Question

You need a capacitance of 40.0 uF, but you don't happen to have a 40.0 uF capacitor. You do have a 90.0 uF capacitor.

Part A 

What additional capacitor do you need to produce a total capacitance of 400 uF? 

Express your answer with the appropriate units. 

Part B 

Should you join the two capacitors in parallel or in series? 

Answer 1

We have a capacitor of 90uF. But we have need of 40uF capacitor.

Let's additional capacitor have capacitance C.

Resultant of both capacitors is 40 uF, that is less than from available capacitance so both capacitors should be in series.

40 = (90 x C) /(90 + C)

90 + C = 2.25C

1.25 C = 90

  C = 72 uF

(b)

In series combination.