In the circuit shown below, a battery of  emf E=36v is connected between P and Q. What is the potential difference between a and b? 

 

For resistors branch
$36-V_{\text{a}}=\frac{6}{6+3}.36$
              $=\frac{6}{9}\times 36$
$36-V_{\text{a}}=24$
$V_{\text{a}}=36-24=12V$
For capacitors branch
$36-V_b=\frac{3}{6+3}\times 36$
$36-V_b=3\times 4$
$V_b=36-12=24V$
$V_{\text{a}}-V_b=12V$