Cosmology | Edexcel International A Level Physics Exam Questions 2019

5 marks

The image shown is a representation of our galaxy, the Milky Way.

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Astronomers think that there is a very large concentration of stars in the central region of the galaxy. Outside this central region the concentration of stars is very much less. All stars in the galaxy are rotating about its centre.

It can be shown that the velocity v of a star a distance r from the centre of the galaxy is given by the expression

$v = \sqrt{\frac{G M}{r}}$

where M is the mass of the galaxy contained in a sphere of radius r.

The graph shows how astronomers expect v to vary with r.

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Explain how the expression gives the variation of v with r shown in the graph.

(3)

ii)

The observed variation of v with r has been added to produce the graph below.

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Suggest why the ‘observed’ velocity varies as shown.

(2)

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2 marks

The ultimate fate of the universe may be a closed universe, but astronomers cannot be sure what their current models predict.

Explain why astronomers cannot be sure that the universe is closed.

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1 mark

The diagram shows the Sun rotating about its axis as viewed from the Earth.

A, B and C are points on the surface of the Sun.

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Light from points A, B and C is analysed. The wavelength λ_α of the alpha line in the hydrogen spectrum is determined for the light from each point.

Complete the table with the letters A, B and C to indicate the point corresponding to each wavelength.

λ_α /nm	Point
656.2837
656.2797
656.2757

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2 marks

Explain why there is a variation in the wavelength of the light from different points on the Sun.

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4 marks

In the laboratory, the wavelength of the alpha line in the hydrogen spectrum is 656.2797 nm.
Assess whether this is consistent with the Sun having a period of rotation approximately equal to 28 days.
radius of the Sun = 7.0 × 10⁸m
1 day = 86 400 s

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Cosmology (Edexcel International A Level Physics)

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