Question: Q. 7. How are protons, which are positively charged, held together inside a nucleus? Explain the variation of potential energy of a pair of nucleons a function of their separation. State the significance of negative potential energy in this region?
[SQP 2018]
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Solution:
Ans. Nuclear force binds the protons inside the nucleus. $1 / 2$
For Graph and explanation, refer to NCERT page No. 445 2 Significance of negative potential energy. Force is attractive in nature.
[CBSE Marking Scheme 2018]
Detailed Answer :
There are two types of forces : coulomb force and nuclear force which are acting between protons in the nucleus. Gravitational force between two protons is very weak.
Between these two forces, nuclear force is stronger as compared to Coulomb force but it is very short range force. Coulomb force which is repulsive in nature for two protons dominant over nuclear force when distance between two protons increases more than few femtometer. As nuclear size is in the order of femtometer, hence in nucleus, nuclear force between protons dominate over the Columbian force. It is attractive in nature and binds together. Moreover neutrons in nucleus also act as glue to held protons together as there is no coulomb force between protons and neutrons but there is attractive nuclear force between them.
It is clear from the graph, as the nucleons come closer under attractive nuclear force, the potential energy decreases and PE becomes more and more negative. When the distance between two nucleons falls below $10^{-15} \mathrm{~m}$, the nuclear force becomes repulsive and potential energy increases rapidly. 2 Hence negative PE shows that force between nucleons is attractive.
