Introduction:
In this scenario, we have a square with three fixed charges +q at each corner and an empty charge Q placed at the fourth corner. We need to determine the position of the charge Q such that there is no net electrostatic force on the diagonal opposite charge.

Analysis:
To solve this problem, we need to consider the forces acting on the charge Q from the three fixed charges. The electrostatic force between two charges is given by Coulomb's law, which states that the force between two charges is directly proportional to the product of the charges and inversely proportional to the square of the distance between them.

Steps to find equilibrium:
- Calculate the forces acting on the charge Q from each of the fixed charges +q.
- Determine the direction of the forces based on the charges (repulsive or attractive).
- Find the net force on the charge Q by taking the vector sum of the individual forces.
- Position the charge Q in such a way that the net force on it is zero, ensuring equilibrium.

Explanation:
To achieve equilibrium, the forces from the three fixed charges must balance out such that there is no net force on the charge Q. This can be achieved by positioning the charge Q at the center of the square, equidistant from each of the fixed charges. In this configuration, the forces from the fixed charges cancel each other out, resulting in a net force of zero on the charge Q.

Conclusion:
By placing the charge Q at the center of the square, we ensure that there is no net electrostatic force on the diagonal opposite charge. This equilibrium position is determined by the balance of forces from the fixed charges, leading to a stable configuration.