Electrostatic force has longer range than strong nuclear force? In my revision guide it says that the electrostatic repulsion between protons in a nucleus has a longer (indefinite, actually) range than the strong nuclear force keeping the nucleons together. So I thought: If this is true, what happens at very large separations, e.g. in stable atoms with a large nucleus. The strong nuclear force wouldn't be holding the nucleons together, so what is keeping them from flying apart?
Thanks
 A: You answered to your own question, I believe.
The electrostatic force has an infinite range and it falls down with distance in an inverse square power law
$$ F_e \propto \frac{1}{r^2} $$
while the strong nuclear force behaves quite differently as it exhibits a property called confinement.
Confinement is still to be understood (it is related to one of the millennium prizes), but heuristically you can think of it as a force (for example acting on a pair of quark and anti-quark) that grows linearly with distance
$$F_s \propto r$$
just like a stretched spring. Although quite stronger than the electrostatic force, it happens that $r$ cannot stretch ad infinitum as the spring breaks into two pieces

and as such there is somehow a maximum distance after which the strong force does not attract any more.
As such nuclei, as you pointed out, will not be stable after some size as the electrostatic force will be the predominant force, which is repelling protons from each other.
Other references:
http://en.wikipedia.org/wiki/Strong_interaction
http://en.wikipedia.org/wiki/Millennium_Prize_Problems#Yang.E2.80.93Mills_existence_and_mass_gap
