[theThinker (OP)]

1) Can I assume that Coulombs law applies in water by replacing the the ε₀ for vacuum with
ε_water= ε₀ * water-dielectric-constant (about 80).
2) If 1) is true, what is the force between 2 charges +Q, +Q on the opposite surface of a water tank (assume ideal conditions) with separation d.

Thinker.

[Bored chemist]

Yes, but not for long.
Water conducts electricity
A capacitor using water as the dielectric will self-discharge with a time constant of the resistivity times the permittivity.
Typically 180 µ seconds.
Less if the water is warmer than 20C or contains impurities.

[theThinker (OP)]

I will edit my post:
1) Can I assume that Coulombs law applies in water by replacing the the ε₀ for vacuum with
ε_water= ε₀ * water-dielectric-constant (about 80).
2) If 1) is true, assume two small metal balls charged with +Q and insulated so that the charge cannot escape. They are place far apart distance d in water. What is the force between the metal balls.
 

Thinker.

[Bored chemist]

You can use coulomb's law to calculate the force.
However, unless it's measured very quickly, the water will conduct it to nothing.

[theThinker (OP)]

You can use coulomb's law to calculate the force.
However, unless it's measured very quickly, the water will conduct it to nothing.

But it has been assumed the charges of the two metal balls are insulated and cannot escape.

[Paul25]

I think in that case a different value is assigned to k