IB Physics HL E.5 Fusion and stars questions

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Type
Multiple choiceWritten
Difficulty
EasyMediumHard

Question 1

Paper 2Easy9 marks

Figure 1 is a schematic Hertzsprung–Russell (HR) diagram. The regions are labelled A–D, and the positions of the Sun and star X are shown.

Luminosity, L/L☉30 00010 0005000300010−410−21102104106Surface temperature, T / KDBCASunX
(a)

Identify the stellar type represented by each of the regions A, B, C and D in Figure 1.

[4]
(b)

Using Figure 1, compare the surface temperature and luminosity of star X with those of the Sun.

[2]
(c)

Stars P and Q are both on the main sequence. P has a greater mass than Q. State which star has the shorter main-sequence lifetime.

[1]
(d)

State the likely final remnant of a low-mass star and of a high-mass star.

[2]

Question 2

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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