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Homework Help: Help with total capacitance in parallel and in series

  1. Mar 18, 2008 #1
    According to its design specification, the timer circuit delaying the closing of an elevator door is to have a capacitance of 33.0 µF between two points A and B.
    (a) When one circuit is being constructed, the inexpensive but durable capacitor installed between these two points is found to have capacitance 35.4 µF. To meet the specification, one additional capacitor can be placed between the two points. To meet the specification, one additional capacitor can be placed between the two points. Should it be in series or in parallel with the 35.4 µF capacitor? (in series)

    What should be its capacitance?

    (b) The next circuit comes down the assembly line with capacitance 32.5 µF between A and B. What additional capacitor should be installed in series or in parallel in that circuit, to meet the specification? (parallel)

    a) Ct = C1 +C2
    33uF = 35.4uF + C2
    isn't C2 = 33fF - 34.5uF ??

    b) (1/Ct) = (1/C1) + (1/C2)
    isnt (1/C2) = (1/33uf) - (1/32.5uF) ??

    Im lost here...any help will be apprecieated
  2. jcsd
  3. Mar 18, 2008 #2
    For a you have the right answers, but you did the method wrong. How did you manage to do that? Series capacitors add like parallel resistances and parallel add like series resistances. Swap your two procedures and you should be good.
  4. Mar 18, 2008 #3


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    Consider the two calculation rules for combining capacitors in series and in parallel. You want the effective capacitance, as the result of putting two capacitors together, to be lower than the capacitance of the single capacitor presently in use. Which combination will allow you to come up with smaller capacitance?

    This is a similar question, but now you want to end up with a larger effective capacitance. Will a series or a parallel combination get you a higher value?
  5. Mar 18, 2008 #4
    thanx for the quick reply guys....dynamicsolo, i should have thought about both of ur conclusions...im dumb :s
  6. Mar 18, 2008 #5


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    Don't take it as a sign of being dumb -- unfortunately, equations and formulas in courses like these are not always taught with consideration of what happens to the numbers put into them...
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