question_answer

Two copper balls, each weighing 10g are kept in air 10 cm apart. If one electron from every \[{{10}^{6}}\] atoms is transferred from one ball to the other, the coulomb force between them is (atomic weight of copper is 63.5)     [KCET 2002]

A)            \[2.0\times {{10}^{10}}\]N     

B)            \[2.0\times {{10}^{4}}\]N

C)            \[2.0\times {{10}^{8}}\]N

D)            \[2.0\times {{10}^{6}}\]N

Correct Answer: C

Solution :

                     Number of atoms in given mass \[=\frac{10}{63.5}\times 6.02\times {{10}^{23}}\] = 9.48 ´ 1022 Transfer of electron between balls \[=\frac{9.48\times {{10}^{22}}}{{{10}^{6}}}\] = 9.48 ´ 1016 Hence magnitude of charge gained by each ball. Q = 9.48 ´ 1016 ´ 1.6 ´ 10?19 = 0.015 C Force of attraction between the balls \[F=9\times {{10}^{9}}\times \frac{{{(0.015)}^{2}}}{{{(0.1)}^{2}}}=2\times {{10}^{8}}N.\]