Hi folks.
I have my examinations soon and there's no one around who can help me.

I stuck with Worksheet "Time constant calculations", Question 28, Figure 4: can't understand how to determine initial and final voltage for a capacitor when it is a part of a resistor series-parallel combination circuit. Why should the answer be V(C initial)=10V and V(C final)=12V????

Your help will be very much appreciated

 

Here is the schematic I'm working on:

http://sub.allaboutcircuits.com/images/quiz/01809x01.png

 

Its quite simple. For the 4th diagram, initially the switch is open, so the voltage across the capacitor is given by 12(5)(5+1) = 10 volts (Potential divider rule. Just remove the switch as it's open).

When the switch is closed, it shorts the 1 ohm resistor, and the capacitor will charge towards 12V. So the final potential will be 12V.

Potential at any time after closing will be given by V = 10(initial voltage) + 12(1-exp(-t/RC))
Where R = 6 ohms, C = the capacitance, and t is the time at any instant in seconds.