Topic 6 · Space physics

Marks: 3910
Questions: 105
Multiple choice: 64
Suggested time: 40 minutes

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Core and Supp.the whole paper · 39 marksCore onlySupp. questions drop out · 10 marks

How hard the questions are

Pitched at the board's own level, not above it. Multiple choice is mostly one step, with a couple of two-step items and one that rewards the candidate who does not take the obvious route. Structured questions open on a 1-mark recall and close on an explanation or a suggestion worth two marks — the marks real candidates most often drop.

44RecallOne idea, one step. The mark is for knowing it.
21RoutineThe standard application — the named equation, the usual graph read.
30DemandingSeveral steps, and you have to choose them. Nothing says which comes first.
10DiscriminatingThe part that separates the top grade: an unfamiliar context, a derivation, or an argument that has to hold together to earn anything.

Multiple choiceRecallCore6.1[1]

What causes the seasons on Earth?

AThe Earth's distance from the Sun changes during the year.
BThe Earth's axis is tilted relative to the plane of its orbit.
CThe Earth spins on its axis once every 24 hours.
DThe Sun's output of energy varies during the year.

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Answer overview

BThe Earth's axis is tilted relative to the plane of its orbit.

Multiple choiceRecallCore6.1[1]

Which list gives four planets in order of increasing distance from the Sun?

AMercury, Venus, Mars, Earth
BVenus, Earth, Mars, Jupiter
CEarth, Venus, Jupiter, Saturn
DMars, Earth, Saturn, Jupiter

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Answer overview

BVenus, Earth, Mars, Jupiter

Multiple choiceRoutineSupp.6.1[1]

A planet orbits its star in a nearly circular path of radius 1.1 × 10¹¹ m, taking 1.9 × 10⁷ s to complete one orbit. What is its average orbital speed?

Fig. 3.1 A planet moving on a nearly circular orbit around its star. The star is a disc at the centre of a dashed circle and the planet is a smaller disc on that circle, level with the star and to its right. The distance from the centre of the star out to the planet is marked 1.1 × 10¹¹ m, an arrow at the planet points along the orbit to show the direction in which it travels, and a note beneath states that one complete orbit takes 1.9 × 10⁷ s.

A5.8 × 10³ m/s
B1.8 × 10⁴ m/s
C3.6 × 10⁴ m/s
D5.8 × 10⁴ m/s

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Answer overview

C3.6 × 10⁴ m/s

Multiple choiceRecallCore6.2[1]

What is a light-year?

Athe time taken for light to travel across the Milky Way
Bthe distance travelled by light in one year
Cthe time taken for light from the Sun to reach the Earth
Dthe age of the light emitted by a distant galaxy

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Answer overview

Bthe distance travelled by light in one year

Multiple choiceRecallCore6.2[1]

A star with a mass much greater than that of the Sun reaches the end of its life. Which sequence of stages is correct?

Ared giant → planetary nebula → white dwarf
Bred supergiant → supernova → neutron star or black hole
Cwhite dwarf → supernova → red supergiant
Dprotostar → white dwarf → black hole

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Answer overview

Bred supergiant → supernova → neutron star or black hole

Multiple choiceDemandingSupp.6.2[1]

Light from a distant galaxy shows a spectral line at a wavelength longer than the wavelength of the same line measured in a laboratory on Earth. What does this show?

Fig. 6.1 Two spectra drawn one above the other against a common horizontal wavelength scale that increases to the right. The upper strip is the spectrum of a source in the laboratory on Earth, with one spectral line marked in it. The lower strip is the spectrum of light received from the distant galaxy, in which the same line appears further to the right. A dashed line dropped from each line marks its position on the scale, and the gap between the two positions is labelled as the increase in wavelength.

AThe galaxy is moving towards the Earth.
BThe galaxy is moving away from the Earth.
CThe galaxy is hotter than the laboratory source.
DLight slows down as it travels across space.

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Answer overview

BThe galaxy is moving away from the Earth.

StructuredRoutineCore6.1[6]

The Earth spins on its own axis and also orbits the Sun. The Moon orbits the Earth, taking about one month to complete an orbit.

