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P7 - DOC


  • pg 1
									P7 Observing the Universe
Workbook answers

1               For example:

                    Telescope                     Type of electromagnetic         Location
                                                  radiation detected

                    the Monument                  visible light                   London

                    Effelsberg                    radio                           Germany

                    Hanbury Brown                 radio                           Australia

                    XMM                           X-rays                          low Eart h orbit

                    Calar Alto                    visible light                   Spain

                    McMath-Pierce solar           visible light                   Arizona, USA

2                     Large telescopes can gather weak radiation from a distant source, producing a brighter
                       image by focussing the radiation.
                      Some telescopes use detectors that are sensitive to radiation from parts of the
                       electromagnetic spectrum that are invisible to the human eye.

3               Ticked phrases:
                      track a distant source…
                      scan a distant source…
                      follow instructions from an astronomer…
                      move quickly and point…
                      operate for more hours…

4               See bottom diagram on Textbook page 184.

5               Its surfac es are more curved, making it fatter.

6    a          P = 1/f where f is measured in metres

     b    i     P = 1/0.5 = +2 diopters (D)

          ii    P = 1/0.4 = +2.5 D

          iii   P = 1/0.2 = +5 D

          iv    P = 1/5.0 = +0.02 D

7               Missing words: same size, bigger, inverted, real

8    a          The lens closest to the eye is the eyepiece; the furthest lens is the objective lens.

     b          The eyepiece is fatter and has a smaller diameter than the objective lens.

9               C, C, D, C, D

Workbook answers – P7 - Observing the Universe                                                Page 1 of 10
10   a         How much nearer the telescope makes the object appear to be located.

     b         It spreads stars out, so you can see that a cluster is made of distinct stars.


                   Telescope              Focal length of        Focal length of          Magnification
                                          objective lens         eyepiece lens

                   A                      50 cm                  5 cm                     50/5 = x10

                   B                      80 cm                  5 cm                     80/5 = x16

                   C                      5.0m                   5 cm                     500/5 = x100

                   D                      1000 mm                4 mm                     1000/4 = x250

                   E                      2000 mm                10 mm                    2000/10 = x200

     d         D

11             It makes stars bright er, so you can see more of them.

12   a         R

     b         P

     c         Q

     d         P would be the objective lens, R the eyepiece.
               Magnification = 500/25 = x20

13             The lens glass refracts (changes the direction of) light of different colours by different
               amounts. This means you cannot form a clear image of the distant object at a single plane.
               It will be blurred. (P upils may show this on the diagram: the image plane would be a vertical
               line drawn bet ween the blue focus and the red focus, and intersecting where the light rays
               A mirror reflects light of all colours exactly the same way, so the image will be sharper.

14             1 The aperture can be larger, giving a brighter image.
               2 The aperture can be larger, giving great er resolving power.
               3 It avoids the problem of distortion produced by uneven composition of lens glass.

15             See top left diagram on Textbook page 192.

16             See diagrams on Textbook page 193.

17             Missing words: resolving power, separate, aperture, wavelength, electromagnetic, diffraction

18                    High resolution telescope — bottom, smaller spots
                      Medium resolution telescope — middle, medium-size spots
                      Low resolution telescope — top, larger spots

19             1 Bigger apertures gather more light from distant sources.
               2 Bigger apertures mean greater resolving power.

Workbook answers – P7 - Observing the Universe                                               Page 2 of 10
20             For example:
               1 Telescope mountings must support the telescope and enable it to move and maintain an
               2 The telescope will expand and contract as its temperature changes. This must not affect
               image formation.


               The star emits light in all directions. The distant telescope gat hers light for such a tiny cone
               of radiation from the star that all rays are effectively parallel.

22   a

     d         It is a real image, because light will form an image on a screen at that location.

23   a

     c         An object in the Solar System, e.g. the Moon, a planet or the Sun

24             A large diameter means it gathers more light. A weak lens means the image will be larger,
               improving the resolution of the stars.

25   a         Rotate by 4/24 x 361 ≈ 60 around the Pole Star

     b         The Eart h rotates by 361 on its axis every 24 hours.

     c         It is aligned with the Earth’s axis (directly above the North Pole).

Workbook answers – P7 - Observing the Universe                                                Page 3 of 10
26   a

     b         The Eart h rotates 361 in 24 hours. The star is in the same position after 360  rotation. 1
               rotation takes 4 minutes.

     c         4 minutes ≈ 1 rotation
               365 days x 1 = 365 ≈ 360, or 1 rotation

27   a         23 hr 56 min
               24 hr 49 min

     b         The stars appear to travel across the sky because of the Earth’s rotation, with 360 taking
               about 23 hours 56 minutes. Meanwhile the Moon is orbiting the Earth once every month, so
               it must drift 360 against the star pattern in this time. This is equivalent to about 13 drift
               each day.

