Question

The charges on two spheres aree +7u and-5uC respectively. they experience a force F. If each of them is given an additional charge of _2uC the new force of attraction will be

Answer 1

Ans: F

F = k x +7 x -5/ r^2 = -35k/ r^2

new charges

+7-2=+5

-5-2=-7

new force

f= k x -7 x +5 / r^2 = -35k/r^2 = F

Answer 2

Given that,

Charge 1, $q_1=7\ \mu C$

Charge 2, $q_2=-5\ \mu C$

To find,

New force when each charge is given additional charge of $-2\ \mu C$.

Solution,

Initial force is given by :

$F=\dfrac{kq_1q_2}{r^2}\\\\F=\dfrac{k\times 7\times 10^{-6}\times 5\times 10^{-6}}{r^2}\\\\F=\dfrac{3.5\times 10^{-11}}{r^2}\ ............(1)$

Charges after giving additional charge,

$q_1'=7-2=5\ \mu C\\\\q_2'=-5-2=-7\ \mu C$

New force,

$F'=\dfrac{kq_1'q_2'}{r^2}\\\\F'=\dfrac{k\times 5\times 10^{-6}\times 7\times 10^{-6}}{r^2}\\\\F'=\dfrac{3.5\times 10^{-11}}{r^2}\ ............(2)$

From equation (1) and (2) we get that that the forces are same when additional charges are given.

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