Question 1

Multiple choiceMedium1 mark

A fusion event in a star is represented below. The labels give the total rest masses before and after fusion. What fraction of the initial rest energy is released?

before fusionreactantstotal rest mass Miafter fusionproductstotal rest mass Mffusionreleased energyMi > Mf
Show answer and explanation

Answer: B. The energy released is the decrease in rest energy, Erel=(Mi−Mf)c2E_{\rm rel}=(M_i-M_f)c^2 The initial rest energy is Ei=Mic2E_i=M_i c^2 so ErelEi=(Mi−Mf)c2Mic2=Mi−MfMi\dfrac{E_{\rm rel}}{E_i}=\dfrac{(M_i-M_f)c^2}{M_i c^2}=\dfrac{M_i-M_f}{M_i}

A: divides the mass defect by the final mass, not the initial mass.
C: gives the fraction of the initial rest energy that remains as product rest energy.
D: takes the reciprocal of the required fraction.
Remember: the energy released in fusion comes from the decrease in rest energy.

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