Question

# A small sphere with mass 1.2 g hangs by a thread between two parallel vertical plates 5 cm apart. The plates are insulating and have uniform surface charge densities +δ and −δ . The charge on the sphere is q=9×10−6C. What potential difference (in V) between the plates will cause the thread to assume an angle of 37∘ with the vertical as shown in figure?

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Solution

## According to the question FBD can be given as From the FBD we can say that tan37∘=FEmg⇒34=qEmg⇒E=3mg4q To find out potential we multiply both sides with ′d′ which is the distance between the plates ⇒E.d=3mgd4q Putting the values V=3×1.2×10−3×10×5×10−24×9×10−6=50 V

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