Two small balls with the same charges, which are in water at a distance of 10 cm from each other

Two small balls with the same charges, which are in water at a distance of 10 cm from each other, are repelled with a force of 4 μN. Find the charge modulus of each of the balls

These tasks: the balls are in the water; r (distance between two small balls) = 10 cm (0.1 m); F (repulsive force of balls, Coulomb force) = 4 μN (4 * 10 ^ -6 N).

Reference values: k (proportionality coefficient) = 9 * 10 ^ 9 N * m2 / Cl2; ε (dielectric constant of water) = 81.

The modulus of the charge of each of the balls is expressed from the formula: F = k * q1 * q2 / (ε * r ^ 2) = k * q ^ 2 / (ε * r ^ 2), whence q = √ (F * ε * r ^ 2 / k).

Calculation: q = √ (4 * 10 ^ -6 * 81 * 0.1 ^ 2 / (9 * 10 ^ 9)) = 1.9 * 10 ^ -8 C (19 nC).