When the charged polystyrene ball of mass 0.14 g is suspended by a nylon thread from a fine glass spring in the absence of any electric field, the spring extends by 30 mm.
If the spring constant of the glass spring is k, `0.14*10^-3*9.8 = k*30*10^-3`
=> `k = (0.14*9.8)/30 = 343/7500`
When there is an electric field of strength `2*10^5 V/m` acting vertically upwards, the spring is extended further by `6*10^-3` m.
If the ball has a charge X C, the force due to the field is 2*10^5*X
`2*10^5*X = (343/7500)*(6/1000)`
=> `X = 1.372*10^-9 C`
The electric charge on the polystyrene ball is `1.372*10^-9` C
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