A LevelPhysicsOCRTopic Questions5. Newtonian World & Astrophysics5.11 EM Radiation From Stars

EM Radiation From Stars (OCR A Level Physics)

Topic Questions

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1a4 marks

This question is about the Sun and its radiation.

a)
i)
Use the data below to show that the luminosity of the Sun is about 4 × 1026 W.

•    radius of Sun = 7.0 × 108 m
•    surface temperature of Sun = 5800 K

[1]

ii)
Sirius, the brightest star in the night sky, has a luminosity 25 times greater than that of the Sun. It has diameter 1.7 times greater than that of the Sun.

Calculate the surface temperature T of Sirius.





T = ..................................... K [3]

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1b6 marks
*b)
A student attends a lecture about the Sun and makes the following notes.

1.
The Sun loses more than 4 × 109 kg of its mass every second to maintain its luminosity.
2.
Treating hydrogen nuclei (protons) as an ideal gas, a temperature of 1010 K provides a kinetic energy of about 1 MeV, which is necessary for fusion.
3.
However, the Sun’s core temperature is only 107 K, so the chance of protons fusing on collision is very small. This explains why the Sun has such a long lifetime.


Explain the principles of physics which are involved in each of the three points.

You should include relevant formulae, but no numbers or calculations are required.

[6]

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2a4 marks

Algol is a triple-star system, with stars Aa1, Aa2 and Aa3 orbiting each other.

This triple-star is 90 light-years from the Earth.

a)
Here is some data on the star Aa1.
• radius = (1.90 ± 0.14) × 109 m
• mass = (6.31 ± 0.42) × 1030 kg.

Calculate the gravitational field strength g at the surface of Aa1 to 3 significant figures.
 
Include the absolute uncertainty in your answer. Assume that the other stars of the system exert negligible gravitational force on Aa1.

g = .............................. ± .............................. N kg–1 [4]

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2b9 marks
b)
The table shows some data about the three stars of Algol.

Star Luminosity of star / LΘ Surface temperature of star / K
Aa1 182 13000
Aa2 6.92 4500
Aa3 10.0 7500


The luminosity of each star is in terms of the solar luminosity LΘ.

i)
Define the luminosity of a star.

[1]

ii)
Use Stefan’s law to determine the ratio fraction numerator r a d i u s italic space o f italic space s t a r italic space A a italic 2 over denominator r a d i u s italic space o f italic space s t a r italic space A a italic 3 end fraction

ratio = .......................................................... [2]

iii)
Use Wien’s displacement law to explain which star would have the longest wavelength at the peak intensity of the emitted electromagnetic radiation.

[2]

iv)
Suggest how an astronomer using just an optical telescope can deduce that the three stars of Algol have different surface temperatures.

[1]

v)
The light from each star passing through a diffraction grating shows an absorption line spectrum.

Explain how a specific absorption line is produced in this type of spectrum in terms of photons and electrons.

[3]

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2c2 marks
c)
The Aa1 star could evolve into a black hole.

State two ways in which the black hole would differ from the Aa1 star.

1. .............................................................................................

2. .............................................................................................


[2]

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3a5 marks
a)
Fig. 21.1 shows some of the energy levels of electrons in hydrogen gas atoms. The energy levels are labelled A, B, C and D.

q21a-paper-1-june-2017-ocr-a-level-physics

Fig. 21.1 (not to scale)

i)
Explain why the energy levels are negative.

[1]

ii)
An electron makes a transition (jump) from level C to level A.

1
Calculate the energy gained by this electron.



energy = ............................. eV [1]

2
Calculate the wavelength in nm of the photon absorbed by this electron.





wavelength = ............................. nm [3]

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3b6 marks
b)
Light from a distant galaxy is passed through a diffraction grating. Fig. 21.2 shows the part of the spectrum of light that shows a strong hydrogen-alpha emission line.

q21b-paper-1-june-2017-ocr-a-level-physics

Fig. 21.2

i)
State how an emission line is produced.

[1]

ii)
State an adjustment that could be made to the experimental arrangement that would space the emission lines more widely.

[1]

iii)
In the laboratory, the wavelength of the hydrogen-alpha emission line is 656.3 nm.

Use Fig. 21.2 to determine the recession velocity of the galaxy.





recession velocity = .................................. m s−1 [3]

iv)
Suggest why hydrogen spectral lines play an important role in determining red shift of galaxies.

[1]

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3c3 marks
c)
Light from a similar star is viewed in a galaxy further away. The star is part of a pair of stars which orbit a common centre of mass.
Describe and explain how the equivalent spectrum might appear.

[3]

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4a1 mark

A group of students are conducting an experiment to determine the wavelength of monochromatic light from a laser.

Fig. 24.1 shows the laser beam incident normally at a diffraction grating.

q24-paper-1-june-2018-ocr-a-level-physics

Fig. 24.1

The students use a diffraction grating with 600 lines mm−1.

They vary the distance x between the grating and the screen from 1.000 m to 2.000 m. They measure the distance y from the central maximum to the second order maximum.

a)
The students decide to plot a graph of y against square root of x to the power of italic 2 plus space y to the power of italic 2 end root.
 
Show that the gradient of the graph is equal to sin θ, where θ is the angle between the central maximum and the second order maximum.

[1]

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4b5 marks
b)
Fig. 24.2 shows the graph plotted by the students.


q24b-paper-1-june-2018-ocr-a-level-physics
Fig. 24.2

i)
Use Fig. 24.2 to determine an accurate value of the wavelength λ of the light from the laser.

λ = ..................................................... m [3]

 

ii)
Suggest why there are no error bars shown in Fig. 24.2.

 [1]



iii)
Suggest how the precision of this experiment may be affected by using a protractor to measure the angle θ.


 [1]

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