JEE Main & Advanced
Physics
Electrostatics & Capacitance
Question Bank
Electric Field and Potential
question_answer
Two small spherical balls each carrying a charge \[Q=10\mu C\] (10 micro-coulomb) are suspended by two insulating threads of equal lengths 1m each, from a point fixed in the ceiling. It is found that in equilibrium threads are separated by an angle \[{{60}^{o}}\] between them, as shown in the figure. What is the tension in the threads (Given: \[\frac{1}{(4\pi {{\varepsilon }_{0}})}=9\times {{10}^{9}}Nm/{{C}^{2}}\]) [MP PET 2001; Pb PET 2003]
A) 18 N
B) 1.8 N
C) 0.18 N
D) None of the above
Correct Answer:
B
Solution :
In the following figure, in equilibrium\[{{F}_{e}}=Tsin\text{ }{{30}^{o}}\], r = 1m Þ \[9\times {{10}^{9}}.\frac{{{Q}^{2}}}{{{r}^{2}}}\]=\[T\times \frac{1}{2}\] Þ \[9\times {{10}^{9}}.\frac{{{(10\times {{10}^{-6}})}^{2}}}{{{1}^{2}}}=T\times \frac{1}{2}\] \[\Rightarrow \]T = 1.8 N