Q: Find the value of source voltage V_s in given figure.

 Solution:

We need to calculate the source voltage. The value of voltage across 2kΩ resistor is 4V, therefore the current passing through it will be

                       I = V/R

                         = (4v/2k)   = 2mA

The similar current is passing through the series combination of 4kΩ & 2kΩ. Thus the value of voltage across 4kΩ resistor

                                                            V = IR

                                                            = (2m) x (4k) = 8 Volts

Hence the total voltage across 2kΩ & 4kΩ resistor

                                                V = (4V +8V) =12V

4kΩ is in series along 2kΩ these can joint as

                                                         = (4k+2k) =6kΩ

6kΩ is in parallel with 6kΩ resistor therefore

                                                 6k||6k = (6x6)/(6+6) = 3kΩ

            The value of voltage across 3kΩ resistor is 12kΩ therefore the source voltage is  

                                                       V = (3/ (3+9)) V_s

                                                       V_s =12 (12/3) = 48V