Bihar BoardEnglish MediumSTD 12 SciencePhysicsNUCLEI3 Marks
Question
Explain the process of thermonuclear fusion with example.
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Answer
→When two light nuclei fuse to form a larger nucleus, energy is released, because binding energy increases during the process. →Some examples of such energy liberating nuclear fusion reactions are : $\begin{array}{l} { }_1 H ^1+{ }_1 H ^1 \rightarrow{ }_1 H ^2+e^{+}+v+0.42 MeV \\ { }_1 H ^2+{ }_1 H ^2 \rightarrow{ }_2 He ^3+n+3.27 MeV \\ { }_1 H ^2+{ }_1 H ^2 \rightarrow{ }_1 H ^3+{ }_1 H ^1+4.03 MeV \end{array}$ →In the first reaction, two protons combine to form a deuteron and a positron with a release of 0.42 MeV energy. →In the second reaction, two deuterons combine to form the isotope of helium ${ }_2 He ^3$. →In third reaction two deuterons combine to form a tritium and a proton. 4.03 MeV energy is released during this process. →For a fusion to take place, the two nuclei must come close enough so that nuclear force is able to affect them. →This force must be strong enough to overcome the repulsive barrier between two positively charged nuclei. →The height of the barrier depends on the charges and radii of the two interacting nuclei. →For example : The barrier height for two protons is $\sim 400 keV$. The temperature required for a proton to overcome this barrier is T . $\begin{aligned} \therefore \quad \frac{3}{2} k T & =400 keV \\ T & =\frac{2 \times 400 \times 10^3 \times 1.6 \times 10^{-19}}{3 \times 1.38 \times 10^{-23}} \\ T & =3 \times 10^9 K \end{aligned}$ →When fusion is achieved by raising the temperature of the system so that particles have enough kinetic energy to overcome the Coulomb repulsive barrier, it is called thermo - nuclear fusion.
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