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# Two small spherical balls whose charges are $20 \mu c$ each are hanged with help of 1 m long thread from roof. In equilibrium both threads are seperated by $60^{\circ}$ Tension in each thread is :

$(A)\;36 \;N \\ (B)\;72 N \\ (C)\; 7.2 N \\ (D)\;0.72 N$

$T \sin 30^{\circ}=F$
=> $T \times \large\frac{1}{2}$$= 9 \times 10^9 \large\frac{Q^2}{r^2}$
$\qquad= \large\frac{9 \times 10^9 (20 \times 10^{-6})^2}{1^2}$
$T= 7.2 N$
Hence C is the correct answer.