Electric charges of $1\mu C,-1\mu C$ and $2\mu C$ are placed in air at the corners A, B and C respectively of an equilateral triangle ABC having length of each side 10 cm. The resultant force on the charge at C is

The correct option is B 1.8 N
$F_A$ = force on C due to charge placed at A
$=9\times {10}^9\times \frac{{10}^{-6}\times 2\times {10}^{-6}}{(10\times {10}^{-2}{)}^2}=1.8N$
$F_B$ = force on C due to charge placed at B
$=9\times {10}^9\times \frac{{10}^{-6}\times 2\times {10}^{-6}}{(0.1{)}^2}=1.8N$

Net force on C
$F_{net}=\sqrt{(F_A{)}^2+(F_B{)}^2+2F_A{F}_{B}cos{120}^{\circ }}=1.8N$