posted by bad at physics .
According to its design specification, the timer circuit delaying the closing of an elevator door is to have a capacitance of 30.0 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 33.6 µF. To meet the specification, one additional capacitor can be placed between the two points. Should it be in series or in parallel with the 33.6 µF capacitor?
What should be its capacitance?
(b) The next circuit comes down the assembly line with capacitance 29.3 µF between A and B. What additional capacitor should be installed in series or in parallel in that circuit, to meet the specification?
a. C = C1*C2/(C1+C2) = 30 uF
33.6*C2/(33.6+C2) = 30
33.6C2 = 1008+30C2
33.6C2-30C2 = 1008
3.6C2 = 1008
C2 = 280 uF In series.
b. C1 + C2 = 30 uF
29.3 + C2 = 30
C2 = 30 - 29.3 = 0.7 uF in parallel.