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# CBSE Class 12-science Answered

Two equal small spheres each weighing 1g , hang by two equal silk threads from the same point. The spheres are charged when in contact and come to rest with their centres 2cm apart and 20cm vertically below the point of support. Find the charge on each sphere.
Asked by ishantkundu31 | 14 Jun, 2021, 05:00: PM
Expert Answer
Figure shows forces acting on charged sphere. Weight mg is acting downward.  Repulsive electrostatic force is shown as Fe .

Tension force T acting along string is resolved as ( T cosθ ) and ( T sinθ ) .

(Tcosθ) component balances weight mg.   (T sinθ) component balances Electrostatic force Fe .

T cosθ = mg ...................... (1)

T sinθ  = Fe ........................(2)

If we eliminate tension T in above equations, we get ,  Fe = m g tanθ .......................(3)

Eqn.(3) is written as

K × ( q2 / d2 ) = ( m g tanθ ) ........................... (4)

where K = 1/( 4 π εo ) = 9 × 109 N m2 C-2 Coulomb's constant  and d = 2 cm  is the distance between charged spheres.

m is mass of sphere , g is acceleration due to gravity and θ is the angle made by the string with vertical

From eqn.(4) , we get

Answered by Thiyagarajan K | 14 Jun, 2021, 09:41: PM

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