Two small insulating spheres are attached to silk threads. The spheres have equa

Question

Two small insulating spheres are attached to silk threads. The spheres have equal masses of 40 g and have electric charges of q1= +2.0 ?C and q2= -2.0 ?C. The spheres are brought into the initial positions shown in Figure (a), with a vertical separation of 15 cm between them.
In the figure, the upper thread is slowly pulled upward, while point B is kept fixed. When point A has been raised through a height h, the lower sphere suddenly falls, as shown in Figure (b). The height h, is closest to:

Two small insulating spheres are attached to silk threads. The spheres have equal masses of 40 g and have electric charges of q1= +2.0 ?C and q2= -2.0 ?C. The spheres are brought into the initial positions shown in Figure (a), with a vertical separation of 15 cm between them. In the figure, the upper thread is slowly pulled upward, while point B is kept fixed. When point A has been raised through a height h, the lower sphere suddenly falls, as shown in Figure (b). The height h, is closest to:

Explanation / Answer

In the above case in initial position the gravitational force is balanced by the electric force which is

greater

But in second case when h is so that electric force decreases to just less than gravitational force the ball moves downward.

I am assuming charges are in C

Taking h in cms

So

Fe = mg

=> 9*109*(2*10-6)2/(15+h)2*10-4 = 40*10-3*9.81

=> (15+h)2 = 917.43

=> h = 30.289-15 = 15.289 cm 15.3 cm

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