Fig. 7.1 A not-to-scale plan view of the Sun, the Earth and the Moon. The Sun is a disc at the centre of a large dashed circle, and the Earth is a smaller disc sitting on that circle to the right of the Sun. A second, much smaller dashed circle is drawn around the Earth with the Moon on it, up and to the right of the Earth. A short arrow on each dashed circle shows the direction in which the Earth travels around the Sun and the Moon around the Earth.

(a)
State State the approximate time taken for one rotation of the Earth on its axis, and for one orbit of the Earth around the Sun.
[2]
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Mark-by-mark answer
1. rotation: 24 hours (1 day)
2. orbit: 365 days (1 year)
(b)
Explain Explain why the Moon can be seen from Earth even though it produces no light of its own.
[2]
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Mark-by-mark answer
1. light from the Sun falls on the Moon
2. and is reflected from the Moon's surface to the Earth
(c)
Explain The Sun is much further from the Earth than the Moon is, yet the Sun and the Moon appear almost the same size in the sky. Explain how this is possible.
[2]
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Mark-by-mark answer
1. the Sun's diameter is very much larger than the Moon's — about 400 times larger
2. and the Sun is about 400 times further away, so the two subtend almost the same angle at the Earth

StructuredDemandingSupp.6.1[7]

The table gives the orbital radius and the orbital period of four planets in the Solar System.

planet	orbital radius / 10¹¹ m	orbital period / 10⁷ s
Earth	1.50	3.16
Mars	2.28	5.94
Jupiter	7.78	37.4
Saturn	14.3	92.9

(a)
Calculate Calculate the average orbital speed of Jupiter.
[3]
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Mark-by-mark answer
1. v = 2πr / T
2. v = 2π × 7.78 × 10¹¹ / 37.4 × 10⁷
3. v = 1.31 × 10⁴ m/s
(b)
Determine Without further calculation, determine whether Saturn's orbital speed is greater or smaller than Jupiter's, and justify your answer using the table.
[2]
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Mark-by-mark answer
1. Saturn's radius is about 1.8 times Jupiter's, but its period is about 2.5 times Jupiter's
2. the period grows by the larger factor, so 2πr/T is smaller — Saturn moves more slowly
(c)
Explain Explain, in terms of gravitational field strength, why planets further from the Sun move more slowly.
[2]
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Mark-by-mark answer
1. the Sun's gravitational field strength decreases with distance from the Sun
2. so a more distant planet is held by a weaker force and a smaller speed is enough to maintain its orbit

StructuredDemandingSupp.6.2[8]

A star forms from a cloud of dust and gas that is pulled together by gravitational attraction. The Sun is a stable star roughly halfway through its life.

(a)
Explain Explain why the Sun is stable at present, in terms of the forces acting within it.
[2]
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Mark-by-mark answer
1. the inward gravitational attraction of the Sun's own mass
2. is balanced by the outward pressure produced by the energy released in the core, so the Sun neither collapses nor expands
(b)
Describe Describe the process by which energy is released in the core of the Sun.
[3]
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Mark-by-mark answer
1. hydrogen nuclei fuse together at the very high temperature and pressure of the core
2. The fusion products combine to form helium nuclei in the Sun's core.
3. the mass of the products is slightly less than that of the reactants, and the difference is released as energy
(c)
Describe Describe what will happen to the Sun once the hydrogen in its core has been used up.
[3]
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Mark-by-mark answer
1. the core contracts and the outer layers expand — the Sun becomes a red giant
2. helium and heavier nuclei fuse in the core, up to the mass of iron
3. the outer layers are then thrown off as a planetary nebula, leaving the hot dense core as a white dwarf

Data analysisDiscriminatingSupp.6.2[12]

Astronomers measure the speed at which distant galaxies are moving away from the Earth and the distance to each galaxy. The table gives values for four galaxies.