28   a         A7, B5, C3, D1, E2, F8, G4, H6

     b         We see the Moon by light reflected from the Sun. As the Moon orbits the Earth, we see
               different amounts of its surface that faces the Sun and so is lit. The shape of the li t surface
               changes too, and these shapes are called ‘phases’. The cycle of shapes seen corresponds
               to one orbit of the Moon around the Earth.

29             See left diagram on Textbook page 206.

30             The night sky is what we see facing away from the Sun, as the Earth rotates daily. As the
               Earth orbits the Sun through the year, the night sky contains different constellations in each
               The left side of the diagram is the winter sky at night; the right side is the summer sky at

Workbook answers – P7 - Observing the Universe                                                 Page 4 of 10
31             The Eart h is nearer the Sun than Mars, so it moves round its orbit more quickly. As the Earth
               overtakes Mars, Mars appears to move backwards against the background star pattern.

32   a    i

     b         When the Moon moves through the large shadow cast by the Earth, the eclipse will be seen
               from many places on the Earth that night. The shadow cast by the Moon in a solar eclipse is
               very small, so the eclipse is only seen at particular places on the Earth.

33             See the lower diagram on Textbook page 209.
               Because the Moon’s orbit of the Earth is tilted relative to the plane of the Earth and Sun, the
               Moon seldom comes between Earth and Sun, and it seldom passes through the Earth’s

34             For example:
               I look at my finger, held an arm’s length away, first with one eye and then with t he other. The
               finger appears to jump from place to place against its background. When astronomers look
               at a star after a 6-month int erval, the Eart h has moved to the opposite side of its orbit.
               Nearby stars will ‘jump’ against the background of distant stars in the same way.

35   a         See the diagram on Textbook page 212.

     b         The positions of the Earth, 6 months apart, form the base of a triangle. A more distant star
               makes a smaller angle at the top of the triangle. This is the parallax angle.

36             Missing words/numbers: 360, 60, 60, parsec, 1 second, 3.26


                Parallax angle          Di stance
                (seconds of arc)        (parsecs)

                0.769                   1.3

                0.1                     10

                0.025                   40

                0.0125                  80

                0.06                    16.7

                0.01                    100

Workbook answers – P7 - Observing the Universe                                                 Page 5 of 10
38             Size, temperature


               The energy radiated by a star spreads out over a larger and larger surface, with distance
               from the star. Greater luminosity means more energy radiated every second. Greater
               distance means less of that energy will be det ected by a telescope.

40             Rigel is hotter that Betelgeuse. Betelgeuse is closer to Eart h than Rigel (about half the
               distance of Rigel).

41             Any star radiates a continuous range of frequencies across the electromagnetic spectrum.
               The peak frequency will depend on a star’s temperature. Using a spectrometer, astronomers
               can measure the int ensity of radiation from a star at each frequency detected by their
               telescope. The peak frequency tells them the star’s temperature.

42             Temperat ure, distance

43   a         The hottest star has the tallest peak; the coolest star has the lowest peak.

     b         Its total luminosity

44   a         It changes in a regular way approximat ely every 5½ days.

     b         Luminosity and period

     c         Correct order: 3, 2, 4, 1, 5

45   a         Missing words: Univers e, nebulae, Shapley, Milky Way, galaxies, Curtis

     b         Edwin Hubble us ed Cepheid variable stars to measure distances to Andromeda. His results
               showed that it is outside the Milky Way.

46   a         A collection of thousands of millions of stars held together by gravity

     b         Infrared radiation is not absorbed by interstellar dust so very distant stars c an be seen.

     c         It takes many people to design a telescope like this, and to analyse the data it produces.

     d         Missing words: parsecs (pc), megaparsecs (Mpc )

47             Missing words: spectra, Cepheid variables, Hubble, away from, speed of recession

Workbook answers – P7 - Observing the Universe                                                Page 6 of 10

                   Speed of recession              Hubble constant                 Di stance

                   5000 km/s                       70 km/s per Mpc                 71 Mpc

                   3500 km/s                       72.9 km/s per Mpc               48 Mpc
                                                           –18         –1                   21
                   7080 km/s                       2.3 x 10        s               3.08 x 10     km
                                                              18       –1                   20
                   2000 km/s                       2.3 x 10–       s               8.70 x 10     km
                                                              –18       –1                  21
                   3000 km/s                       2.44 x 10        s              1.23 x 10     km

49   a         Present in the star: calcium , helium , hydrogen , iron , sodium 

     b         This is a very young star. Helium has been produced by the fusion of hydrogen, but the
               fusion of helium has not yet started, so there are no heavier elements present.

50   a         Electrons

     b         Only phot ons associated wit h changes in electron energy levels can be produced. This
               means light of particular frequencies in a line spectrum. A continuous spectrum contains
               light of all frequencies across a range.

     c         If the atom absorbs photons with en ergy of 13.6 or more, an electron will have sufficient
               energy to leave the atom. What is left behind is an ion. The radiation is ionizing.
               If the photons have energies less than 13.6, electrons can be excited but none will leave the
               atom. This radiation is non-ionizing.

