The electrical repulsion of protons
Two electrical charges of the same sign repel each other. This repulsion increases with the inverse of the square of the distance, as dictated by Coulomb’s law. The protons (in blue) are affected by this repulsion inside the nucleus, as opposed to the red neutrons, whose absence of charge makes them immune. Without the strong nuclear forces capable of overcoming these repulsions, the nucleus would explode. The strong force needs to be very intense to hold the protons together in such a small volume.
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