With a potential difference of 21.9V across the combination, what is the potenti

Question

With a potential difference of 21.9V across the combination, what is the potential at point a with S open? (Let V = 0 at the negative terminal of the source.) I found the answer to be 7.30 V. I also found that the potential at point b with the switch open is 14.6 V. When the switch is closed, the final potential at point b is 7.30 V.

So, now I am down to the fourth part of this problem which reads: How much charge flows through the switch S after it is closed? I have tried 8.03 E-6 C and 4.818 E-6 C -- both of which are incorrect. I would appreciate some assistance on this last question.

Explanation / Answer

Part IV:

Before the switch is closed, the potential at point b = 14.6 V, then the potential difference across capacitor C2, V2 = 14.6 V and across C1, V1 = 21.9-14.6 = 7.3 V.

The charge on C1, Q1=C1*V1 = 7.3*0.44 = 3.212 uC

The charge on C2, Q2 = 14.6*0.22 = 3.212 uC

Now switch is closed and new charges across capacitors are:

Q2 = 0.22*7.3 = 1.606 uC

Q1 = 14.6*0.44 = 6.424 uC.

The charge flow through switch will be sum of change of charges of negative terminals of C1 and positive terminal of C2.

Then the charge flow = final sum - initial sum = (-6.424+1.606)-(-3.212+3.212) = -4.818 uC

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