51   a         So that the only interactions taking plac e are between alpha particles and gold at oms in the

     b                Observations: Most radiation is only slightly deflected. Fewer than 1 alpha particle in
                       8000 is deflected by an angle greater than 90.
                      Conclusions: The nucleus, containing protons (positive charge), is very small compared
                       to the size of atoms. Alpha particles are repelled by the nucleus if they get close
                       enough. Most alpha particles pass through the atom without getting close to the

     c         Largest to smallest: gold atom, alpha particle, gold nucleus, electron

52   a         Protons, neutrons

     b         The mass of a nucleon is (about 2000 times) greater than the mass of an electron.

Workbook answers – P7 - Observing the Universe                                                   Page 7 of 10

                                                 Name of force         Particles that the        Range of the
                                                                       force acts on             force
                    Force holding particles      strong                neutrons, protons         10 m
                    together in nucleus
                    Force pushing particles      electrical            protons                   10       m
                    apart in nucleus             (electromagnetic)

53                    Repulsion: An electrical force bet ween objects with like charges pushes them apart.
                       Nuclei contain protons, whic h have positive charge.
                      Attractive force: The strong nuclear force between all nucleons (protons and neutrons)
                       pulls them toget her.
                      Kinetic energy: The energy associated with the motion of an object = mv /2 where m is
                       its mass and v is its speed.
                      Conditions: Pressures and temperatures are very high, so collisions between nuclei are
                       hard enough to bring them within range of the strong force.

54   a          Pressure, temperature, volume, mass

     b                The billions of molecules in a gas… — move around freely in what is mostly empty
                      The speed of the molecules… — is related to the temperature of the gas.
                      As the molecules move around, they… — collide with each other and with the walls of
                       their container.
                      Each collision with the walls… — causes a tiny force.
                      The tiny forces from molec ular collisions with the walls… — together produce gas
                       pressure on the walls.

     c    i     Collisions happen more frequently (because there is less space to move around in), so the
                force on the container walls is greater (pressure rises).

          ii    Collisions happen more frequently (because the molecules travel faster to their next
                collision), so the force on the container walls is great er (pressure rises).

          iii   The molecules are travelling faster, so must travel larger distanc es between collisions if the
                pressure remains the same. The volume must be larger.

          iv    The molecules are no longer moving so there is no pressure resulting from collisions.

55   a          Missing values: –273, 0

Workbook answers – P7 - Observing the Universe                                                  Page 8 of 10

                   Temperature (K)        Temperature (C)

                   373                      100

                    35                    –238

                   783                      510

                   173                    –100

                   258                    –15

                    77                    –196

56             Missing words: colour, luminosity, H–R diagram, whit e dwarf, red giants, data, models, main
               sequence, small part

57   a                  A — core — fusion of hydrogen takes place
                        B — radiative zone — energy is carried by photons
                        C — convective zone — energy is transferred by convective cells
                        D — photosphere — energy radiat es into space

     b         Missing words: mass, rate it radiates energy

58   a         Correct order of statements: 4, 3, 1, 2

     b         Gravity causes the volume of hydrogen to get smaller. As the volume is reduced, pressure
               and temperature rise.

     c         E very bit of mass is attracting every other bit, pulling them toget her. When they are all as
               close as possible, the resulting shape is a sphere.

59   a                  Similarities: all stars are formed from clouds of dust and hydrogen. All stars are main
                         sequence stars for some time.
                        Differences: What happens at the end of their life cycle depends on star mass.

     b         Missing words: hydrogen, decreases, gravitation al, kinetic, increases, helium, carbon,
               oxygen, red giant

60   a         Missing words: white, dwarf

     b         Correct order of statements: 4, 2, 3, 1

61   a         For example:
               1 ESO operates observat ories in Chile from its headquarters in Germany, where data is
               2 Astronomers plan observations but most telescopes are computer -controlled. Computers
               are also used to analyse the dat a collected.

Workbook answers – P7 - Observing the Universe                                                  Page 9 of 10
     b         For example:
               They can be operated remotely.
               They can aut omatically track stars as they cross the sky.
               They can be set to find a particular star with known coordinates.


                Advantage or disadvantage?           Statement

                A                                    They avoid absorption…

                D                                    They are expensive…

                A                                    They can det ect radiation…

                D                                    Servicing relies on…

                A                                    Orbit allows imaging…

                D                                    If things go wrong…

                D                                    Instruments can quickly…

                D                                    Launching limits the size…

     d         For example:
               1 Shares cost of building and operating observatories
               2 Huge amounts of data need analysing
               3 Provides access to data from different kinds of observatories, so full spectrum of a star
               can be studied

62   a         For example:
               1 Altitude – higher altitudes have less atmosphere above them
               2 Optical observatories – less windy sites where density of air is more stable, away from
               light pollution, climate where cloud cover is infrequent

     b         For example:
               1 Accessibility for astronomers and technicians, and provisions
               2 Water supply nearby
               3 Environmental and social impact
               4 Cost of building, operating and maint enance
               Also working and living conditions for employees

Workbook answers – P7 - Observing the Universe                                           Page 10 of 10

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