IB Physics SL E.5 Fusion and stars questions

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Question 1

Paper 2Medium7 marks

In the core of a young star, deuterium nuclei fuse with protons according to

12H+11H→23He+γ.{}^{2}_{1}\mathrm{H}+{}^{1}_{1}\mathrm{H}\rightarrow{}^{3}_{2}\mathrm{He}+\gamma .

Figure 1 shows the binding energy per nucleon for the nuclei relevant to this reaction. The plotted values are approximate.

01234012345678Nucleon number, ABinding energy per nucleon / MeV11H21H32He42He

Use 1 MeV=1.60×10−13 J1\ \mathrm{MeV}=1.60\times10^{-13}\ \mathrm{J}

(a)

Use Figure 1 to determine the energy released per reaction. Give your answer in MeV to two significant figures.

[3]
(b)

At a particular stage, this reaction occurs at a rate of 2.0×1037 s−12.0\times10^{37}\ \mathrm{s^{-1}} Calculate the energy released per second by these reactions. Give your answer in W to two significant figures.

[2]
(c)

The reaction rate then falls to half its value in (b), while the energy released per reaction remains unchanged. Determine the new energy released per second and explain how it compares with your answer to (b).

[2]

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