faster. Which of the following will accomplish it?

    A) Same R, lower C.
    B) Same R, higher C.
    C) Higher R, same C.
    D) Lower R, same C.
    E) Lower R, lower C.

    2. Relevant equations
    I=V/R*(1-e^(-t/RC)) for a charging RC circuit.

    3. The attempt at a solution
    In my textbook, it says that resistance is small, than it's easier for current to flow. I think the answer is D, but someone else told me A. Can someone help?
  2. jcsd
  3. Apr 1, 2012 #2


    User Avatar

    Staff: Mentor

    If the resistance is made smaller, won't the maximum current be larger?
  4. Apr 1, 2012 #3
    You have two requirements:
    1) That maximum current stay the same, and
    2) That the time constant be reduced.

    1) What are the factors that determine maximum current?
    1a) What must be done to keep maximum current the same?

    2) What are the factors that determine charging time?
    2a) Keeping the solution of 1a, what can you do to shorten the charging time?
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