Four identical capacitors are connected in series with a 10 V battery as shown in the figure. The point N is earthed. The potentials of points A and B are 

All capacitors have equal capacitance. Hence, equal potential drop (=2.5V) will take place across all capacitors.

$V_N-V_B=2.5V$

$0-V_B=2.5V$

Further, $V_A-V_N=3\left(2.5\right)V$

$=7.5\mathrm{V}$

$\therefore V_A=+7.5\mathrm{V}$   $(\text{as }{V}_N=0)$