I can't understand how to calculate the voltage between a and b in this open circuit. Resistances like the 9 Ohms and 13 Ohms can be ignored since no current is passing through (In case we connect a Voltmeter, we consider infinite resistance). But, how can I analise or simplify the other circuits?
- What does the 13 ohm resistor bring to the party?
- What does the 9 ohm resistor bring to the party?
- What effect does the 18 ohm resistor have on the 15V source?
- What does the 4 ohm and 11 ohm bring to the party?
- The voltage across the 10 ohm resistor is ONLY due to which current source?
- Ditto the 5 ohm resistor
- Simplify based on these hints.
The answer is 35V from a brief mental add-up of things - it is THAT simple (assuming I've read your scrawl correctly).
The current sources are tricky!
However, it should be straight-forward.
All of the current from the 6 A current source HAS to be going through the 10 ohm resistor. Be sure you understand why that is true before going further. The voltage across the 10 ohm resistor is \$60 \$ Volts.
Also, All of the current from the 8 A current source is going through the 5 ohm resistor. the voltage across the 5 ohm resistor is \$40 \$ Volts.
You already pointed out that the voltage across the 13 ohm and the 9 ohm resistor is zero. The voltage across the 18 ohm resistor is 15 volts by definition.
The voltage at point a with respect to point b is:
\$Vab = + 15 + 60 - 40 = + 35 \$ Volts