Could someone please explain whats going on for t > 0. For t = 0-, I understand that the capacitor turns into an open circuit and Voc is straightforward to calculate. However, for t > 0, it appears the 12 V voltage supply was removed from the figure in order to find the voltage across the capacitor. Why was the 12 V voltage supply removed from figure.

A similar problem;

In this situation, I'm confused as to why the R3 = 4k ohms and the 6 V power supply were removed in order to find the Voltage across the capacitor for t > 0 but the R1 = 3 k ohms is included.

Now lastly this problem,

It looks slightly different than the previous 2 problems because in this situation, the 12 V power supply is included in the circuit diagram to calculate Vc but yet again the R1 = 3 k ohms is included.

All three situations have caused a difficult conundrum I am battling with as to why

**for t > 0 some parts of the circuits are cut off**.

Any help is greatly appreciated.

Thanks.