Question
Explain Yukawa's Principle for the origin of nuclear forces.
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Answer
Yukawa's Theory for the Origin of Nuclear Forces : In 1935, Japanese scientist Yukawa attributed the origin of these forces to a new particle called 'π-meson' which, later on, was discovered experimentally in cosmic rays. The rest mass of a π-meson is a little more than the rest mass of electron but less than the mass of proton or neutron. Mesons are of three types, positively charged, negatively charged and uncharged $\left(\pi^{+}, \pi^{-}, \pi^0\right)$. The magnitude of charge on a charged meson is equal to electronic charge. According to Yukawa, there is a cloud of π-mesons around every nucleon (proton and neutron). The difference between proton and neutron is only due to different structures of meson clouds around them. There is a continuous exchange of π-mesons between protons and neutrons due to which they continue to be converted into one another.
According to Yukawa, when a proton and a neutron interact, the proton emits a positively charged π-meson which is absorbed by the neutron. As a result, the proton is converted into neutron and the neutron is converted into proton:
$p - \pi^{+} \rightarrow n; n + \pi^{+} \rightarrow p$
Similarly, when a neutron emits a negatively charged π-meson which is absorbed by a proton, then the neutron is converted into proton and proton is converted into neutron:
$n-\pi^{+} \rightarrow p ; p+\pi^{-} \rightarrow n$
The exchange of $\pi^{-}$ and $\pi^{+}$ mesons between protons and neutrons is responsible for the origin of nuclear forces between them. Similarly, nuclear forces between two protons and between two neutrons are generated by a continuous exchange of $\pi^{0}$-mesons between them. Thus, the basis of nuclear forces is the exchange of mesons and hence these are also called 'exchange forces'.
Since mesons are continuously moving between the nucleons, so their mass is not added to the mass of the nucleus.
According to Yukawa, when a proton and a neutron interact, the proton emits a positively charged π-meson which is absorbed by the neutron. As a result, the proton is converted into neutron and the neutron is converted into proton:
$p - \pi^{+} \rightarrow n; n + \pi^{+} \rightarrow p$
Similarly, when a neutron emits a negatively charged π-meson which is absorbed by a proton, then the neutron is converted into proton and proton is converted into neutron:
$n-\pi^{+} \rightarrow p ; p+\pi^{-} \rightarrow n$
The exchange of $\pi^{-}$ and $\pi^{+}$ mesons between protons and neutrons is responsible for the origin of nuclear forces between them. Similarly, nuclear forces between two protons and between two neutrons are generated by a continuous exchange of $\pi^{0}$-mesons between them. Thus, the basis of nuclear forces is the exchange of mesons and hence these are also called 'exchange forces'.
Since mesons are continuously moving between the nucleons, so their mass is not added to the mass of the nucleus.
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