Fig. 10.1 An empty graph grid on which the readings in the table can be plotted. The horizontal axis is labelled distance d / 10²⁴ m and runs from 0 to 10 with a numbered gridline at every unit; the vertical axis is labelled speed of recession v / 10⁶ m/s and runs from 0 to 25 with a numbered gridline every 5. No points are plotted on the grid.

galaxy	distance d / 10²⁴ m	speed of recession v / 10⁶ m/s
P	1.2	2.6
Q	3.5	7.7
R	6.0	13.2
S	9.4	20.7

(a)
Describe Describe the relationship between the speed of recession and the distance shown by the table.
[2]
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Mark-by-mark answer
1. the speed of recession increases as the distance increases
2. and the increase is proportional — v/d is the same for all four galaxies
(b)
Determine Determine a value for the Hubble constant H₀ from these data, and give its unit.
[3]
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Mark-by-mark answer
1. H₀ = v / d
2. using galaxy S: H₀ = 20.7 × 10⁶ / 9.4 × 10²⁴
3. H₀ = 2.2 × 10⁻¹⁸, unit s⁻¹ (per second)
Examiner insight
Working from a single well-separated point, or from the gradient of a v–d graph, both earn full marks. Taking the mean of the four ratios is also accepted.
(c)
Estimate Use your value of H₀ to estimate the age of the Universe. Give your answer in seconds.
[2]
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Mark-by-mark answer
1. age ≈ 1 / H₀
2. age ≈ 1 / 2.2 × 10⁻¹⁸ = 4.5 × 10¹⁷ s
(d)
Explain Explain how the relationship in (a) supports the idea that the Universe began with the Big Bang.
[3]
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Mark-by-mark answer
1. every distant galaxy is moving away from us, and the further away it is the faster it recedes, so the Universe is expanding
2. tracing the motion backwards, all the galaxies were once much closer together
3. so the expansion began from a single point at a definite time in the past
Examiner insight
The last mark is the one that closes the argument. Candidates who describe the expansion but never run it backwards have given evidence for expansion, not for a beginning.
(e)
Suggest The astronomers measured only four galaxies. Suggest two reasons why a value of H₀ obtained this way is less reliable than one obtained from several hundred galaxies.
[2]
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Mark-by-mark answer
1. the distance to a galaxy is very difficult to measure and carries a large uncertainty, so a value from four galaxies depends heavily on whether those four happen to be well measured
2. galaxies also have their own motion within their local cluster on top of the expansion, and averaging over many galaxies is what reduces that additional scatter
Examiner insight
The only place on this paper where the marks are for judging data rather than using it — which is what Topic 6 offers in place of a practical, since space physics is not examined on Paper 5 or Paper 6.

The cheapest marks on any paper

What the command words are asking for

Every board publishes these and marks to them. A candidate who explains where the question said state has spent four minutes earning one mark; one who states where it said explain has earned none.

Calculate: Work out from given facts, figures or information.
Define: Give the precise meaning.
Describe: State the points of a topic; give characteristics and main features.
Determine: Establish an answer using the information available.
Estimate: Suggest an approximate value.
Explain: Set out purposes or reasons; make relationships evident; give why and/or how.
Give: Produce an answer from a given source or recall.
Identify: Name, select or recognise.
Show (that): Provide structured evidence that leads to a given result.
State: Express in clear terms.
Suggest: Apply knowledge and understanding to situations where there is a range of valid responses in order to make proposals.

What is being tested

IGCSE assessment objectives, and how this paper divides between them

Every candidate sits two theory papers and one practical-skills paper. Core takes Papers 1 and 3 and is capped at grade C; Extended takes Papers 2 and 4 and reaches A*. Both then take either Paper 5 (practical test) or Paper 6 (alternative to practical), which carries 20% either way. Theory papers give you no formula sheet.

AO118 marks · 46%8 marks · 80%

Knowledge with understanding

Recall, describe, explain and use physics ideas, terminology, instruments and conventions.

Across the whole qualification: 50%

AO219 marks · 49%2 marks · 20%

Handling information and problem-solving

Locate and interpret information, translate between forms, calculate, reason, and apply physics to unfamiliar situations.

Across the whole qualification: 30%

AO32 marks · 5%0 marks · 0%

Experimental skills and investigations

Plan, use apparatus, record and present observations, analyse, evaluate, and suggest improvements.

Across the whole qualification: 20%