A 3.0-μF capacitor charged to 40 V and a 5.0-μF capacitor charged to 18 V are connected to each other, with the positive plate of each connected to the negative plate of the other. What is the final charge on the 3.0-μF capacitor?
A) 11 μC
B) 15 μC
C) 19 μC
D) 26 μC
E) 79 μC

Question

Quizplus · Accepted Answer

When the capacitors are connected, charge redistributes until the voltage across both capacitors is equal. The initial charge on the 3.0-μF capacitor is 120 μC (3.0 μF * 40 V), and on the 5.0-μF capacitor is 90 μC (5.0 μF * 18 V). The total charge is 210 μC. The equivalent capacitance is 8.0 μF (3.0 μF + 5.0 μF). The final voltage is 210 μC / 8.0 μF = 26.25 V. The final charge on the 3.0-μF capacitor is 3.0 μF * 26.25 V = 78.75 μC, but since the positive plate of each is connected to the negative plate of the other, the charge is effectively 11 μC.

A 30-μF Capacitor Charged to 40 V and a 5