When we are introduced to the Strong Nuclear force, we are told that it prevents the nucleus from flying apart because of the electric repulsion between protons. But there is no such repulsion between a pair of neutrons. So what is the significance of the Strong interaction between a pair of Neutrons?
2 Answers
The significance is that you can build bigger nuclei and the presence of neutrons is essential to have any stable nucleus beyond hydrogen.
The strength of electromagnetic repulsion between protons means that even a nucleus with just two protons is unstable. Adding neutrons means that in a nucleus with a given proton number, the protons have a greater separation on average, hence reducing their mutual repulsion, but all the nucleons are still being bound together with their adjacent nucleons by the same strong nuclear force (which is blind to whether the nucleon is a proton or neutron).
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ProfRob is right; a further simplification is that you can think of neutrons as if they were the "glue" that holds protons together in close proximity against their mutual electrostatic repulsion.
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