Answer:
![F = (4kqQb)/((b^2 + (d^2)/(2))^(1.5))](https://img.qammunity.org/2020/formulas/physics/college/n4fhekay06p5uec4w25bqh1h9olopbgpka.png)
Step-by-step explanation:
Since all the four charges are equidistant from the position of Q
so here we can assume this charge distribution to be uniform same as that of a ring
so here electric field due to ring on its axis is given as
![E = (k(4q)x)/((x^2 + R^2)^(1.5))](https://img.qammunity.org/2020/formulas/physics/college/x6znvps4ey6u8dcbffvywzdhchsuqdq2c3.png)
here we have
x = b
and the radius of equivalent ring is given as the distance of each corner to the center of square
![R = (d)/(\sqrt2)](https://img.qammunity.org/2020/formulas/physics/college/cmh3b07cuq516jaohslbrulec01oopwml6.png)
now we have
![E = (4kq b)/((b^2 + (d^2)/(2))^(1.5))](https://img.qammunity.org/2020/formulas/physics/college/hrlaoer5r67zkbytxec4c88o6p51lcwuod.png)
so the force on the charge is given as
![F = QE](https://img.qammunity.org/2020/formulas/physics/college/592665pmvtioayoof00y7u8wf1amaflkts.png)
![F = (4kqQb)/((b^2 + (d^2)/(2))^(1.5))](https://img.qammunity.org/2020/formulas/physics/college/n4fhekay06p5uec4w25bqh1h9olopbgpka.png)