Strong Nuclear Force and Electrostatic Force

[garytse86]

there are two forces, the nuclear force which binds protons and neutrons together. However as the size of the nucleus gets larger the electrostatic force of repulsion in the nucleus overcomes the strong nuclear force. so larger atoms breakdown to form smaller atoms, why does the electrostatic force dominate the strong nuclear force. Yes, the electrostatic force will increase in strength because of the increasing number of protons, but surely wouldn't the strength of strong nuclear force increase as well?

 

[mathman]

I'm not sure if this is the entire answer, but the electromagnetic force obeys an inverse square law, while the (derived, i.e. between nucleons) strong force falls off like an exponential over square law. In other words the latter falls off faster.

 

[mormonator_rm]

It is the weak force that is responsible for nuclear decay. The weak force may increase as an atom increases in proton or neutron number in a fashion that, put together with the growing electro-magnetic potential, counters the exponential potential of the residual strong force. This necessitates nuclear decay.

 

[mathman]

There are several types of nuclear decay. The weak force is related to beta emission (neutron becomes proton). Other types of decay, which are related to the strong force, are alpha emission (He4 nucleus emitted) and spontaneous fission (very heavy nucleus breaks up into at least two smaller nuclei and several neutrons). I believe the original question (strong force versus electromagnetic) had to do with the latter.

 

[Yustas]

To answer the original question, nuclear force has a property of "saturation" related to the short-range nature of it mentioned earlier (classical analogy: if you have many marbles in a sack and ask a question how many marbles will any given marble touch at one time, the answer will always be the same -- only the once in its visinity, regardless how many marbles are in there). In other words, due to saturation, strong force acts on nearby nucleons only, while electromagnetic force grows with addition of new protons leading to the instability of very large-mass nuclei.

 

[mormonator_rm]

Yes, this is due to the short life-time (and hence short range) of pions which mediate between nucleons. The residual strong force potential drops off very quickly between nucleons at increasing distances.