In the figure, charge q2 is in static equilibrium. What is q1?
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Since F due to point charges = kq1q2/r^2
There are two forces on q2; one is from the -2 nC on q2 and q1 on q2. Also, notice that q1 and q2 must be positive and larger in magnitude than the -2 nC in order to be attracted to the -2 nC, yet be repeled by the larger positive charge of q1.
Keeping in mind that q2 is positive,
kq2(-210^-9)/(0.1 m)^2 = kq1*q2/(0.2 m)^2
q1 is therefore +8*10^-9 C or +8 nC